Extended Interactions between Metal Ions. in Transition Metal Complexes 9780841202245, 9780841201712, 0-8412-0224-9

Table of contents :
Title Page......Page 1
Copyright......Page 2
ACS Symposium Series......Page 3
FOREWORD......Page 4
PdftkEmptyString......Page 0
PREFACE......Page 5
Literature Cited......Page 6
1 Extended Interactions in Transition Metal Oxides and Chalcogenides......Page 8
Literature Cited......Page 22
2 High Temperature Crystal Chemistry of V2O3 through the Metal-Insulator Transition and of (Cr0.01 V0.99)2O3, an Insulator......Page 23
Results......Page 24
Acknowledgements.......Page 28
Literature Cited.......Page 29
3 Synthesis and Properties of Some New Organic Intercalation Complexes of Tantalum Sulfide......Page 30
Results and Discussion......Page 31
Summary......Page 38
Literature Cited......Page 39
Acknowledgment......Page 40
Discussion......Page 41
Literature Cited......Page 46
1. Introduction......Page 47
2. Eigenfield Equations for Exact Calculation......Page 48
3. Frequency-Shift Perturbation......Page 52
Literature Cited.......Page 57
Structural Background......Page 58
Interactions in Cr(urea) 6X3......Page 61
Additional Studies of Interactions in M(urea) 6X3 Lattices......Page 70
Literature Cited......Page 72
Strong, Chemically-Significant Interactions Through a Bridging Ligand. μ-οxο-Bridged Complexes of Ruthenium (III).......Page 73
Weak Metal-Metal Interactions in Ligand-Bridged Ruthenium Complexes.......Page 75
The Effects of Weak Metal-Metal Interactions in Polymeric, Ligand-Bridged Compounds.......Page 78
Literature Cited......Page 82
Introduction......Page 83
Nickel(II) Dimers......Page 84
Copper(II) Dimers......Page 91
Cobalt(II) and Manganese(II) Dimers......Page 97
Literature Cited......Page 100
Introduction......Page 101
Glycinato Complex......Page 102
Phenanthroline Complexes......Page 104
Oxalato Complex......Page 106
Interpretation......Page 108
Literature Cited......Page 113
10 Superexchange Interactions in Copper(II) Complexes......Page 115
A. Structural and Magnetic Data.......Page 116
B. Correlation of Structural and Magnetic Properties.......Page 125
C. Rationalization in Terms of a Molecular Orbital Model.......Page 127
A. Structural and Magnetic Data.......Page 131
III. The Polymeric Compound [Cu(pyrazine)(NO3)2]n and Related Chains.......Page 139
V. Literature Cited......Page 145
Biscyclopentadienyl Titanium Derivatives......Page 149
RMX3 Complexes......Page 161
Literature Cited......Page 169
12 Optical Properties of Linear Chains......Page 171
Literature Cited......Page 188
13 Spectroscopic and Magnetic Properties of CsMI3 Type Transition Metal Iodides......Page 189
Literature Cited......Page 199
Crystal Structures......Page 201
Properties of [(CH3) 3NH]CoX3·2H2O (X=Cl, Br)......Page 203
Properties of [(CH3) 3NH]MnX3·2H2O (X=Cl, Br)......Page 206
Literature Cited......Page 211
Introduction......Page 212
Chemical Preparation......Page 213
Susceptibility and Magnetization Studies......Page 214
Mössbauer Studies......Page 218
Molecular Structure Studies......Page 220
Magnetic Behavior and Structure......Page 225
Literature Cited......Page 226
Acknowledgements......Page 227
16 The Unusual Magnetic Properties of an Imidazolate Bridged Polymer of an Iron(III) Hemin......Page 228
Literature Cited......Page 239
1. Introduction......Page 241
2. Localized and Collective States......Page 242
3. Types of Transition Metal Chain Compound......Page 243
4. Single Valence Metal Chain Compounds......Page 246
5. Mixed Valence Metal Chain Compounds......Page 253
Literature Cited......Page 259
Introduction......Page 261
K2PtBr4 and K2PtCl4: Minimum Extended Interactions......Page 264
Magnus1 Green Salt. Pt(NH3)4PtCl4.......Page 268
Pt2Br62- Spectra......Page 271
Pt(en)Cl2 and Pt(en)Br2. Transitions to Ionic States......Page 273
Summary......Page 280
Literature Cited......Page 282
Abstract......Page 283
Introduction......Page 284
Specular Reflection Spectroscopy......Page 285
The Glyoximates......Page 289
The Magnus' Salts......Page 292
Tetrachloroplatinate(II) Ion and Related Systems......Page 295
Characterization of the Excited State......Page 298
General Discussion......Page 302
Acknowledgment:......Page 305
Literature Cited......Page 307
Introduction......Page 308
The Electronic Structure of Planar d8 Complexes......Page 309
Results and Discussion......Page 311
Literature Cited......Page 320
I. Introduction......Page 321
II. Electronic structure of suitable metal complexes......Page 322
IV. Choice of suitable transition metal compounds......Page 325
V. Systematic Variation of Ligand Parameters......Page 326
VI. Preparation of "Mixed Valence" Solids......Page 331
Acknowledgements......Page 335
"Literature Cited,"......Page 336
Introduction......Page 338
Materials Preparation......Page 343
Experimental Results......Page 344
Discussion......Page 350
Literature Cited......Page 355
23 Studies on Some 1-D Metal Group VIII Complexes......Page 357
Literature Cited......Page 362
I. Introduction......Page 363
II. General Remarks......Page 364
III. Band-Structure Energy UB in the Tight-Binding Model......Page 366
IV. Beyond the Tight-Binding Approximation......Page 370
V. The Covalent Energy UC......Page 373
VI. Electrochemical Oxidation of Crystalline K2Pt(CN)4 to a Solid-State Galvanic Cell......Page 374
VII. Summary and Conclusions......Page 375
Literature Cited......Page 376
Acknowledgments......Page 378
25 The Peierls Transition in One-Dimensional Solids......Page 379
References......Page 382
26 The Preparation of and the Anisotropic Dielectric Properties of K2Pt(CN)4Br0.30·3H2O......Page 383
Literature Cited......Page 387
Introduction......Page 389
Method of Calculation......Page 390
Results and Discussion......Page 391
Acknowledgments......Page 397
Literature Cited......Page 398
One-dimensional Systems......Page 399
Organic Linear Systems......Page 400
Transition Metal Complexes and TCNQ Salts......Page 401
Pseudo-One-Dimensional Systems......Page 403
The STEP Arrangement......Page 405
The LADDER Arrangement......Page 407
Literature Cited......Page 408
C......Page 410
I......Page 411
M......Page 412
R......Page 413
T......Page 414
Ζ......Page 415

Citation preview

Extended Interactions between Metal Ions in Transition Metal Complexes LeonardV.Interrante,Editor

A symposiu the D i v i s i o n of Inorganic Chemistry at the 167th Meeting of the American Chemical Society, Los Angeles, Calif., April 3 - 5 , 1974.

ACS

AMERICAN

SYMPOSIUM

CHEMICAL

SERIES

SOCIETY

WASHINGTON, D. C. 1974

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5

L i b r a r y o f Congress CIP D a t a Extended interactions between metal complexes. ( ACS symposium series; 5 )

metal

ions

in

transition

Includes bibliographical references and index. 1. T r a n s i t i o n metal compounds—Congresses. 2. Com­ plex compounds—Congresses. 3. Solid state c h e m i s t r y — Congresses. I. Interrante, Leonard V . , 1 9 3 9 ed. II. American C h e m i c a l Society. D i v i s i o n of Inorganic Chemistry. III. Series: A m e r i c a n C h e m i c a l Society. ACS symposium series; 5. QD172.T6E49 I S B N 0-8412-0224-9

Copyright ©

546'.6 ACSMC8

5

1-408

74-30455 (1974)

1974

American Chemical Society All Rights Reserved PRINTED IN THE UNITED STATES OF AMERICA

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

ACS Symposium Series Robert F. Gould, Series Editor

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

FOREWOR The

ACS

S Y M P O S I U M SERIES was

founded in 1974

to provide

a medium for publishing symposia quickly in book form. The format of the SERIES parallels that of its predecessor, ADVANCES IN C H E M I S T R Y SERIES, except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready

form.

As a further

means of saving time, the papers are not edited or reviewed except by the symposium chairman, who

becomes editor of

the book. Papers published in the A C S

S Y M P O S I U M SERIES

are original contributions not published elsewhere in whole or major part and include reports of research as well as reviews since symposia may embrace both types of presentation.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

PREFACE he papers that comprise this volume span two major areas of research — s o l i d state chemistry and physics, and the study of transition metal complexes. The former has been concerned largely with relatively simple ionically or covalently bound inorganic solids such as the metal oxides, halides, and sulfides, where extended interactions are the rule rather than the exception. On

the other hand, until quite recently, the study of

transition metal complexes has been almost entirely a study of molecular systems, where the interaction

i

th

solid stat

generall

f th

weak van der Waals typ closely those of the constituent molecular units in solution. In the last few years it has become clear that there are a number of transition metal complexes which do not fit this description very well. In particular, these materials usually exhibit some of the characteristics typical of molecular systems but display certain optical, magnetic, or electrical properties which evidence "extended

interactions" of appre-

ciable magnitude in the solid state. The study of such complexes constitutes the main subject of this volume. Many of these systems show quite anisotropic and even pseudo-"onedimensional" solid state behavior, reflecting strong intermolecular interactions of a highly directional character in the crystal. Such one-dimensional systems, based on both metal complexes and organic charge-transfer compounds, are currently of considerable interest within the solid state physics community and are under active study in laboratories here and abroad (I, 2).

The intense interest in these materials can be attributed,

in part, to suggestions regarding the possibility of superconductivity in such systems (3, 4).

The actual likelihood of superconductivity here is

a matter of some controversy; however, the wealth of new

concepts and

the physical understanding that has already resulted from the study of these systems, not to mention the high conductivities and highly anisotropic optical, magnetic, and electronic properties that many of them exhibit, promise continued growth of interest. One

important conclusion from the work on these and other types

of transition metal complex solids is that interactions between the metal ions in the crystal, propagated

either directly or through a bridging

group, are often of major importance in determining the solid state properties. The nature of these metal ion interactions and the manner in ix

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

which they relate to the observed properties is the principal focus of the papers in this volume. The first three chapters, which are concerned with transition metal oxides and sulfides, introduce the topic of extended interactions in transi­ tion metal compounds from a solid state viewpoint and provide an account of current work on the simple inorganic, non-molecular systems. The discussion of transition metal complexes begins with Chapter 4, which deals with exchange interactions in, and pathways for exchange in some weakly

associated molecular systems.

The next two papers

describe the application and interpretation of electron spin resonance measurements for studying exchange interactions in such systems. Chapters 7 through 11 deal with systems in which the exchange interactions are localized largely within dimeric or higher polymeric units and provide some fundamenta metal ion, the bridging group

geometry

type and magnitude of the exchange process. Chapters 10 and 11 also introduce the topic of extended exchange interactions in one-dimensional, ligand-bridged systems—a topic which is developed further in Chapters 12-17. Beginning in Chapter 17, which provides a general survey of current work on one-dimensional metal complex systems, the discussion shifts to direct metal-metal interactions in solids containing stacked planar metal complexes.

The remaining chapters continue this discussion, concluding

with some recent work on the mixed valence platinum salts, which have lately been of much interest for their one-dimensional metallic char­ acteristics. The authors include both academic and industrial research scientists from the United States and Europe and represent virtually every phase of the current work on transition metal complex solids. The symposium on which this volume is based was supported, in part, by grants and travel assistance provided by the United States Air Force, through its European Office for Aerospace Research and Develop­ ment, General Electric Corporate Research and Development, and the Inorganic Division of the American Chemical Society. This support made it possible for several European authors to travel to the symposium and contributed greatly to the international flavor and overall success of the program. Literature

Cited

1. Zeller, H. R., Advan.

Solid

State

Phys.

2. Garito, A. F., Heeger, A. J., Acc. Chem.

(1973) 13, 31.

Res. (1974) 7, 232.

χ

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3. Little, W. Α., Ed., Proc. Intern. Superconductors, J. Polymer

Symp. Phys. Chem. Problems Sci. C (1969), No. 29.

Possible

Org.

4. Coleman, L. B., Cohen, M. J., Sandman, D. J., Yamagishi, F. G., Garito, A. F., Heeger, A. J., Solid

State

Comm.

(1973) 12, 1125. L. V. I N T E R R A N T E

Schenectady, Ν. Y. November 6, 1974

xi

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1 Extended Interactions in Transition Metal Oxides and Chalcogenides AARON WOLD D i v i s i o n of E n g i n e e r i n g , B r o w n U n i v e r s i t y , Providence, R.I. 02912

As e a r l y as 1937 i n t r a n s i t i o n metal o x i d e s , in which the d-bands o f the t r a n s i ­ t i o n metal ions were partially filled, the p o t e n t i a l energy b a r r i e r between atoms was high enough t o reduce the c o n d u c t i ­ vity by an enormous amount. This was probably the first i n d i ­ c a t i o n that the Block-Wilson band theory of s o l i d s (1,2) could not d e s c r i b e i n a realistic way the t r a n s p o r t p r o p e r t i e s o f t r a n s i t i o n metal compounds. Indeed, the classical band theory had p r e d i c t e d that these compounds with partially filled d-bands would show high electrical c o n d u c t i v i t y (σ = n e μ); the num­ ber o f c a r r i e r s n would show temperature independence and the m o b i l i t y μ would decrease as the temperature Τ increased. The electrical c o n d u c t i v i t y observed f o r these compounds would be de­ r i v e d from these latter temperature dependencies. In 1957 (3) Morin r e p o r t e d on the electrical p r o p e r t i e s of s e v e r a l vanadium and t i t a n i u m o x i d e s , namely VO, V O , VO and Ti 0 . These oxides c o n t a i n 3,2,1 and 1-d e l e c t r o n s per t r a n s i ­ t i o n metal c a t i o n r e s p e c t i v e l y . These compounds show m e t a l l i c behavior a t high temperatures and then become semiconducting when cooled through a critical temperature Τ , (114°K, 153°K, 340°K and 450°K). For most o f these compounSs, the electrical resistivity was observed t o drop by s e v e r a l orders o f magnitude over a s m a l l temperature range. The oxides of vanadium show a r e d u c t i o n i n symmetry a s s o c i a t e d with the resistivity changes. The lattice parameters o f T i O vary q u i t e r a p i d l y i n the vici­ nity of the t r a n s i t i o n temperature ( T ) but there is no change i n symmetry (4). Figure 1 summarizes these f i n d i n g s . I t should a l s o be i n d i c a t e d that f o r pure s t o i c h i o m e t r i c s i n g l e c r y s t a l s , the temperature range over which the t r a n s i t i o n occurs is g r e a t l y reduced and the magnitude o f the d i s c o n t i n u i t y i n electrical con­ d u c t i v i t y i s increased by a f a c t o r o f 1 0 . The 3d t r a n s i t i o n metal oxides may t h e r e f o r e be e i t h e r me­ tallic at a l l temperatures, semiconducting at all temperatures or undergo a semiconductor

°3>

2°3

MnS, MnS , FeS « 2

2

-CLASS I I I T r a n s i t i o n a l Compounds

3d-compounds:

Vo, V Og, V 0

T i

2°3>

NiS, Ud-compound s :

T i

3°5*

V ^ , ν^0 , ν 0

2 $

?

F

e

β

2 V

CrS, FeS.

NbO . 9

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

E X T E N D E D INTERACTIONS

4

V Q '.(3d) 2

V0 ;(3d)'

2

TLCUSd)

2

3

B E T W E E N M E T A L IONS

1

log ο* IOV ^. I Ο* Electrical Conductivity! I O ΙΟ σ (σ ohm"'cm") - i o 4 _ _ J L i o IO -ΙΟ" T ^450°K Τ T ^34Q°K τ Τ,*Μ50*Κ Τ (Z>i?J-?J),(2S)*C29-30)J(7),(35-37) P^O 6?i 17-21Y22 - 23V References (22),(3j-33)*,(34) 0

1

J

J

t

t

X X IO*

XX ΙΟ*

XX ΙΟ*

Magnetic 16501 TOO Susceptibility], 1050 X 200 J (x in eg*, ΙΟΟ ΙΟΟ mole"' ) "673Ύ°Κ io iSolooT^ 33 ΙΟΟ 3 4 0 450Τ°Κ J(I2)(42-43)(26)^I)/221)(I3-I4)/3J)*(45) Γ(12)(3β)*.(3?-4Ι) References Κ4ΪΧ(44) Specific Heat) 81 (C in cals, mole** deg"')| p

^Lo700colt/»o»«

Τ

Figure

Ι.

*M.o750ce»»/eol*

20

15

28 20 Τ Κ

Τ Κ

κ

Electrical conductivity, cific heat vs. temperature

magnetic susceptibility, and spe­ for V>0, , VO>, and Ti 0. {

Figure

IL

:

Structure

s

of ReO 2

30 "

Re * 6

[2>[6] σ ·

j[2][2][2]

ζ-JT 1

(E

E

m,

FERML LEVEL

*[2][21Κ

Ε - E, )

}

1

*cf

/

[2>[3]

Figure

III.

Electron

energy diagram for ReO.,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1.

WOLD

Oxides

and

5

Chalcogenides

cubic bronze, Ma WO^, and the ordered p e r o v s k i t e Sr^M ReO have been i n v e s t i g a t e d by Goodenough and co-workers (9)· The one e l e c t r o n energy diagram f o r ReO« i s shown i n Figure III. Because o f the o c t a h e d r a l c r y s t a l f i e l d the d - e l e c t r o n energy s t a t e s o f the rhenium are s p l i t i n t o a l e s s s t a b l e , doubly degenerate s t a t e (e ) and a more s t a b l e t r i p l y degenerate ( t ^ )· The outer e l e c t r o n Inergy l e v e l s of the anion are s i m i l a r l y s flown on the r i g h t s i d e o f the f i g u r e . The overlap o f c a t i o n t and anion p^ o r b i t a l s r e s u l t s i n the formation o f bonding and Intibonding bands extending throughout the c r y s t a l (10^. The Fermi l e v e l may be l o c a t e d by l o o k i n g a t the number o f outer elec^roçs per molecule. Consider the ReO^ u n i t f o r rhenium 5d 6s . Ther 2s 2p c o n t r i b u t e [6] e l e c t r o n s per molecule. Upon f i l l i n g the a v a i l a b l e energy l e v e l s , the l a s t e l e c t r o n occupies the ir*band. This band i s p a r t i a l l y f i l l e d and causes the observed m e t a l l i c behavior. The o r i g i n o f the m e t a l l i c c o n d u c t i v i t y o f cubic Na^WO^ has been p o s t u l a t e d by s e v e r a l models: 1. The d i r e c t overlap of sodium 3p o r b i t a l s (11) 2. The d i r e c t overlap o f tungsten t o r b i t a l s l e a d i n g t o tungsten-tungsten bonds (12) 3. The covalent mixing o f tungsten t o r b i t a l s and oxygen ρ orbitals t o form p a r t i a l l y f i l l e d b a n d s ( 1 3 ) . Goodenough and h i s co-workers (9_) performed a unique ex­ periment which i n d i c a t e d t h a t the t h i r d p o s s i b i l i t y was indeed the most probable mechanism f o r the observed m e t a l l i c con­ d u c t i o n i n these m a t e r i a l s . The compound Sr M c crystallizes i n the ordered p e r o v s k i t e s t r u c t u r e (see F i g u r l IV;. The B - s i t e c a t i o n s , Mg and Re, order such that each rhenium has only magne­ sium nearest c a t i o n neighbors and v i c e versa ( 9 ) . Such an arrangement would s t i l l allow f o r the overlap of rhenium t ~ or­ b i t a l s across a cube f a c e . Consequently, m e t a l l i c behavior would be expected i f Model 2 a p p l i e d . However, i f the e l e c t r i c a l pro­ p e r t i e s are a r e s u l t of c a t i o n t and oxygen interactions (Model 3 ) , Sr MgReOg should be a iemi-conductor, s i n c e Mg does not possess t e l e c t r o n s . The observed semi-conductive be­ havior (E = 0.§leV) and temperature dependent paramagnetism con­ f i r m t h a t the e l e c t r o n s are l o c a l i z e d r a t h e r than occupying c o l l e c t i v e energy bands ( 9 ) . Thus, model 3 i s c o n s i s t e n t with the i n t e r a c t i o n s present i n p e r o v s k i t e - l i k e m a t e r i a l s and best e x p l a i n s the observed p r o p e r t i e s . g

g

2

δ

g

π

R e 0

2

2

2

2

II. Vanadium (IV) Oxide and S u b s t i t u t e d Compounds. The high temperature, m e t a l l i c VO phase has a t e t r a g o n a l r u t i l e l i k e (1*0 s t r u c t u r e (space group p4 /mnm). As can be seen from Figure V the s t r u c t u r e may be d e s c r i b e d as s t r i n g s o f edge-shared octahedra j o i n e d by corners extending i n the c d i r e c t i o n . The V-V d i s t a n c e s are equivalent w i t h i n the s t r i n g s (2.87 8 ). 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6

E X T E N D E D INTERACTIONS

B E T W E E N M E T A L IONS

• Re O o Figure

IV.

Structure

of

Sr MgReO« 2

V 0 (tetragonal) 2

• Vanadium Ο Oxygen Figure V.

Rutile

structure

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1.

WOLD

Oxides

and

Chalcogenides

7

Below 340°K, VO^ has been reported to have a d i s t o r t e d r u ­ t i l e s t r u c t u r e with monoclinic symmetry (P2^/c. This d i s t o r t i o n (see F i g u r e VI) causes the vanadium atoms l o c a t e d i n the s t r i n g s of VOg octahedra t o occur as doublets ( s i m i l a r t o MoO^). The V-V d i s t a n c e s are no longer equal but are 2.658 and 3.12^ (1^0· As pointed out by Heckingbottom and L i n e t t (15), i n the low temper­ ature semiconducting phase the c a x i s i s t i l t e d causing the vana­ dium atoms to be d i s p l a c e d from the center o f i t s octahedra. The band s t r u c t u r e f o r t e t r a g o n a l V 0 has a l s o been examined by Goodenough (14) and i s shown i n Figure V I I . Because of edge sharing of VO^ octahedra i n the t e t r a g o n a l phase, an o r t h o r hombic component o f the c r y s t a l f i e l d removes the d-state de­ generacy. The two e o r b i t a l s , normally o c c u r i n g i n an o c t a ­ h e d r a l f i e l d , are s p f i o r b i t a l s s p l i t i n t o tw ρ o r b i t a l and a d n o r b i ! a l d i r e c t e d along the c r u t i l e a x i s . When the 17 outer e l e c t r o n s per molecule i n V0- are placed i n t o t h i s energy scheme the f i n a l e l e c t r o n enters the over­ lapping π* and d i i bands. These o v e r l a p p i n g , non-degenerate bands, being o n l y ' p a r t i a l l y f i l l e d , r e s u l t i n m e t a l l i c conductiv­ i t y i n the t e t r a g o n a l phase (14). Below the t r a n s i t i o n temperature VO^ has a m o n o c l i n i c a l l y d i s t o r t e d r u t i l e s t r u c t u r e c h a r a c t e r i z e d by the formation o f V-V p a i r s along the a monoclinic a x i s ( «i ) * a consequent d o u b l i n g g f the c r y s t a l o g r a p h i c u n i t c e l l ' ( 1 6 ) . This displacement o f the V ion from i t s center of symmetry i s b e l i e v e d to be caused by a f e r r o e l e c t r i c - type d i s t o r t i o n (14). The e f f e c t o f t h i s d i s t o r t i o n on the band s t r u c t u r e o f VO^ i s shown i n Figure V I I I . The d j i band i s s p l i t i n two and the Fermi energy i s lowered below'the bottom of the π» band. As a consequence o f doubling the c e l l , the lower X band i s f i l l e d completely and semi-conducting behavior r e s u l t l (14). 2

c

a x : L S

r u t

a n c

e

+

I I I . O x y f l u o r i d e s . At low l e v e l s of f l u o r i n e , the oxyf l u o r i d e s u s u a l l y possess s t r u c t u r e s q u i t e s i m i l a r to the parent oxide. The c e l l parameters (orthorhombic indexing) a = 7.356 A, o °*> °- r e p o r t e d f o r WO F are q u i t e s i m i l a r to those observed by S l e i g h t (17) f o r monoclinic W0 a = 7.301 X b = 7.538 8, c = 3.844 ^7 3 = 90.89°. S l e i g h t d i d not r u l e out the p o s s i b i l i t y that WO^ gg^Q ^ could be monoclinic with the d e v i a t i o n o f the monoclinic angïe from 90° being too s m a l l to d e t e c t . Higher s u b s t i t u t i o n s of f l u o r i n e i n the WO^ F^ systems s t a b i l i z e s the c u b i c Re0 s t r u c t u r e (17, 18).~¥his s t r u c t u r e i s analogous t o the p e r o v s k i t e type a l k a l i metal bronzes with a l l A s i t e s vacant. In the tungsten o x y f l u o r i d e system the cubic phase extended from χ = 0.17 to 0.66. A one-electron energy diagram would be much simpler f o r 3 x x ^ corresponding cubic tungsten bronzes Na^WO^. I t Is not necessary t o consider i n t e r a c t i o n s i n v o l v i n g A s i t e c a t i o n s s i n c e these p o s i t i o n s are empty·

b

=

7 , 1 + 6 9

c

=

3 , 8 1 4 6

0

g 6

Q

Q l +

3

3

W 0

F

t

n

a

n

o r

t

n

e

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8

E X T E N D E D INTERACTIONS

£

S

/ y

i

B E T W E E N M E T A L IONS

= 2c

yf

—

/A

rutile axes Figure

V

Figure

7=

monoclinic axes

VI. Monoclinic structure temperature) for VO.

I

V0_

(low

(

VII. Electron energy for tetragonal VO

diagram

:

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1.

WOLD

Oxides

and

Chalcogenides

9

The s u b s t i t u t i o n o f f l u o r i n e ajlso r e s u l t s i n a d d i t i o n a l elec­ trons because o f the formation o f W i o n s . Thus, the compounds WOg F have χ a d d i t i o n a l e l e c t r o n s per molecule. Although i t mighi £e expected that the more e l e c t r o n e g a t i v e f l u o r i d e ion would have a l o c a l i z i n g e f f e c t on the a d d i t i o n a l e l e c t r o n s , me­ t a l l i c c o n d u c t i v i t y has been observed f o r the c u b i c o x y f l u o r i d e bronzes. By comparison with the WO. F system, i n which l a r g e f l u o r i n e s u b s t i t u t i o n s s t a b i l i z e a high symmetry c u b i c phase, there may a l s o be an analogous decrease i n the semiconductor metal t r a n s i t i o n with i n c r e a s i n g f l u o r i n e s u b s t i t u t i o n i n 2 - x . For V 0 , the monoclinic t o t e t r a g o n a l t r a n s i t i o n temper­ ature decreases with i n c r e a s i n g anion s u b s t i t u t i o n i . e . the high temperature t e t r a g o n a l phase becomes more s t a b l e (19). For the system V 0 from the r e s i s t i v i t y dat i n Table II f o r d i f f e r e n t values of χ i n V0F . I t can be seen 2—χ χ from these curves t h a t a m e t a l l i c t o semiconductor t r a n s i t i o n occurs a t a temperature which decreases with i n c r e a s i n g values of x. T h i s change i n the e l e c t r i c a l p r o p e r t i e s o f V 0 ^F^ can be explained by the corresponding t r a n s i t i o n from the monoclinic phase t o the t e t r a g o n a l phase which has been observed by means of low-temperature X-ray a n a l y s i s . A l i n e a r r e l a t i o n s h i p e x i s t s be­ tween the value of χ and the t r a n s i t i o n temperature (T ) shown i n F i g u r e X; t h i s e x t r a p o l a t e s t o the c o r r e c t t r a n s i t i o n temperature f o r pure V0 . For the higher f l u o r i n e compounds, the t r a n s i t i o n r e g i o n i s c o n s i d e r a b l y broadened and the t r a n s i t i o n p o i n t was chosen as the f i r s t d e v i a t i o n from l o g - l i n e a r behavior. A s i m i ­ l a r l i n e a r r e l a t i o n s h i p e x i s t s between the volume of the t e t r a ­ gonal c e l l and the value o f x, again e x t r a p o l a t i n g t o the value of the pure VC> phase a t χ = 0 (see F i g u r e X I ) . The same behavior has been observed i n compounds corresponding t o the formula V W 0 (0$x^0.067) by Nygren and I s r a e l s s o n (19). x-x x . i s seen, t h e r e f o r e , that the a d d i t i o n o f f l u o r i n e tends to s t a b i l i z e the high temperature, higher symmetry, r u t i l e phase. The compositions c o n t a i n i n g l a r g e r amounts of s u b s t i t u t e d f l u o r i n e shows p r i m a r i l y m e t a l l i c behavior. However, f o r a l l compositions s t u d i e d there s t i l l appears t o be a d i s c o n t i n u i t y i n the r e s i s t i ­ v i t y a t the expected t r a n s i t i o n temperature ( T ) . The m e t a l l i c behavior observed i n these m a t e r i a l s may be ex­ p l a i n e d on the model presented by Goodenough (13) and Rogers (20). In t h e i r model the band formed between the overlap o f the t σ o r b i t a l s p a r a l l e l t o the c r y s t a l l o g r a p h i c c d i r e c t i o n s p l i t s 'into a more s t a b l e , p a i r - l o c a l i z e d , bonding V-V s t a t e and a higher l e s s s t a b l e σ* s t a t e . The lower l y i n g V-V l e v e l i f f i l l e d with one e l e c t r o n per vanadium and hence the semiconducting p r o p e r t i e s of the monoclinic V 0 may be e x p l a i n e d . The s u b s t i t u t i o n of f l u o r i n e f o r oxygen i n V 0 r e s u l t s i n the c r e a t i o n o f a d d i t i o n a l unpaired d - e l e c t r o n s . Despite the tendency f o r the more e l e c t r o ­ negative anion t o l o c a l i z e d - e l e c t r o n s , i t i s apparent t h a t f o r V 0

P

2

2

2

2

2

t

t

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10

E X T E N D E D INTERACTIONS

BETWEEN

M

monoclinic

tetragonal

V

2°4

Figure

/

VO F 2-X X ι /'

L0Gp|

X-0-208

0

2 b

L 10

20

x. SO

7\

X0

I

-2

2

J

I

Figure

I

6

4

IX.

8

Log

P

I

10

vs. W/Ί

L

12

Ιθ/

Γ
. ,F,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

M E T A L IONS

WOLD

1.

Oxides

and

11

Chalcogenides

T a b l e 2.

C e l l Parameters of VO_» F C o m p o u n d s V

C e l l Parameters (A)

X

A c t i v a t i o n Energy (eV) Τ (°K)

Compound 4.530 - 4

V0„

V0

1.97 0.03

V0

1.96 0.04

2 %9 - 3 e

340

0.5 j u s t below Τ

F

4.552 -

F

4.554 ί 4

2.854 ί 3

0.06

282

VO

F 1.86 0.14

+ 4.562 - 4

+ 2.876 - 5

< 0.01

155

VO

F 1.79 0.21

+ 4.569 - 4

+ 2.886 - 3

< 0.01

65

Figure

X.

Transition

temperature

vs. composition

for

VO>.F.

r

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

12

EXTENDED

INTERACTIONS B E T W E E N M E T A L

IONS

the amount o f f l u o r i n e which has been s u b s t i t u t e d , the conduction paths have not been s u b s t a n t i a l l y a l t e r e d . IV. T r a n s i t i o n Metal Chalcogenides. Binary and ternary t r a n s i t i o n metal chalcogenides e x h i b i t a v a r i e t y o f d i f f e r e n t s t r u c t u r e s which a r e o f t e n q u i t e complex. This has been a t t r i b u ­ ted i n p a r t t o the considerable covalent nature o f the metals u l f u r bonds. Many o f these compounds a l s o e x h i b i t semimetallic or m e t a l l i c p r o p e r t i e s and t h i s i n d i c a t e s that not a l l o f the bonding e l e c t r o n s behave as i n simple covalent c r y s t a l s . A c h a r a c t e r i s t i c o f s u l f i d e minerals i s t h a t p a r t i a l or complete replacement o f e i t h e r the c a t i o n s or anions i s p o s s i b l e without a change i n the c r y s t a l s t r u c t u r e o c c u r i n g . Among the many chalco­ genides studied a t Brow cogenides, with the p y r i t a t t r a c t i v e s i n c e t h e i r e l e c t r o n i c p r o p e r t i e s can be r a d i c a l l y changed by s u b s t i t u t i n g f o r e i t h e r the c a t i o n or anion i n the host compound. This group o f compounds combine a simple s t r u c t u r e (Figure XII) with a wide v a r i e t y o f magnetic and e l e c t r i c a l properties(21). Of p a r t i c u l a r i n t e r e s t i s the e f f e c t o f c a t i o n and anion sub­ s t i t u t i o n on the ferromagnetic compound CoS^. A one-electron energy scheme f o r CoS has been described by B i t h e r e t a l (22) and i s shown i n F i g u r e ^ X I J I . ^ In t h i s model σ-d sp o r b i t a l s on the metal atom and sp o r ­ b i t a l s on the anions a r e assumed. S u l f u r has s i x valence e l e c ­ trons that a r e shared among four t e t r a h e d r a l bonds. Each s u l f u r c o n t r i b u t e s one e l e c t r o n t o the S-S bond and the remaining f i v e t o the three M-S bonds. Since a l l the M-S bonds are e q u i v a l e n t , each s u l f u r c o n t r i b u t e s 1 2/3 e l e c t r o n s t o each o f them. Cobalt i s coordinated t o s i x s u l f u r atoms a t the apices o f an octahedron and these s u l f u r atoms thus c o n t r i b u t e 6 x 1 2/3 = 1 0 e l e c t r o n s t o the bonding i n each octahedron. These ten e l e c t r o n s plus the two 4 s e l e c t r o n s o f the t r a n s i t i o n metal j u s t f i l l the ground s t a t e a(s-p) and σ e manifold o f s t a t e s . The remaining d - e l e c trons o f the metal occupy the next-lowest a v a i l a b l e l e v e l s . Since c o b a l t possesses seven d - e l e c t r o n s , the one unpaired e l e c t r o n i n d i c a t e d by the magnetic moment o f CoS^ suggests that the t l e v e l s a r e f i l l e d with s i x e l e c t r o n s ( s p i n - p a i r e d ) . The u n p a i r i d e l e c t r o n t h e r e f o r e occupies the σ* e s t a t e . In the presence o f a s u f f i c i e n t l y strong covalent i n f e r a c t i o n the σ* e l e v e l w i l l broaden i n t o a band o f c r y s t a l l i n e s t a t e s ; the e l e c trons a r e no longer bound to s p e c i f i c s i t e s but a r e f r e e t o move through the c r y s t a l under the i n f l u e n c e o f an e l e c t r i c f i e l d . This concept already has been discussed by Goodenough (23,24). t o e x p l a i n the occurrence o f m e t a l l i c c o n d u c t i v i t y i n oxides and sulfides. For both compounds CoS and CoSe , as w e l l as members o f the 2

2

g

0

0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1.

WOLD

Oxides

and

Chalcogenides

13

Figure Large

CoS

Co

s-s

6

]^s,p)

8 y P3

XII. Pyrite structure. halls are sulfur; small halls, cobalt.

•,\σΚβς)

^

2

f

3a*(S-S)

\\ÉrfCoS2> / e g

"χ

»

\ \b — \V"

Figure

XIII.

À

^ u

Inn* il * 9 \\ 2

Electron

energy diagram for CoS,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

14

E X T E N D E D INTERACTIONS B E T W E E N

METAL

IONS

system CoS-_ Se , the only p a r t i a l l y f i l l e d s t a t e s are the o*e s t a t e s . This implies that i n these m a t e r i a l s the o*e states c o n s t i t u t e a band; the degree of covalence i s l a r g e . % h e occur­ rence o f an energy d i f f e r e n c e between the t ^ and a*e s t a t e s that i s l a r g e r than the intraatomic exchange enerfy ( l o w - l p i n con­ f i g u r a t i o n s i n l i g a n d f i e l d parlance) f o r the s o l i d s o l u t i o n s and CoS^ i s c o n s i s t e n t with the presence o f strong covalent i n t e r ­ actions. The stronger the bonding between the c a t i o n i c e or­ b i t a l s and the a n i o n i c o r b i t a l s , the l a r g e r w i l l be the s p l i t t i n g between the t ^ and the o*e s t a t e s . Since the l e v e l s are e s s e n t i a l l y noibonding they^are l i t t l e a f f e c t e d in^energy; hence, the energy d i f f e r e n c e between the t ^ and the o*e s t a t e s (which corresponds to the l i g a n d f i e l d s p l i f t i n g , or 10D§) becomes l a r g e r with i n c r e a s i n covalence CoAsS has been reporte conductor where the low s p i n s t a t e c o b a l t (d ) i s a l s o present i n an octahedral f i e l d . S u b s t i t u t i o n of a r s e n i c f o r s u l f u r i n CoS> r e s u l t s i n a continued depopulation o f e l e c t r o n s from the σ* a n t i bonding band ( 2 6 ) u n t i l f o r the end member CoAsS the σ* band i s empty. The p o s s i b i l i t y c^f r e p o p u l a t i n g t h i s band by p r o g r e s s i v e s u b s t i t u t i o n of n i c k e l d f o r c o b a l t d has r e c e n t l y been studied (27). The s u b s t i t u t i o n of a small amount o f n i c k e l (x = 0.05) f o r c o b a l t i n the system Co^ ^Ni^AsS changed the e l e c t r i c a l pro­ p e r t i e s from semiconducting to m e t a l l i c . H a l l - e f f e c t measure­ ments a l s o showed that the number o f c a r r i e r s ( e l e c t r o n s ) was p r o p o r t i o n a l to the n i c k e l concentration. These r e s u l t s are con­ s i s t e n t with the i n t r o d u c t i o n of e l e c t r o n s i n t o the σ* band as n i c k e l i s s u b s t i t u t e d i n t o CoAsS. g

This work was supported by the U.S. Army Research O f f i c e , Durham, the National Science Foundation and the M a t e r i a l s Research Laboratory Program at Brown U n i v e r s i t y .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1. WOLD

Oxides

and

Chalcogenides

15

Literature Cited 1 de Boer, J. Η., and Verwey, E. J. W., Proc. Phys. Soc. London (1937) A49, 59 2 Block, F., Z e i t s , f., Phys. (1928) 52, 555 3 Morin, F. J., Phys. Rev. L e t t e r s (1959) 3, 34 4 Pearson, A. D., J. Phys. Chem S o l i d s (1958) 5, 316 5 A d l e r , D., Rev. Mod. Phys. (1968) 40, 714 6 Goodenough, J . Β., Mat. Res. B u l l (1967) 2, 37 7 Goodenough, J . G., Mat. Res. B u l l (1967) 2, 165 8 Braeken, Z. Krist., (1931) 78, 484 9 F e r r e t t i , Α., Rogers, D., Goodenough, J . B., J. Phys. Chem. S o l i d s (1965) 26, 2007 10 Banks, Ε., and Wold, Α., "Preparative Inorganic Reactions", 4, 237 I n t e r s c i e n c e P u b l i s h e r s , New York, 1968 11 MacKintosh, A. R., 12 Sienko, M. J., "Pape 39, 224 Adv. Chem., Amer. Chem. Soc., Wash., D. C. 1963 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Goodenough, J. B., B u l l Soc. Chim. F r . , (1965) 1200 Goodenough, J. B., S o l i d State Chem., (1971) 3, 490 Heckingbottom, L i n e t t , J. V., Nature (London) (1962) 194, 678 Hagg, G., Z., Physik Chem. (1935) B29 , 192 S l e i g h t , A. W., Inorg. Chem., (1969),8, 1764 P i e r c e , J . W., McKinzie, H. L., V l a s s e , Μ., Wold, Α., J. S o l i d State Chem., (1969) 1, 332 Nygren, Μ., I s r a e l s s o n , Mat. Res. Bull., (1969) 4, 881 Rogers, D. B., Shannon, R. D., S l e i g h t , A. W., G i l l d o n , J . L., Inorg. Chem., (1969) 8, 841 H u l l i n g e r , F., J. Phys. Chem. S o l i d s (1965) 26, 639 B i t h e r , Τ. Α., Bouchard, R. J., Cloud, W. Η., Donohue, P. A. and Siemond, W. J., Inorg. Chem. (1968) 7, 2208 Goodenough, J . B., "Magnetism and the Chemical Bond", I n t e r science P u b l i s h e r s , Inc., New York, (1963) Goodenough, J . B., Proc. Colloque I n t . Orsay 1965 (1967) C.N.R.S. No. 157, 263 Nahigian, H., Steger, J., McKinzie H., A r n o t t , R. J., Wold, Α., to be published Goodenough, J. B., J . S o l i d State Chem. (1971) 3, 26 Steger, J. J., Nahigian,Η., A r n o t t , R. J., Wold, Α., t o be published

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2 VO

H i g h Temperature Crystal Chemistry of

2

the Metal-Insulator Transition and of (Cr . 0

through

3

V

01

0.99

)2Ο ,

an Insulator WILLIAM

R. R O B I N S O N

P u r d u e University, W e s t Lafayette, Ind. 47907

A variety of interestin are observed upon heating the chromium doped V 2 0 3 system (Cr Vl-x) O . At 155-170°K, those systems with χ ≤ 0.018 undergo an antiferromagnetic i n s u l a t o r to metal t r a n s i t i o n (1). This t r a n s i t i o n i s accompanied by a change from a monoclinic s t r u c t u r e to a rhombohedral s t r u c t u r e isomorphous with α-corundum. Those systems with x > 0.018 e x h i b i t an antiferromagnetic i n s u l a t o r to i n s u l a t o r t r a n s i t i o n at about 170°K and maintain t h e i r negative thermal c o e f f i c i e n t s o f resistivity at elevated temperatures. The electrical behavior o f systems containing l e s s chromium (x < 0.018) i s q u i t e d i f f e r e n t at higher temperatures. Pure V O e x h i b i t s a continuous electrical t r a n s i t i o n i n the range 225-325°C which r e s u l t s in an increase i n resistivity of about one order o f magnitude ( 1 , 2 , 3 , 4 ) . The doped systems with x < 0.018 e x h i b i t an e l e c t r i c a l e f f e c t which was o r i g i n a l l y ascribed to a Mott t r a n s i t i o n (1) But which more r e c e n t l y has been described as an e x t r i n s i c e f f e c t r e s u l t i n g from the c o e x i s t ­ ence o f two phases (4). Above the t r a n s i t i o n a t 155-170°K, a l l of the various elec­ trical m o d i f i c a t i o n s of (Cr Vl ) 0 are rhombohedral and i s o ­ morphous with corundum. However, the s i z e o f the rhombohedral cell v a r i e s between two extreme v a l u e s . V0 at room temperature and the low temperature form o f (Cro. V ) 0 (the α form) have u n i t cell parameters with a approximately 4.95A and c approximately 14.00A i n the hexagonal indexing of the rhombo­ hedral system. V 0 at high temperature, the high temperature form o f (Cr V ) 0 (the β form), and (Cr V ) 0 have a approximately 5.00Α and c approximately 13.94A. The change i n the V 0 c e l l parameters i s continuous with temperature while the primary change i n the (Cr0 V ) 0 c e l l parameters occurs with the α-β s t r u c t u r a l t r a n s i t i o n . In view o f the s t r u c t u r a l changes accompanying the semicon­ ductor to metal t r a n s i t i o n upon heating Ti 0 (5,6) or doping with vanadium to give (V Ti ) 0 where x ≤ 0.1 (7,8), we thought i t i n t e r e s t i n g to follow the s t r u c t u r a l changes accompanying the e l e c t r i c a l t r a n s i t i o n i n V 0 in the 225°-325°C range. Since x

2

3

2

3

x

-x

2

3

2

o1

2

0.o1

2

0.99

o.99

3

2

3

3

2

0.o4

3

0.96 2

3

3

.01

0.99

2

3

2

x

l-x

2

3

3

2

3

16

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3

2.

ROBINSON

Crystal

Chemistry

of

17

V 0, À

{

6 - ( C r . 0 ^ 0 . 9 9 ) 2 0 3 does not change much i n t h i s range, i t was s e l e c t e d as a reference m a t e r i a l . 0

Experimental. Samples of V2O3 and ( C r 0 . 0 ^ 0 . 9 9 ) 2 0 3 (4) were provided by Professor J . M. Honig, Department o f Chemistry, Purdue U n i v e r s i t y . A small s i n g l e c r y s t a l o f each material was cut from the boule and mounted along the 210 d i r e c t i o n i n evacuated s i l i c a c a p i l ­ l a r i e s using the procedure described by Brown, Sueno, and Prewitt (9). These c r y s t a l s were used i n a l l subsequent s t u d i e s . The u n i t c e l l dimensions reported i n Table I and Figure 1 were determined by l e a s t - s q u a r e s refinement of the 2Θ values of 15-20 r e f l e c t i o n s , p r i n c i p a l l y o f the type h k l , l y i n g i n the region 50° < 2Θ < 5 5 ° . The 2Θ values were 3ëtermined on a P i c k e r d i f f r a c t o m e t e r equippe average peak p o s i t i o n at p o s i t i v e and negative 2Θ was determined using the quarter height technique and graphite monochromated MoKa r a d i a t i o n . At each temperature i n d i c a t e d i n Table I, about 360 i n t e n ­ s i t i e s l y i n g i n a quadrant o f r e c i p r o c a l space with 5° < 2Θ < 62° were c o l l e c t e d . The d i f f r a c t o m e t e r was operated i n a Θ - 2Θ mode using a graphite monochromator and MoΚα r a d i a t i o n . Following a p p l i c a t i o n o f absorption c o r r e c t i o n s and the Lp c o r r e c t i o n , e q u i v a l e n t r e f l e c t i o n s were averaged g i v i n g about 100 independent r e f l e c t i o n s a t each temperature. Both i s o t r o p i c and a n i s o t r o p i c refinements were c a r r i e d out f o r each s e t of i n t e n s i t y data i n space group RJc using the V and 0 p o s i t i o n s reported by Newnham and deHaan (10) as s t a r t i n g param­ eters. Three c y c l e s o f refinement i n each case r e s u l t e d i n con­ vergence and gave f i n a l R values o f 0 . 0 2 - 0 . 0 4 . Selected s t r u c ­ t u r a l parameters are reported i n Table I and Figures 2 and 3. Results The c r y s t a l s t r u c t u r e o f V 0 at 23° was found to be i d e n t i c a l w i t h i n experimental e r r o r to that reported by Dernier (11) and Newnham and deHaan (10). The s t r u c t u r e of V 0 at 600°C ancT the s t r u c t u r e o f B - ( C r . o T V o . 9 9 ) 2 0 3 were found to be i d e n t i c a l to t h a t of (Cro.otCo.sehOaUl). The s t r u c t u r e s of a l l o f T h e s e systems, l i k e that of c o r u n ­ dum, c o n s i s t s of an approximately hexagonal c l o s e s t packed array of oxide ions with metal ions i n two-thirds of the octahedral holes (Figure 4 ) . Each metal ion has four near metal neighbors; one sharing an octahedral face of the c o o r d i n a t i o n polyhedron (Mi-M i n Figure 4) and three sharing edges of the octahedron 2

3

2

3

0

2

(M1-M3 i n Figure 4 ) .

The nonlinear v a r i a t i o n o f the u n i t c e l l parameters o f V 0 upon heating (Table 1 and Figure 1) i s s i m i l a r to t h a t p r e v i o u s l y reported (l_ 12). The l i n e a r increase with temperature of the c e l l parameters of B - ( C r . 0 ^ 0 . 9 9 ) 2 0 3 p a r a l l e l s that of 2

t

0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

s

2

0

2.697(1 2.880(1 2.051(1 1.968(1 2.676(3 2.804(1 2.889(1 2.952(1

2

*

2.747(1) 2.917(1) 2.061(1) 1.976(1) 2.661(2) 2.792(1) 2.897(1) 3.004(1)

23° 4.9974(1) 13.9260(6)

ο A l l e n t r i e s i n A.

2

3

Μχ-Μ M!-M Μχ-Oj Μχ-00i-0 Οχ-Ο* Οχ-Os 0.,-Os

7*iex ^hex

0

2.746(1) 2.919(1) 2.061(1) 1.977 1 2.663(2) 2.793(1) 2.898(1) 3.006(1)

113° 5.0018(1) 13.9238(7)

2.705(1) 2.888(1) 2.053(1) 1.970(1) 2.675(3) 2.802(1) 2.891(2) 2.962(2)

115° 4.9605(1) 13.9857(6) 2.734(1) 2.916(1) 2.064(1) 1.977(2) 2.680(3) 2.799(2) 2.897(1) 2.997(2)

2.728(1) 2.910(1) 2.061(1) 1.975(2) 2.676(3) 2.799(2) 2.895(2) 2.990(1)

2.718(1) 2.900(1) 2.057(2) 1.973(1) 2.675(4) 2.800(1) 2.893(1) 2.978(1)

2.739(1) 2.920(1) 2.062(2) 1.979 1) 2.670(2) 2.796(1) 2.899(1) 3.006(1)

310° 5.0052(1) 13.9318(6)

400° 5.0013(1) 13.9416(5)

0

and ( C r . 0 ^ 0 . 9 9 ) 2 0 3

300° 4.9912(2) 13.9489(6)

3

212° 4.9776(2) 13.9647(7)

2

C r y s t a l l o g r a p h i c Data f o r V 0

23° 4.9492(2) 13.998(1)

(Cr .oiV .99)203

0,-0s 0„-0

x

0 -0>

Mt-0, Μχ-0,, 0j-0

M1-M3

Μχ-Μ

V2O3

Table I:

2.738(1) 2.924(1) 2.066(1) 1.981(1) 2.681(3) 2.800(1) 2.901(1) 3.007(1)

600° 5.0140(2) 13.9427(7)

2.

ROBixsox

Crystal

Chemistry

of V,0.

19

{

A I4.0C%,

-V 0 2

3

- l%Cr-V 03 2

13.9813.96-

-hex

13.94 13.92 A* 5.00 -.. 4.98 4.964.94100 Figure

1.

300 T(°C)

Variation of unit cell with temperature

500 parameters

A 2.92

MO)- M(3)

2.91 2.90 2.89 238jh 2.74

M(D-M(2)

2.73 2.72 2.71 2.701 200

Figure

2. Variation tances with

400 T(°C) of metal-metal temperature

600

dis-

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

20

E X T E N D E D INTERACTIONS B E T W E E N

METAL

ο 2 07 h

1.96 i

0

.

i

200

.

i

.

400

1

600

T(°C) Figure

3.

Figure 4.

Variation with

Projection

of metal-oxygen temperature

of the corundum

distances

structure

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

ROBINSON

2.

Crystal

Chemistry

of

21

VjO.

{

(Cr .o»»Vo 96)203 (1) although the c e l l dimensions d i f f e r s l i g h t l y . It i s apparent from the data that the u n i t c e l l dimensions of the pure and doped systems converge at elevated temperatures. The coordinates o f the metal and oxygen atoms of V 0 and S - ( C r . o i V o 9 9 ) 2 0 3 a l s o converge with i n c r e a s i n g temperature. The changes i n m e t a l - m e t a l , metal-oxygen, and oxygen-oxygen distances which r e s u l t are given i n Table 1. P l o t s of metal-metal and metal-oxygen distances are presented i n Figures 2 and 3. Both the M!-M2 and M i - M 3 e q u i l i b r i u m distances increase with temperature i n V 0 . The M i - M 2 change from 2.697(1)A at 23° to 2.738(1)A a t 600° represents a change o f about 40 e . s . d . ' s as does the increase from 2.880(1)A a t 23° to 2.924(1)A a t 600° i n the M i - M 3 d i s t a n c e . Over the temperature range 23° to 310°C the M1-M3 e q u i l i b r i u m distance o f 6 - ( C r 01V0 9 9 ) 2 0 3 increases s l i g h t l y from 2.917(1)A to 2.920(1)A*whilé the distance decreases from 2.747(1) even though the c a x i s which i s p a r a l l e l to the Μ χ - Μ 2 d i r e c t i o n is increasing. No s i g n i f i c a n t changes i n the metal-oxygen d i s t a n c e s i n the doped system are observed upon h e a t i n g . The maximum change i s three e . s . d . ' s . Small changes i n the metal-oxygen d i s t a n c e s i n V 0 are observed. The M1-O1 d i s t a n c e v a r i e s by 0.015A from 2.051(1)A to 2.066(1)A while M i - 0 - v a r i e s from 1.968(1)A to 1.981(1)A, a change of 0.013A. The only s i g n i f i c a n t change i n oxygen-oxygen distances i s i n the 0i»-0 d i s t a n c e i n V 0 . This d i s t a n c e changes from 2.952(1 )A a t 23° to 3.007(1)A at 600°C. The other changes are 0.009A or less. In g e n e r a l , the s t r u c t u r a l changes i n V 0 upon heating involve a s l i g h t reduction of the spacing between the a p p r o x i ­ mately hexagonally c l o s e s t packed planes of oxygen atoms with a concommitant increase i n the average oxygen-oxygen separation i n the planes. One can d i s t i n g u i s h two types of oxygen-oxygen d i s t a n c e s w i t h i n a given p l a n e ; distances o f the type 0 i - 0 along the edge o f a shared octahedral face and distances o f the type Ο1-Ο3 or 0i*-0s along the edges o f an unshared face. It i s expansion of the oxygen-oxygen distances along the edges of the unshared face which i s r e f l e c t e d in the increase i n the a a x i s . P a r a l l e l i n g the increase i n the a axis i s the M i - M 3 s e p a r a t i o n . The M i - M 2 d i s t a n c e increases at the same rate as the M1-M3 d i s t a n c e . The net r e s u l t o f these changes i s that a t high temperatures V 0 , which i s i s o s t r u c t u r a l with a - ( C r . 0 ^ 0 . 9 9 ) 2 0 3 (13) a t room temperature, becomes i s o s t r u c t u r a l with 0

E

2

0

3

e

2

3

0

2

3

5

2

3

2

3

2

2

3

0

B^TCro.oiVo.99)203 and (Cro.owVo.96)203.

Acknow!edgements. T h i s work was done i n the Department o f Earth and Space S c i e n c e s , SUNY, Stony Brook, N.Y. We wish to thank Professor C. T . P r e w i t t f o r h i s a s s i s t a n c e with t h i s p r o j e c t and the

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

22

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

National Science Foundation (Grant No. GP-17554) and Purdue U n i v e r s i t y f o r support of the work. Literature Cited. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

McWhan, D. B. and Remeika, J. P . , Phys. Rev. B, (1970), 2, 3734. Foex, Μ., Compt. Rend., (1946), 223, 1126. M o r i n , F. J., Phys. Rev. Lett., (1959), 3, 34. Honig, J. M . , Chandrashekhar, G. V . , Sinha, A. P. B . , Phys. Rev. Lett., (1974), 32, 13. Honig, J. Μ., and Reed, T . Β . , Phys. R e v . , (1968), 174, 1020. Simonyi, E., and Raccah, P. Μ., B u l l . Am. Phys. Soc., (1973), 11, 339. Chandrashekhar, G. V . , Won C h o i , Q . , Moyo, J., and Honig, J. Μ., Mat. Res. Bull., Robinson, W. R . , J. S o l i d State Chem., (1974), 9 , 255. Brown, G. E., Sueno, S . , and P r e w i t t , C. T., Amer. M i n e r a l . , (1973), 58, 698. Newnham, R. E . and deHaan, Y. M . , Z. Kristallog., (1962), 117, 235. D e r n i e r , P. D . , J. Phys. Chem. S o l i d s , (1970), 31, 2569. E c k e r t , L . J. and B r a d t , R. C . , J. A p p l . P h y s . , (l973), 44, 8. Robinson, W. R . , Unreported Observations.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3 Synthesis and Properties of Some N e w

Organic

Intercalation Complexes of Tantalum Sulfide R. L .

HARTLESS

and A . M .

TROZZOLO

B e l l Laboratories, M u r r a y Hill, N.J. 07974

Intercalation of layered compounds by metals has been p r e v i o u s l y r e p o r t e d ( 1 - 5 ) . A number o f i n o r g a n i c and biological m o l e c u l e s have been found to i n t e r c a l a t e c l a y s and g r a p h i t e ( 6 , 7 ) . The first intercalation of layered transition m e t a l c h a l c o g e n i d e s by o r g a n i c m o l e c u l e s was r e p o r t e d by Weiss and Rudhardt ( 8 ) . They intercalated TiS w i t h h y d r a z i n e and aliphatic amides and o b s e r v e d i n c r e a s e s o f s e v e r a l angstroms in the interlayer s p a c i n g o f the c h a l c o g e n i d e . More r e c e n t s t u d i e s show t h a t a v a r i e t y o f o r g a n i c m o l e c u l e s intercalate layered transition m e t a l c h a l c o g e n i d e s (9-11). The o r g a n i c m o l e c u l e s p e n e t r a t e the i n t e r l a y e r p l a n e s and form a p e r i o d i c crystalline structure. In a d d i t i o n to i n c r e a s e d i n t e r p l a n a r s p a c i n g , the i n t e r c a l a t e d complexes showed enhancement o f the critical superconducting temperature ( Tc) above t h a t o b s e r v e d f o r the unintercalated chalcogenide. Tantalum sulfide crystallizes i n several polytypes which are c h a r a c t e r i z e d by layer-like structures i n which the m e t a l atoms are l o c a t e d between a l t e r n a t e sulfur layers. The bonds forming the i n t r a p l a n a r l a y e r s are s t r o n g ( p r i m a r i l y c o v a l e n t ) bonds. Each s u l f u r - t a n t a l u m - s u l f u r l a y e r ( i n t e r p l a n a r ) i s bonded to a d j a c e n t l a y e r s by weak van der Waals attraction. Intercalation ( i n s e r t i o n ) o f atoms o r m o l e c u l e s between the l a y e r s o c c u r s by c l e a v a g e a l o n g the p l a n e s which have the low energy i n t e r a c t i o n . F i g u r e 1 g i v e s a schematic d e s c r i p t i o n o f the s t r u c t u r e o f the l a y e r e d transition metal chalcogenide prior to intercalation where M i s the transition m e t a l atom and X i s the c h a l c o g e n atom. Shown are t h r e e i n t r a p l a n a r l a y e r s s e p a r a t e d by the van der Waals gap. The intraplanar l a y e r s are a p p r o x i m a t e l y 6 Å t h i c k and the i n t e r l a y e r distance between transition m e t a l atoms i s a p p r o x i mately 6Å. Upon intercalation of the organic molecule, 2

o

23

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

24

EXTENDED

INTERACTIONS B E T W E E N

METAL

IONS

the van der Waals gap expands as s c h e m a t i c a l l y r e p r e ­ sented i n F i g u r e 2. T a b l e 1 shows the s y n t h e s i s con­ d i t i o n s and parameters f o r some p r e v i o u s l y i n t e r c a l a t e d complexes ( 9 ) . The i n t e r l a y e r d i s t a n c e f o r the p y r i d i n e - T a S g c o m p l e x i n c r e a s e s ~Joh t o g i v e a m e t a l m e t a l d i s t a n c e o f ^12A. In the case o f the o c t a decylamine-TaS^ complex, the enhanced i n t e r l a y e r d i s t a n c e was Λ,^ΟΑ which was a t t r i b u t e d t o two l a y e r s o f the amine o r i e n t e d end-to-end and p e r p e n d i c u l a r to the TaS2 p l a n e s ( F i g u r e 3 ) . In g e n e r a l , o r g a n i c m o l e c u l e s capable o f i n t e r c a l a t i n g T a S a r e Lewis bases c o n t a i n ­ i n g an sp3 o r s p h y b r i d i z e d n i t r o g e n . T h i s study i n v o l v e s i n t e r c a l a t i o n o f the 2Hp o l y t y p e o f tantalum s u l f i d e ( T a S ) w i t h o r g a n i c molecules o f variou increased i n t e r l a y e s u p e r c o n d u c t i n g temperatures. The e x t e n t o f i n t e r p l a n a r l a y e r expansion and T enhancement v a r i e s w i t h the p a r t i c u l a r i n t e r c a l a t e , thus a l l o w i n g p o s s i b l e chemical c o n t r o l of T . 0

2

2

2

c

c

Experimental The powdered tantalum s u l f i d e was p l a c e d i n a 4mm p y r e x tube w i t h the l i q u i d i n t e r c a l a t e o r a benzene s o l u t i o n o f the s o l i d i n t e r c a l a t e . The tube was evacuated and s e a l e d and the r e a c t i o n s c a r r i e d out i n i s o t h e r m a l baths a t 25, 100, 150 and 200°. Inter­ c a l a t i o n was e v i d e n t when the volume o f the chalcogenide increased. A f t e r i n t e r c a l a t i o n , the excess o r g a n i c r e a c t a n t s were removed by washing w i t h benzene f o l l o w e d by d r y i n g a t reduced p r e s s u r e . S u p p o r t i n g evidence f o r i n t e r c a l a t i o n was o b t a i n e d by powder x-ray d i f f r a c t i o n w i t h a G u i n i e r camera u s i n g CUKQ r a d i a t i o n . The c r i t i c a l s u p e r c o n d u c t i n g tem­ p e r a t u r e s were determined on powdered complexes by AC s u s c e p t i b i l i t y measurements. R e s u l t s and D i s c u s s i o n S e v e r a l mono- and d i s u b s t i t u t e d amines form complexes w i t h T a S ( T a b l e 2 ) . In g e n e r a l the amines which i n t e r c a l a t e d had p K > 9 . D i e t h y l a m i n e inter­ c a l a t e s i n two hours a t ambient temperatures to y i e l d a comçlex w i t h an i n t e r l a y e r d i s t a n c e i n c r e a s e (σ) o f 3.69 A and an onset o f s u p e r c o n d u c t i v i t y ( T ) o f 3.0°K. The d i b e n z y l amine complex was s u p e r c o n d u c t i n g at 2 7 ° K and the i n t e r l a y e r s p a c i n g was i n c r e a s e d by 6.39A. P i p e r i d i n e i n t e r c a l a t e s T a S and i n c r e a s e s the i n t e r l a y e r d i s t a n c e by 4.20A, but the complex i s not 2

a

Q

&

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3.

HARTLESS A N D TROZZOLO

X

X

25

M

X

X

χ

VAN DER

X

χ

WAALS BOND

χ

Μ

Μ

X

X

VAN DER WAALS BOND

X

χ

"χ

Χ

Μ

Figure

Sulfide

X

M

X

Tantalum

Μ X

X

1.

Schematic

of structure

of TaS >

X

Χ

Χ

ο 6Α

Μ

M X

Χ

II

Χ

1

X

Χ

Χ Μ

M Χ

Χ

Χ

Figure 2. Schematic of structure of TaS> substituted pyridine complexes

TABLE I SYNTHESIS OF T a S

TaS

2

CH -(CH ), -NH 5

2

7

2

2

COMPLEXES

TEMP. (°C)

TIME (DAYS)

8(A)

T (»K>

—

—

6.05

0.8

200

1

40

30

0

5 81

3.5

50

3.0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

26

E X T E N D E D INTERACTIONS B E T W E E N

T 6A i

METAL

IONS

x~^( χ M M X X X

0

25 A

Figure

50A

3. Schematic of structure TaS> (octade calamine

of

X X X M M X X X

TABLE 2 TdS - AMINE COMPLEXES 2

SYNTHESIS INTERCALATE

TEMP. C O

T I M E (DAYS)

B(l)

T (*K)

25

3.6a

3.0

150

4.02

150

6.39

2.7

25

6.15

2.4

3

4.20

NS

8

3.65

1.8

0

CH,CHo CH3CH2 CHJCH2CH2CH2

>

>-H

CH3CH2CH2CH2

;N-H

0-CH

0 - C H

2

2

C M N~N

-
J-NH

H

O

s

N—NH CH

Qj-NH

0

2

3

2

25

14

13.00,6.26

100

5

6.26,4.08

2.9,2.4

2.9

25

10

3.14,1.66

2.4,1.8

150

5

3.48,2.09

2.5,1.6

0

H-C-NH-NH-C-H

TABLE 4 CORRELATION OF PKa .CHARGE DENSITY (ACT) AND THE TEMPERATURE FOR THE ONSET OF SUPERCONDUCTIVITY (To) INTERCALATE H N-NH 2

CH

pKq

Δσ

T CK)

2

7.93

.3795

4.9

2

7.21

.3608

4.0

2

7.87

.3485

3.3

5.2

.3429

2.0

Q

3

^N-NH CH 3

CH X

3

N~NH

N-NH,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3.

ιιARTLESS A N D T R O Z Z O L O

Tantalum

29

Sulfide

intercalate. The c r i t i c a l s u p e r c o n d u c t i n g t e m p e r a t u r e s f o r t h e h y d r a z i n e , 1 , 1 - d i m e t h y l h y d r a z i n e and m e t h y l h y d r a z i n e complexes a r e 4 . 9 * 3-3°K, whereas t h e p K s a r e 7.93* 7.21 and 7.87 r e s p e c t i v e l y . T h i s has been o b s e r v e d by D i S a l v o f o r amine-TaS2 complexes i n w h i c h an i n c r e a s e i n p K may produce an i n c r e a s e o r d e c r e a s e i n T . T h i s i s c o n f i r m e d by n u c l e a r q u a d r u p o l e r e s o n a n c e d a t a where t h e e l e c t r o n d e n s i t y i n t h e nonbonding n i t r o g e n o r b i t a l s o f h y d r a z o n e s does n o t c o r r e l a t e w i t h T o f t h e i n t e r c a l a t e d complexes. Bray and Sauer have o b s e r v e d t h e same e f f e c t f o r p y r i d i n e - T a S complexes ( 1 6 ) · The c o r r e l a t i o n o f n u c l e a r q u a d r u p o l e resonance d e r i v e d charge d e n s i t i e s w i t h To i s c o m p l i c a t e d by t h e f o r m a t i o n o f two phas two d i s t i n c t i n t e r l a y e complex formed between T a S and 1 - p h e n y l - l methylhydrazine has i n t e r l a y e r distance increases o f 13.00 and 6.26 A and t h e Sauer and Bray o c c u p a t i o n number f o r t h e h y d r a z i n e i s Ο.388. The t e m p e r a t u r e f o r t h e o n s e t o f s u p e r c o n d u c t i v i t y f o r t h e complex s h o u l d be > 4.9°K, b u t t h e t r a n s i t i o n o c c u r s a t 2.9°K. The s y n t h e s i s c o n d i t i o n , i n t e r l a y e r d i s t a n c e i n c r e a s e , and t h e t e m p e r a t u r e f o r t h e o n s e t o f s u p e r ­ c o n d u c t i v i t y o f some Ta.S amide complexes a r e i n c l u d e d i n T a b l e 5. The i n t e r c a l a t i o n o f N-methylformamide under v a r i o u s r e a c t i o n c o n d i t i o n s p r o d u c e s d i f f e r e n t complexes. The complex formed a t ajnbient t e m p e r a t u r e s had an i n t e r l a y e r s p a c i n g o f 3.65 A and T = 4.2°K. The complex formed a t 100° had an i n t e r l a y e r s p a c i n g o f 3.00 A and was s u p e r c o n d u c t i n g a t 2.4°K. I f t h e r e a c t i o n i s c a r r i e d o u t a t 90° w i t h u l t r a s o n i c m i x i n g of the reactants, the i n t e r l a y e r distance increases to ^50 Κ and t h e complex i s s u p e r c o n d u c t i n g a t 2.7°K. This evidence supports the previous o b s e r v a t i o n that t h e r e i s no c o r r e l a t i o n between t h e i n t e r l a y e r d i s t a n c e and t h e t e m p e r a t u r e f o r t h e o n s e t o f s u p e r c o n d u c t i v i t y (9). D i - n - b u t y l formamide, N - m e t h y l f o r m a n i l i d e , and d i f o r m y l h y d r a z i n e a l s o f o r m complexes w i t h T a S . D i f o r m y l h y d r a z i n e formed a two phase complex w i t h i n t e r l a y e r s p a c i n g i n c r e a s e s o f 3.48 and 2 . 0 9 Κ and showed two t r a n s i t i o n s f o r t h e o n s e t o f s u p e r ­ c o n d u c t i v i t y a t 2.5 and 1.6°K. I n t e r p r e t a t i o n o f t h e r e s u l t s o f t h e amide complexes i s c o m p l i c a t e d by t h e t e n d e n c y o f t h e s e m o l e c u l e s t o decompose under t h e r e a c t i o n c o n d i t i o n s . Elemental a n a l y s i s i n d i c a t e s the p o s s i b i l i t y o f t h e i n t e r c a l a t i o n o f more than one s p e c i e f o r t h e N-methylformamide complex. The t e n d e n c y o f amide i n t e r c a l a t e s t o decompose has p r e v i o u s l y been o b s e r v e d by D i S a l v o . a

n

d

a

a

0

Q

2

2

0

2

Q

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

30

E X T E N D E D INTERACTIONS B E T W E E N

METAL

IONS

TABLE 5 T 0 S - AMIDE COMPLEXES 2

SYNTHESIS INTERCALATE Η 0 I II CHj-N—C—H

CH CH CH CH2 3

2

2

TEMP. C O

TIME (DAYS)

25 100 90

30MINS. 14 14

8(A)

3.65 3.00 ~50

T ÇK) 0

4.2 2.4 2.7

- P = O

CH

3

N

/ \ CH CH 3

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2.5,1.6

3.

HARTLESS AND TROZZOLO

Tantalum

Sulfich

31

The p r e c e d i n g p a r a g r a p h r e f e r r e d t o the f o r m a t i o n o f a two-phase complex between T a S and d i f o r m y l ­ h y d r a z i n e i n w h i c h the complex had two i n t e r l a y e r s p a c i n g s and two t e m p e r a t u r e s f o r the o n s e t o f s u p e r ­ c o n d u c t i v i t y . The f o r m a t i o n o f two phase complexes appears t o be f a i r l y g e n e r a l and t h e r e are i n t e r e s t i n g r e l a t i o n s h i p s between σ, To, and r e a c t i o n c o n d i t i o n s (Table 6 ) . Reaction of 1-methyl-l-phenylhydrazine w i t h T a S a t ambient t e m p e r a t u r e s y i e l d e d a two-phase complgx w i t h i n t e r l a y e r d i s t a n c e i n c r e a s e s o f 13*00 and 6.26 A w h i c h was s u p e r c o n d u c t i n g a t 2.9°K. R e a c t i o n at h i g h e r t e m p e r a t u r e s y i e l d e d a complex i n w h i c h the 13.00 A d i s t a n c e d i s a p p e a r e d and a new d i s t a n c e (4.08 K) appeared. The 6.26 A i n t e r l a y e r d i s t a n c e i n c r e a s e was p r e s e n t i b o t h complexes Th appearance and d i s a p p e a r a n c i n the f o r m a t i o n o d i e t h y methylamidophosphat complexes. The complex formed a t 100° had one i n t e r ­ l a y e r d i s t a n c e o f 4.85 $ and one t e m p e r a t u r e f o r the o n s e t o f s u p e r c o n d u c t i v i t y . The complex formed a t 150° had 3.95 A and 1.5°K f o r the i n t e r l a y e r d i s t a n c e i n c r e a s e and s u p e r c o n d u c t i n g t r a n s i t i o n , r e s p e c t i v e l y . At i n t e r m e d i a t e r e a c t i o n c o n d i t i o n s , the complex formed had b o t h i n t e r l a y e r d i s t a n c e i n c r e a s e s and b o t h superconducting t r a n s i t i o n s . From p r e v i o u s r e s u l t s t h e d e c r e a s e i n the t r a n s i t i o n t e m p e r a t u r e w i t h a d e c r e a s e i n i n t e r l a y e r s p a c i n g cannot be a t t r i b u t e d s o l e l y t o the i n t e r l a y e r d i s t a n c e change. I t seems r e a s o n a b l e t h a t the i n c r e a s e d tempera­ t u r e s and r e a c t i o n t i m e s a l l o w the i n t e r c a l a t e t o r e a c h an e q u i l i b r i u m o r i e n t a t i o n where e l e c t r o n d o n a t i o n t o the u n f i l l e d m e t a l l i c bands i s l e s s e f f i c i e n t , t h e r e b y c a u s i n g a d e c r e a s e i n the t r a n s i t i o n temperature. 2

2

Summary A number o f o r g a n i c i n t e r c a l a t e d complexes o f the 2H-phase o f T a S have been p r e p a r e d w h i c h have i n ­ c r e a s e d i n t e r l a y e r s p a c i n g s and enhanced c r i t i c a l superconducting temperatures. There i s a d i r e c t c o r r e l a t i o n between the n u c l e a r q u a d r u p o l e resonance d e r i v e d charge d e n s i t i e s o f the i n t e r c a l a t e w i t h the enhanced s u p e r c o n d u c t i n g t e m p e r a t u r e s o f h y d r a z i n e complexes. Some o r g a n i c i n t e r c a l a t e s form two phase complexes w h i c h may have one o r two t e m p e r a t u r e s f o r the o n s e t o f s u p e r c o n d u c t i v i t y . 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

32

EXTENDED INTERACTIONS BETWEEN METAL IONS Literature

Cited

1. Voorhoeve, J . M. and Robbins, M., J. Solid State Chem., (1970), 1, 134. 2. Huisman, R., Kadijk, F., and Jellinek, F., J. Less-Common Metals, (1970), 21, 187. 3.

Omloo, W. P. F. Α. Μ., and Jellinek, F., J. LessCommon Metals, (1970), 20, 121.

4. Rudorff, W., Chimia (1965), 19, 489. 5. Rudorff, W. and Sich H H. Angew Chem. (1959) 71, 724. 6.

Freeman, A. G. and Johnston, J. H., Carbon (Oxford), (1971), 9, 667.

7. Grim, R. E.,"ClayMinerology,"McGraw-Hill, New York, N.Y., 1968. 8. Weiss, A. and Rudhardt, R., Z. Naturforsch., Β (1969), 24, 265, 355, 1066. 9. Gamble, F. R., DiSalvo, F. J., Klemm, R. Α., and Geballe, T. H., Science (1970), 168, 568. 10.

Gamble, F. R., Osiecki, J. H., Cais, M., Pisharody, R., DiSalvo, F. J., and Geballe, T. H., Science, (1971), 174, 493.

11.

DiSalvo, F. J., Schwall, R., Geballe, T. H., Gamble, F. R., and Osiecki, J. Η., Phys. Rev. Lett., (1971), 27, 310.

12.

Ehrenfreund, E., Gossard, A. C., and Gamble, F. R., J. Phys. Rev. Β (1972) 5, 1708.

13.

Beal, A. R. and Liang, W. Y., Phil. Mag. (1973), 27, 1397.

14.

Bardeen, J., Cooper, L. N. and Schrieffer, J. R., Phys. Rev., (1957), 108, 1175.

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Sauer, Enrique G. and Bray, P. J., J. Chem. Phys., (1972), 56, 820.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3.

16.

HARTLESS

AND

TROZZOLO

Tantalum

Sulfide

B r a y , P. J. and S a u e r , E n r i q u e G., Communications, (1972), 11, 1239.

33

Solid

State

Acknowledgment The a u t h o r s acknowledge helpful d i s c u s s i o n s w i t h F. J. D i S a l v o , Jr., G. W. Hull, T. H. G e b a l l e , D. W. Murphy and F. J. Padden, Jr. We particularly thank G. W. Hull f o r t h e superconductivity data.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4 Intermolecular Magnetic Exchange Interactions in Molecular Crystals J. M .

GROW

and H .

H.

WICKMAN

O r e g o n State U n i v e r s i t y , C o r v a l l i s , O r e . 97331

Abstract Intermolecular exchange interactions have been studied in a representative, homologous series of metal complexes: iron(III) bis (dithiocarbamates) . Several of these molecular crystals exhibit cooperative m a g n e t i c order at t e m p e r a t u r e s b e l o w approximately 4 °K. Exchange paths include weak interactions between formally non-bonded molecular units. The m a g n e t i c phenomena a r e sensitive to zero-field splittings within the orbital-singlet, spin-quartet ground term o f iron(III). Discussion Molecules containing a single paramagnetic ion s u r r o u n d e d by b u l k y , organic ligands often form magnetically dilute molecular crystals. For metalm e t a l d i s t a n c e s g r e a t e r t h a n a b o u t 7 A, t h e d i p o l a r f i e l d s a r e s i g n i f i c a n t a s an o r i g i n o f m a g n e t i c o r d e r o n l y a t t e m p e r a t u r e s b e l o w -0.01 °K. Hence, most such m e t a l complexes remain s i m p l e paramagnets t o q u i t e low temperatures. However, s e v e r a l homologues o f t h e iron(III) halo-bis-dithiocarbamatesorder magnetically a t t e m p e r a t u r e s i n t h e r a n g e 1-4 K (_l-_3) . We summarize h e r e c e r t a i n s t r u c t u r a l o r c r y s t a l l o g r a p h i c f e a t u r e s o f t h e s e c o m p l e x e s w h i c h a p p e a r t o be i m p o r t a n t d e t e r m i n a n t s f o r exchange paths i n these molecular crystals. I n a d d i t i o n , we n o t e t h a t v a r i a b l e ground term c r y s t a l f i e l d interactions may enhance o r depress the c o o p e r a t i v e t r a n s i t i o n s f o r a f i x e d c r y s t a l s t r u c t u r e o r s e t o f exchange p a t h s . The s t r u c t u r a l f o r m u l a f o r an i r o n ( I I I ) b i s - d t c i s g i v e n i n F i g . 1. Much o f t h e c r y s t a l l o g r a p h i c i n f o r m a t i o n on b i s - d t c s has b e e n r e p o r t e d by H o s k i n s , e

34

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

complex of a halobisdithiocarbamatoiron(IH) formula Structural 1. Figure

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

36

EXTENDED

INTERACTIONS B E T W E E N

METAL

IONS

M a r t i n , W h i t e , a n d c o - w o r k e r s (4_-6_) . F o r example, the dimensions g i v e n i n the F i g u r e are f o r the d i e t h y l chloro derivative = e t h y l , i = l , 4 ; X = CI) (_5) . The i r o n ( I I I ) i s s i t u a t e d n e a r t h e c e n t r o i d o f a r e c t a n g u l a r p y r a m i d w h o s e a p e x may be o c c u p i e d b y a m o n o v a l e n t a n i o n ( h a l i d e , SCN", e t c . ) and whose b a s e c o n s i s t s o f 2 p a i r o f s u l f u r atoms f r o m t h e d t c ligands. T h i s s y m m e t r y l e a d s t o an o r b i t a l - s i n g l e t , s p i n - q u a r t e t g r o u n d t e r m (£) . The f o u r - f o l d s p i n d e g e n e r a c y i s removed by s p i n - o r b i t e f f e c t s , t o g e t h e r w i t h m e t a l - l i g a n d i n t e r a c t i o n s (7_-8_) , a n d two K r a m e r s d o u b l e t s , s e p a r a t e d b y ~2 t o 20 cm" , a r e commonly observed (9). The f o r e g o i n g l e v e l s t r u c t u r e i s a t t r a c t i v e f o r magnetism s t u d i e s First th low-symmetr environ ment i s f e l t w i t h i rank c r y s t a l f i e l d p o t e n t i a l , y y spin-Hamiltonian c o e f f i c i e n t s . Second, the magnetic e x c h a n g e i s r e p r e s e n t e d by an i s o t r o p i c Heisenberg i n t e r a c t i o n , owing t o the s m a l l o r b i t a l c h a r a c t e r o f the ground term. These i n t e r a c t i o n s o f t e n t u r n out t o be s m a l l c o m p a r e d w i t h t h e c r y s t a l f i e l d s p l i t t i n g s ; t h e r e s u l t i s t h a t t h e m a g n e t i c p r o p e r t i e s may be d i s c u s s e d w i t h i n a s i n g l e ground Kramers d o u b l e t . The well-known Hamiltonian f o r the s p i n - q u a r t e t m a n i f o l d is 1

The the

p a r a m e t e r s D and Ε d e s c r i b e t h e c r y s t a l f i e l d and remaining term r e p r e s e n t s magnetic exchange. B o t h f e r r o m a g n e t i c , c h l o r o - b i s - d i e t h y l d t c (1) , and a n t i f e r r o m a g n e t i c , i o d o - b i s - d i e t h y l d t c ( 3 ) , h o m o l o g u e s a r e known. The t r a n s i t i o n t e m p e r a t u r e s a r e 2.48 °K a n d 1.9 °K, r e s p e c t i v e l y . M o r e r e c e n t l y we have o b s e r v e d c o o p e r a t i v e t r a n s i t i o n s i n the f o l l o w i n g complexes: Fe(morphylyldtc) I, Fe(morphylyldtc)2Br, Fe(morphylyldtc) C1, Fe(pyrrolidyldtc) C1, F e ( p y r i o l i d y l d t c ) I , and F e ( p y r i d y l d t c ) C 1 . In these complexes, t h e m a g n e t i c o r d e r has been o b s e r v e d by Mossbauer spectroscopy (1^,2^) . A l l t r a n s i t i o n s occur b e l o w 4 °K. C r y s t a l s t r u c t u r e d a t a p r o v i d e s a means f o r i d e n t i f y i n g exchange p a t h s between atoms i n a d j a c e n t molecules. O n l y a few s t r u c t u r e s h a v e b e e n r e p o r t e d , b u t i n c l u d e d a r e t h e f e r r o m a g n e t i c and a n t i f e r r o ­ m a g n e t i c d e r i v a t i v e s m e n t i o n e d a b o v e (5^-6^) . A d e t a i l e d d i s c u s s i o n of the o r i e n t a t i o n s o f the m o l e c u l e s i n c o n n e c t i o n w i t h exchange paths i s 2

2

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4.

GROW A N D

/ntcrmolccular

WICK Μ A Ν

37

Exchange

h

a

v

e

a v a i l a b l e e l s e w h e r e O'^'É.) · We recently obtained s t r u c t u r a l i n f o r m a t i o n on t h e SCN-bis-diethyldtc, b r o m o - b i s - d i e t h y l d t c , and isodo-bis-morpholyldtctoluene adduct. The l a t t e r e x h i b i t s a m a g n e t i c t r a n s i t i o n at approximately 2 °K. The f o l l o w i n g p i c t u r e e m e r g e s f r o m a s t u d y o f t h e structural information. As e x p e c t e d , i r o n - i r o n d i s t a n c e s i n a l l complexes are i n e x c e s s o f 7 Â. In t h o s e c o m p l e x e s w h o s e s t r u c t u r e i s known a n d w h i c h show a m a g n e t i c t r a n s i t i o n , o r e x c h a n g e b r o a d e n e d e p r spectra, there e x i s t s a j u x t a p o s i t i o n of molecules s u c h t h a t t h e r e i s an i n t e r m o l e c u l a r S » - S l i n k a g e . T h a t i s , we may w r i t e t h e i n t e r m o l e c u l a r i n t e r a c t i o n a s F e - S » S - F e ^ w i t h t h e Fe-S u n i t s b e l o n g i n g t o d i s t i n c t molecules. The S***S 4.5 A. The e x c h a n g l a t t i c e i n rather complicated paths: however, simple d i m e r i n t e r a c t i o n s may also occur. Because of the packing c h a r a c t e r i s t i c s of the p a r t i c u l a r l a t t i c e d e t e r m i n e d t h u s f a r , we h a v e n o t o b s e r v e d m o l e c u l e s a r r a n g e d so t h a t the p o t e n t i a l s t r o n g exchange p a t h Fe-X-^Fe-X i s favored. We h a v e a l s o n o t o b s e r v e d p a c k i n g w h i c h p r o d u c e s s a n d w i c h e d d t c l i g a n d s , s u c h as overlapping aromatic r i n g s t r u c t u r e s . O u r pmr shifts (3) show a d e r e a l i z a t i o n o f t h e i r o n s p i n d e n s i t y t o the d t c p e r i p h e r y , but the major d e r e a l i z a t i o n i s t o the immediate i r o n l i g a n d s . This i s of course cons i s t e n t w i t h the Fe-S- »S-Fe type exchange paths. I t i s i n t e r e s t i n g t h a t the e x i s t e n c e o f subs t a n t i a l exchange i n t e r a c t i o n s i s a necessary but not s u f f i c i e n t c o n d i t i o n f o r magnetic o r d e r i n the t e m p e r a t u r e r a n g e o f o u r e x p e r i m e n t s , Τ > 1°K. This may be s e e n b y c o n s i d e r i n g t h e e f f e c t o f g r o u n d K r a m e r s l e v e l a n i s o t r o p i e s on the c o l l e c t i v e m a g n e t i c p r o p e r t i e s of the complexes. T h a t i s , we c o n s i d e r a s i t u a t i o n where a f i x e d i n t e r m o l e c u l a r exchange e x i s t s b e t w e e n m o l e c u l e s i n two i s o m o r p h o n s l a t t i c e s . The m o l e c u l a r l a t t i c e s a r e assumed t o d i f f e r o n l y i n t h e type of s p l i t t i n g t h a t occurs i n the ground S = 3/2 manifold. I n Eq. ( 1 ) , i f D i s l a r g e i n m a g n i t u d e and d o m i n a t e s t h e r h o m b i c c r y s t a l f i e l d and t h e exchange t e r m , we may r e w r i t e the exchange i n t e r a c t i o n w i t h i n t h e g r o u n d K r a m e r s d o u b l e t u s i n g an e f f e c t i v e s p i n S = 1/2 (low t e m p e r a t u r e a p p r o x i m a t i o n ) . This i l l u s t r a t e s the magnetic a n i s o t r o p y o f the o r i g i n a l IS = 3/2, M = ± 3/2> Kramers d o u b l e t . The r e s u l t i s e

e e

e

g

TC

I

=

-2

I

9J

S S i j iz iz

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(2)

38

E X T E N D E D INTERACTIONS

B E T W E E N M E T A L IONS

This i s the usual Ising Hamiltonian (E i s n e g l e c t e d ) . U p o n c h a n g i n g t h e s i g n o f D, b u t m a i n t a i n i n g t h e r e l a t i v e magnitudes o f the i n t e r a c t i o n s , the a l t e r n a t i v e ground doublet y i e l d s the a n i s o t r o p i c Heisenberg interaction "K* = -2

Τ

J..[S.

S. + 4 ( S . S. +S.

S. ) ] .

(3)

I t i s e a s i l y shown (10) t h a t i n t h e m o l e c u l a r f i e l d approximation the r a t i o o f magnetic t r a n s i t i o n temperatures w i t h and w i t h o u t t h e c r y s t a l f i e l d D term (exchange f o r c e s c o n s t a n t ) , i s g i v e n by t h e expression Τ

(D)/T

I n E q . (4) , IJ» i s the i o n i c expectation value S a t t h e magfietië t r a n s i t i o n t e m p e r a t u r e . For D-*+l

e =(p 4w°)+X(^ + ew)+ Σ X e ^ ^

[3-5]

,

v

n

n

i

n>1

The d i f f e r e n c e between t h e u s u a l s o r t o f p e r t u r b a t i o n t r e a t m e n t and t h i s one i s s i m p l y t h a t h e r e we l e t χ have c o r r e c t i o n s t o a l l o r d e r s a s f o r c e d by t h e c o n s t r a i n t t h a t w has o n l y a f i r s t o r d e r c o r r e c t i o n , w h i l e n o r m a l l y one i n t r o d u c e s a l l t h e c o r r e c ­ t i o n s t o χ a s a f i r s t - o r d e r t e r m , t h e r e b y c a u s i n g w t o have c o n ­ t r i b u t i o n s i n a l l orders. Now e x p a n s i o n o f [3-2] and i s o l a t i o n o f t h e c o e f f i c i e n t o f λ f o r each η leads t o t h e f o l l o w i n g .

[ 3

"

6 ]

( H ° ^ - w ° ) ν -Κ Η ' -μ»-ôw) v°=0 ,,

,

,

,

,,

0

(H°-^)u +(H -^ )u +(H -e")u =0 [ 3

"

7 ]

1

1

( H°-u,-w°) v"+( H -μ -6w) v»+{ H"-^) v°=0

In g e n e r a l , f o r n>2, we have 0

η

(η

,,

(Η -μ)ϋ< >-ΚΗ'-μ')υ - + Σ ( H j

[3-8] (Η°-μ-*#°)ν

(η,

ίη

=

4

(j )

(n

J > )u "

j

U

2

1,

-ΚΗ'-μ'-β«)ν " + Σ (H J =2

( j )

-e(j))v

( n

"

j )

=0

Each o r d e r i s r e p r e s e n t e d by a c o u p l e d p a i r o f e q u a t i o n s . Taking t h e inner product o f t h e f i r s t equation i n each p a i r w i t h u° and t h e s e c o n d w i t h v°, s u b t r a c t i n g , and r e a r r a n g i n g w i t h t h e aid o f [3-1], provide equations f o r the successive c o r r e c t i o n s t o the f i e l d . [3-9]

J

n

For convenience,

n

we l e t (u^ ^,u°)=( v^ ^,v°) = 6 no 1

x'=[&y-KF' -F' )+x°(G' -G )]/(G° -G° ) ^ uu vv' uu vv' vv uu' v

v

[3-10] ,

l

0

0

G

„ - G v„ v. ^ ' uu „ r ' vvvv ) ,,_ F' u'u -F'v ' v ^ u ' u v- 'G v„ ' ) + x ( G u'u < (G -G° ) vv uu' \/\/ llll' l v

l l |

l

u

x

etc.

,

H e r e we u s e t h e c o n d e n s e d n o t a t i o n (u ,Gu°)=G

, , etc. u u The c o m p l e t e p r e s c r i p t i o n f o r a p e r t u r b a t i o n s o l u t i o n i s now clear. One f i r s t c a l c u l a t e s ( u °, v°, M-,w°, bu) by e x a c t d i a g o n a l i z a t i o n o f H°. Then one f i n d s x by [ 3 - 9 ] , μ' f r o m one o f t h e e q u a t i o n s w h i c h i s a p r e c u r s o r t o [ 3 - 9 ] , and u and ν' f r o m [3-6], The p r o c e s s c a n c o n t i n u e t o any o r d e r . I t i s a l s o pos­ s i b l e , as i n t h e s t a n d a r d p e r t u r b a t i o n developments, t o d e r i v e 1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5.

BELFORD E T A L .

Field-Swept

EPR

47

Spectra

t h e f i r s t 2n+l c o r r e c t i o n s t o t h e e i g e n f i e l d f r o m t h e f i r s t η corrections t o the eigenvectors. A good s t r a t e g y o f c a l c u l a t i o n i s t o w o r k i n t h e b a s i s i n w h i c h H° i s d i a g o n a l ; t h e n many o f t h e r e l a t i o n s h i p s t a k e a s i m ­ p l e form. I n o u r f i r s t programs u s i n g f r e q u e n c y - s h i f t p e r t u r b a ­ t i o n s , we s t a r t by a p p l y i n g t o a l l m a t r i c e s i n t h e p r o b l e m t h e s i m i l a r i t y t r a n s f o r m a t i o n w h i c h d i a g o n a l i z e s H°. From t h i s p o i n t u n t i l t h e end (where e i g e n v e c t o r s a r e transformed back i n t o t h e o r i g i n a l b a s i s f o r o u t p u t ) we w o r k i n t h i s b a s i s a n d f i n d t h a t t h e programs t h e r e b y c a n be made v e r y e c o n o m i c a l . We have programmed t h e method up t o f o u r t h o r d e r i n v a l u e s a n d t h i r d order i n vectors. 3.3 Example o f F r e q u e n c y - S h i f t P e r t u r b a t i o n a n d C o m p a r i ­ son w i t h E i g e n f i e l d R e s u l t s We have u s e d a ^ r e q u e n c y - s h i f t p e r t u r b a t i o n program t s c r i b e d i n t h e next pape e x a m p l e s d e s c r i b e d i n S e c t i o n 2.3 p r o v i d e d s t a n d a r d s o f c o m p a r i ­ son f o r t h e f r e q u e n c y - s h i f t p e r t u r b a t i o n program. For t h e s e s t u d i e s , o n l y t h e f r e q u e n c y s h i f t was t a k e n a s t h e p e r t u r b a ­ tion — i.e., F'^^O. No a c t u a l d e g e n e r a c i e s o f H° c a u s e d trouble. The d i a g o n a l i z a t i o n s o f H° u s e d t h e w i d e l y a v a i l a b l e EISPAC m a t r i x e i g e n v a l u e package. G e n e r a l l y , t h e p e r t u r b a t i o n program, up t h r o u g h f o u r t h - o r d e r f i e l d c o r r e c t i o n s , r e q u i r e s o n l y a b o u t 1$ o f t h e p r o c e s s i n g t i m e expended by t h e e i g e n f i e l d program, and i t c a n be made more e f f i c i e n t . It also requires o n l y a f r a c t i o n (~10#) o f t h e s t o r a g e s p a c e u s e d by t h e e i g e n ­ f i e l d program. For t h e most p a r t , agreement between t h e e x a c t ( e i g e n f i e l d ) and t h e p e r t u r b a t i o n f i e l d s was good t o 6, 7, o r 8 s i g n i f i c a n t d i g i t s ( i . e . , t h e d i f f e r e n c e s were ~ 1 0 ~ gauss o r l e s s ) . A t y p i c a l r e s u l t i s g i v e n b e l o w ; θ=51°: 3

M,

,m

x°=1022.94, χ'=59.5029, x"=-0. 1604, x = 0 . 0 0 6 6 , x = - 0 . 0 0 0 3 x=l082.288879. (Compare e x a c t = l 0 8 2 . 2 8 8 8 9 1 ) Intensity: 0.1013 (Compare exact=0.1013) F i n a l v e c t o r s ( a l l i n agreement w i t h t h e e x a c t the four places p r i n t e d ) :

result t o

u=(.0720, .0672, .1350, .2371, . 0672,-.0938,-.1747,-.4974, . 1350, -.1747, . 1826, . 3233, . 2371,-. 4974, . 3233,-. 1728) v=(.0026, . 0 0 8 6 , . 0 2 5 6 , . 0 6 1 2 , . 0 0 8 6 , .0259, .0707, .1571, .0256, .0707, .1812, .3827, .0612, .1571, .3827, .7780) I n a l m o s t a l l c a s e s , t h e r e s u l t s c o u l d be i m p r o v e d by a very simple, r a p i d e x t r a p o l a t i o n o f t h e t h i r d and f o u r t h order f i e l d c o r r e c t i o n s toward i n f i n i t e order. The i d e a b e h i n d t h e e x t r a p o l a t i o n i s t h a t t h e s u c c e s s i v e p e r t u r b a t i o n s may a p p r o x i ­ mate a g e o m e t r i c s e q u e n c e , i . e . , x ^ ^ = r x ^ where r i s n e a r l y J

+

American Chemical Society Library 1155 13th between St. N.Metal W. Ions; Interrante, L.; In Extended Interactions ACS Symposium Series; American Society: Washington, DC, 1974. Washington, D.Chemical C. 20036

48

EXTENDED INTERACTIONS B E T W E E N

constant f o r j>3.

By t a k i n g x 5

6

t u r b a t i o n s e r i e s x^ ^+x^ ) + . . . examples

,,,,

/x

111

t o be t h i s

sums t o ( x

l l l l

2

ratio,

METAL

IONS

the per­

,

) /(x" -x"").

Some

follow:

Σ ο χ ^ ) =1022.94+173.0747-2.0890+0.7314-0.2216=1194.4355 x( e x t r a p o l a ted) = 1194.4870 x( e x a c t ) = 1 1 9 4 . 4 8 5 7 . Σ

4 ( J) Χ

.94+97.7430-0.6986+0.0915-0.0157=1120.0602 x(extrapolated)=l120.0625 x(exact)=1120.0626.

=1022

EQX^ ^=1022.940+407.033+3.635-15.536+10.061=1428. 133 x(extrapolated)=l424.178 x(exact)=1425.149. J

The l a s t c a s e i n t h e p r e v i o u s p a r a g r a p h i l l u s t r a t e s a s l o w c o n v e r g e n c e p r o b l e m w h i c h o c c u r r e d o c c a s i o n a l l y and which was i n a few c a s e s m a n i f e s t e d b These c a s e s o c c u r r e d whe s t a t e v e c t o r s i n v o l v e d i n a t r a n s i t i o n was c h a n g i n g v e r y r a p i d l y w i t h f i e l d , s o t h a t t h e range o f c o n v e r g e n c e o f the p e r t u r b a t i o n method was p a t h o l o g i c a l l y s m a l l — i . e . , o n l y a s m a l l 6w c o u l d be t o l e r a t e d . T h i s t e n d s t o happen c l o s e t o t h e c r o s s i n g p o i n t o f two i n t e r a c t i n g s t a t e s . We s u f f e r no s e v e r e d i f f i c u l t i e s f r o m t h i s p r o b l e m , b e c a u s e i t i s e a s y t o r e c o g n i z e such c a s e s and t o c o r r e c t them by t r y i n g a new x°. However, f o r a p r a c t i ­ c a l s e l f - c o n t a i n e d program, one would need t o d e v i s e an a l g o ­ r i t h m f o r a u t o m a t i c r e c o g n i t i o n and a d j u s t m e n t . Incidentally, in the p a t h o l o g i c a l c a s e s t h e v e c t o r s and i n t e n s i t i e s a r e f a r w o r s e than the f i e l d s . As e x p e c t e d , h e r e t h e a p p r o x i m a t e method, though f a s t e r , i s much l e s s f o o l p r o o f t h a n t h e e x a c t ( e i g e n ­ f i e l d ) method. In some c a s e s , t h e same t r a n s i t i o n was computed s t a r t i n g from two o r t h r e e r a t h e r d i s t a n t v a l u e s o f x°. I n most c a s e s , the agreement was r e a s o n a b l e . Of c o u r s e , l a r g e f i e l d s h i f t s from x° produced l e s s a c c u r a t e r e s u l t s t h a n s m a l l s h i f t s , the p a t h o ­ l o g i c a l c a s e s m e n t i o n e d above e x c e p t e d . Some c o m p a r i s o n s f o l l o w : F o r θ=88.5°, x ( e x a c t ) = 2 8 5 8 . 5 4 3 0 , întensity=l.0007. T h r e e p e r t u r ­ b a t i o n t r i a l s f r o m d i f f e r e n t s t a r t i n g f i e l d s produced J

Σοχ^ * ^ =2200+655. 8418+3. 7346- 1.3842+0. 4590=2858. 6512 x ( e x t r a p o l a ted)=2858.5269 ; I n t e n s i t y = 0 . 9 9 8 5 J

Σοχ^ * ) =2900-41.4622+0.0051+0.0001+0.0000=2858. 5430 x( e x t r a p o l a ted)=2858.5430 ; I n t e n s i t y = l . 0 0 0 0 J

Σοχ^ * ^ =3600-742.3981+0.6875+0. 1904+0.0483=2858. 5281 x(extrapolated)=2858.5445; Intensity=0.9990 A l l t h e t h i r d - o r d e r v e c t o r s a g r e e d t o w i t h i n a few p a r t s i n t h e f o u r t h d e c i m a l p l a c e f o r e a c h v e c t o r component; i n e a c h c a s e t h e t r a n s i t i o n a r o s e f r o m the 5th-»10th l e v e l s o f t h e s t a r t i n g Hami1tonian.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5.

BELFORD

Field-Swept

E T AL.

EPR

49

Spectra

3.4 D e g e n e r a t e S t a t e s . Suppose t h a t a f i e l d p o s i t i o n must be c o m p u t e d f r o m a n i n i t i a l l y R e g e n e r a t e s e t o f t r a n s i t i o n s . C o n s i d e r { U|,..., u }-*[ V|,... ,v }, where t h e s e t { u°) i s a p - f o l d Q

0

degenerate

s e t o f e i g e n s t a t e s o f H° and { v } a q - f o l d s e t . 0

[3-11]

H u°=yj,u!; Η^°=(μτΗ*°)ν]

T h e r e a r e s e v e r a l ways t o p r o c e e d . Normally, a degenerate per­ t u r b a t i o n t r e a t m e n t w o u l d c a l l f o r d i a g o n a l i z a t i o n o f H' w i t h i n d e g e n e r a t e b l o c k s o f H°. However, t h a t i s n o t a l w a y s p o s s i b l e > a p r i o r i , because H i s F'+x°G'+χ'ΰ , i n w h i c h x i s not y e t known. The d e s i r e d s e p a r a t i o n c a n be o b t a i n e d by d i a g o n a l i z i n g b o t h t h e f u } and t h e { v } s u b - b l o c k s o f F + x G ' and t h e n diagonalîzing G w i t h i remainin degenerat blocks If degeneracies p e r s i s t , the An a l t e r n a t i v e f o d e g e n e r a t s m a l l e i g e n f i e l d problem. B e g i n n i n g w i t h [ 3 - 1 1 ] , one g e t s t h e f o l l o w i n g p a i r o f f i r s t - o r d e r equations analogous t o those f o r the nondegenerate case. n e r e

1

0

0

1

0

,

,

,

0

l

E.b.(H -u, )u?+(H°-M )u =0 i

[3-12] (H -M, -6W)V .+(H -^-W°)V =0

Y_.c.

,

i

Taking the inner product left,

i

0

j

and t h e second

o f the f i r s t

e q u a t i o n w i t h u° on t h e

by v£, and d e n o t i n g t h e s u b m a t r i c e s

0

0

w i t h i n t h e { u } and { v } b l o c k s a s h

U

and h

V

of H

1

and t h e v e c t o r s o f

c o e f f i c i e n t s b. and c. a s b and c r e s p e c t i v e l y , we g e t t h e f o l ­ lowing matrix

equations. u

[3-13] (Here

v

(h%"I )b=0; I

U

V

( h -(μ,'+ôw) I ) c=0

d e n o t e s t h e u n i t m a t r i x h a v i n g t h e same o r d e r a s h V

U

—

U

i . e . , p.) A p p l y i n g 0 I c t o t h e f i r s t and I b © t o t h e second o f t h e s e e q u a t i o n s and s u b t r a c t i n g y i e l d s [3-14] . (h 0 U

[3-14]

V

I -I

U

V

©

U

h +6wI ©

U

V

I ) (b © U

c) =0 U

The f o r m o f h i s now f ^ x ' g " , where f and g a r e s u b m a t r i c e s w i t h i n t h e { u } b l o c k o f F + x ° G and G ° , r e s p e c t i v e l y . Thus [3-14] c a n be r e a r r a n g e d t o [ 3 - 1 5 ] . 0

,

l

[3-15] U

( f ©

V

I -I

U

©

V

U

f +ôwI ©

V

I ) (b©

U

c) =x « ( I ©

V

U

g -g ©

V

I ) (b©

c)

T h i s i s a s m a l l e i g e n f i e l d e q u a t i o n based on t h e t r a n s i t i o n s from the { u } t o f v } b l o c k s ; t h e e i g e n f i e l d s a r e t h e f i r s t - o r d e r 0

0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

50

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

t r a n s i t i o n f i e l d s f o r t h e s e t r a n s i t i o n s , and t h e e i g e n v e c t o r s are the c o r r e c t z e r o ' t h order t r a n s i t i o n v e c t o r s . Acknowledqements. T h i s r e s e a r c h was s u p p o r t e d by t h e P e t r o l e u m R e s e a r c h t h e N a t i o n a l S c i e n c e F o u n d a t i o n , and t h e Advanced R e s e a r c h j e c t s Agency. Literature 1. 2. 3. 4. 5. 6.

7. 8. 9.

Fund, Pro­

Cited.

B e l f o r d , G. G., Belford, R. L., and Burkhalter, J. F., J . Magn. Resonance (1973) 11, 251. Hempel, J . C., Morgan, L. O., and L e w i s , W. B., I n o r g . Chem. ( 1 9 7 0 ) 9, 2064. B y f l e e t , C. R., Chong C. Α., J . Magn. Resonance ( 1 9 7 0 ) 2, 69. Abragam, A. and B l e a n e y , Β., "Electron P a r a m a g n e t i c Reso­ nance o f Transition I o n s , " C l a r e n d o n P r e s s , O x f o r d , 1970. B e l f o r d , R. L. and Belford, G. G., J . Chem. Phys. ( 1 9 7 3 ) 59, 853. K o r n , G. A. and K o r n , Τ. Μ., "Mathematical Handbook f o r Scientists and Engineers," § 13.2-10, M c G r a w - H i l l , New Y o r k , 1961. M o l e r , C. B. and S t e w a r t , G. W., SIAM J. Numer. Anal. (1973) 1O, 241. Chang, C.-C., M.S. T h e s i s i n Computer S c i e n c e , U n i v . o f Illinois a t Urbana-Champaign, 1974. D a v i s , P. H. and Belford, R. L., this volume.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6 Multisite Magnetic Interactions in HexaureaMetal(III) Halide Lattices PHILLIP

H.

DAVIS

and

R. L .

BELFORD

U n i v e r s i t y of Illinois, U r b a n a , Ill. 61801

Introduction X-ray crystallographic studies carried out i n our l a b o r a t o r y and o t h e r s on a number of salts o f t h e t y p e M(urea) X3 , where M = Al, Ti, V, Cr, o r Fe and X = halide o r perchlorate, have shown them to be isomorphous (1-7). Furthermore, where complete x - r a y structure d e t e r m i n a t i o n s w e r e done, t h e m o l e c u l a r g e o m e t r i e s o f the v a r i o u s M(urea) 6 3 + c o m p l e x i o n s w e r e f o u n d to be virtually identical (2-7). The crystal structure is s u c h as t o s u g g e s t t h e possibility o f o b s e r v i n g weak o n e - d i m e n s i o n a l interactions be­ tween t h e m e t a l i o n s w h i c h o c c u r in infinite linear chains paral­ lel t o t h e crystallographic c-axis. The existence of the iso­ morphous series p r o v i d e s an opportunity f o r a variety o f compara­ tive studies. For example, by v a r y i n g t h e concentration of the p a r a m a g n e t i c s p e c i e s i n binary s y s t e m s involving one o f t h e para­ magnetic i o n s doped into one o f t h e d i a m a g n e t i c a l u m i n u m salts, it w o u l d be possible to v a r y t h e effective length o f the chain along which interactions might t a k e p l a c e . A l s o of interest a r e studies of interactions among different paramagnetic ions i n binary and t e r n a r y s y s t e m s (e.q. Cr:Ti(urea) X and Cr:Ti :Al(urea) X ). 6

6

6

Structural

3

3

Background

The above s a l t s c r y s t a l l i z e i n t h e t r i g o n a l ( rhombohedral ) s p a c e g r o u p R3C w i t h a p p r o x i m a t e u n i t c e l l p a r a m e t e r s a = 17.5A and c = 14.OA. T a b l e I p r o v i d e s a c o m p a r i s o n o f u n i t c e l l parameters f o r the v a r i o u s s a l t s . The b a s i c s t r u c t u r e o f t h e c o m p l e x i o n i s shown i n F i g u r e 1. Each t r i p l y p r i m i t i v e u n i t c e l l c o n t a i n s s i x f o r m u l a u n i t s . The m e t a l atoms o c c u p y t h e s i x - f o l d s e t o f s p e c i a l p o s i t i o n s a o f 32 symmetry. W h i l e t h e c o m p l e x i s t h u s c r y s t a l l o g r a p h i c a l l y c o n ­ s t r a i n e d t o p o s s e s s D symmetry, t h e c o o r d i n a t i o n p o l y h e d r o n i s v e r y n e a r l y a r e g u l a r o c t a h e d r o n . Two t y p e s o f d i s t o r t i o n o f an 3

51

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

INTERACTIONS B E T W E E N

METAL

IONS

Table I Comparative Unit

C e l l P a r a m e t e r s f o r S a l t s M(urea) ό Χ 3

Compound

8

·(*)

£ (A°)

Ref.

Al(urea) I

3

17.539( 3)

13. 855( 8)

6

Ti(urea) I

3

17.67(2)

14. 15(2)

3

6

6

Ti(urea) (C10 )

( 300°K)

18. 132(5)

14. 149(5)

4

Ti(urea) (C!0 )

( 90°K)

17.57

13. 84

5

17.53

13.87

6

17.49(2)

14.38( 3)

7

6

4

6

4

Cr(urea) I 6

3

3

3

V(urea) I

3

(300°K)

V(urea) I

3

(90°K)

6

6

Where known, e s t i m a t e d s t a n d a r d d e v i a t i o n i n l a s t cant d i g i t given i n parentheses.

signifi­

o c t a h e d r o n a r e p o s s i b l e w h i c h m a i n t a i n a t l e a s t D symmetry. The f i r s t i n v o l v e s the compression or e l o n g a t i o n o f t h e octahedron along a t h r e e - f o l d a x i s and i s m a n i f e s t e d i n a d e v i a t i o n o f t h e a n g l e (commonly d e n o t e d Θ) between an M-0 bond and t h e t h r e e - f o l d a x i s from t h e o c t a h e d r a l v a l u e o f 5 4 . 7 4 ° . The s e c o n d d i s t o r t i o n i n v o l v e s a t w i s t i n g about t h e t h r e e - f o l d a x i s o f o p p o s i t e f a c e s o f the octahedron r e s u l t i n g i n a d e v i a t i o n from 6 0 ° o f the a n g l e ( u s u a l l y d e n o t e d [-s,-s

5

+

2

U r ^ ( ^ ]

(3)

and c o m p a r i s o n w i t h t h the i d e n t i f i c a t i o n d = value o f r t h e c r y s t a l l o g r a p h i c a l l y observed metal-metal separa t i o n , we c a l c u l a t e d = - 5 0 . 7 5 x 1 0 " cm" . We n o t e a t t h i s p o i n t t h a t t h e above p h y s i c a l r e a s o n i n g im­ p l i e s t h a t d i s n e g a t i v e . The i n t r a i o n i c z e r o f i e l d s p l i t t i n g p a r a m e t e r , J), i s a l s o a s i g n e d q u a n t i t y , but t h e o r d i n a r y e p r e x ­ p e r i m e n t p e r m i t s o n l y t h e d e t e r m i n a t i o n o f i t s magnitude. Be­ cause o f the i n t e r a c t i o n o f t h e v a r i o u s z e r o f i e l d s p l i t t i n g t e r m s , t h e p o s i t i o n s and s p a c i n g s o f l i n e s i n t h e s p e c t r u m o f t h e c o u p l e d i o n s depend on b o t h t h e m a g n i t u d e s and r e l a t i v e s i g n s o f t h e i n t e r i o n i c i n t e r a c t i o n p a r a m e t e r s d^ and and t h e i n t r a i o n i c i n t e r a c t i o n p a r a m e t e r , JD. Knowledge o f t h e s i g n o f d^ s h o u l d t h u s e n a b l e us t o d e t e r m i n e t h e s i g n s o f £ and J. A comparison o f observed s p e c t r a l features w i t h those c a l ­ c u l a t e d f o r a p u r e l y d i p o l a r i n t e r a c t i o n model and f o r e a c h o f t h e p o s s i b l e s i g n s f o r j) i s f o u n d i n T a b l e I I I . Considering the s i m p l i c i t y o f t h e model and t h e f a c t t h a t t h e v a l u e o f d was c a l ­ c u l a t e d from f i r s t p r i n c i p l e s r a t h e r than being f i t t e d t o t h e ob­ s e r v a t i o n s , t h e agreement between t h e o b s e r v e d and c a l c u l a t e d s p e c t r a ( e s p e c i a l l y f o r t h e c a s e when d and £ a r e o f o p p o s i t e s i g n ) i s good. Examination o f the vectors f o r the states involved i n the t r a n s i t i o n s d e n o t e d 1 and 6 shows them t o be e s s e n t i a l l y ( i n t h e |M ,,M > b a s i s ) ι J r i I 3 ι 3 1 . _L π ι I Ί / J l"2'"? " l"2 "2 " /2 2 '2 " \ ~ 2*2?* 4

S

1

S2

3

, :

A.

i

1

fl

>

3

1

,

ι

1

3

\

> j

1

l |

fl

3

1

,

3

1

>

I

1

3

ι

W i t h i n each p a i r t h e e f f e c t o f t h e i s o t r o p i c exchange t e r m i n ( 2 ) i s t o s h i f t t h e e n e r g y o f e a c h s t a t e by t h e same amount and i n t h e same d i r e c t i o n . The p o s i t i o n s o f and s p a c i n g between t h e s e two l i n e s w i l l t h u s be d e t e r m i n e d a l m o s t s o l e l y by t h e m a g n i t u d e o f t h e d i p o l a r term i n ( | ) . That t h e s e two l i n e s a r e f i t t e d

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6.

DAVIS

AND

BELFORD

Multisite

Interactions

59

Table I I I C o m p a r i s o n o f Observed and C a l c u l a t e d C o u p l e d Cr( u r e a )

3 + 6

-Cr( u r e a ) Calculated

Transi t ion

Observed Field (gauss)

EPR S p e c t r a f o r 3 +

Pairs

6

Field*

5

3

(gauss)

D = -483.4 d = - 50.75 0.0 J

D = 483.4 d = -50.75 J = 0.0

D = -483.4 d = - 50.75 J = 45. 11

1

3047 ( 2 )

3047 ( 4 )

3047 ( 4 )

3047 ( 4 )

2

3107 ( 1 0 )

3117 ( 7 )

3145 ( 9 )

3107 ( 10)

3

3169 ( 1 )

3231

3168 ( 2 )

4

3420 ( 1)

5

3482 ( 10)

3472 ( 7 )

3444 ( 9 )

3482 ( 10)

6

3540 ( 2 )

3542 ( 4 )

3542 ( 4 )

3542 ( 4 )

(3)

S p e c t r a here l i m i t e d t o d i a g n o s t i c group o f s i x t r a n s i t i o n s c e n t e r e d a r o u n d 3300 g a u s s ( a t 9.1 GHz) when m a g n e t i c f i e l d d i r e c t i o n i s p a r a l l e l t o z_-axis. R e l a t i v e i n t e n s i t y i n paren­ theses f o l l o w i n g f i e l d . 'For a l l c a l c u l a t i o n s g = 1.9752. R e m a i n i n g s p i n p a r a m e t e r s e x p r e s s e d i n u n i t s o f 1 0 " cm"'.

Hamiltonian

4

q u i t e w e l l and t h e r e m a i n i n g f o u r l i n e s l e s s w e l l by t h e p u r e l y d i p o l a r i n t e r a c t i o n model i n d i c a t e s 1) t h a t t h e m a g n i t u d e o f t h e d i p o l a r c o u p l i n g c a l c u l a t e d from t h e c r y s t a l s t r u c t u r e i s ap­ p r o p r i a t e and 2) t h a t t h e r e i s a s i g n i f i c a n t amount o f e x c h a n g e coupiing present. I f t h e other parameters o f t h e s p i n Hamiltonian a r e held constant, the f i e l d s at which the s i x d i a g n o s t i c t r a n s i t i o n s oc­ cur a r e each l i n e a r i n t h e exchange parameter over a small range (\A\ < \A\)> f a c i l i t a t i n g t h e p r e c i s e d e t e r m i n a t i o n o f J_. W i t h t r i e i n c l u s i o n o f e x c h a n g e a f i t between t h e o b s e r v e d and c a l ­ c u l a t e d s p e c t r a i n t h e d i a g n o s t i c r e g i o n was o b t a i n e d w h i c h was e x a c t , w i t h i n e x p e r i m e n t a l u n c e r t a i n t y ( s e e T a b l e I I I ) . The parameters o f best f i t , which c o n s t i t u t e a unique s e t , a r e g = 1.9752, D = - 4 8 3 . 4 x 1 0 " cm" , d = - 5 0 . 7 5 x 1 0 " cm" and J = 45. l l x l O " cm" . I t i s worth noting t h a t w h i l e i n t h e absence o f e x c h a n g e t h e b e s t agreement between o b s e r v e d and c a l c u l a t e d f i e l d s f o r t h e d i a g n o s t i c t r a n s i t i o n s was o b t a i n e d when t h e p a r a m e t e r s J) and d w e r e o f o p p o s i t e s i g n , t h e b e h a v i o r o f t h e t r a n s i t i o n f i e l d s w i t h t h e i n c l u s i o n o f exchange i s c o n s i s t e n t o n l y w i t h t h e s e p a r a m e t e r s h a v i n g t h e same s i g n . The p o s i t i v e v a l u e o b s e r v e d f o r J c o r r e s p o n d s t o an a n t i f e r r o m a g n e t i c c o u p l i n g . 4

4

1

4

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

60

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

T a b l e I I I compares o b s e r v e d and c a l c u l a t e d s p e c t r a i n t h e d i a g n o s t i c region f o r several combinations of spin Hamiltonian p a r a m e t e r s , w h i l e F i g u r e 5 i l l u s t r a t e s t h e agreement between t h e c a l c u l a t i o n e m p l o y i n g t h e p a r a m e t e r s o f b e s t f i t and t h e e n t i r e s p e c t r u m o b s e r v e d when t h e m a g n e t i c f i e l d i s p a r a l l e l t o z_. In T a b l e IV o b s e r v e d t r a n s i t i o n f i e l d s a t t h e Q-band f r e q u e n c y ( 3 5 . 1 GHz) and t h o s e c a l c u l a t e d f r o m t h e p a r a m e t e r s o f b e s t f i t ( o b t a i n e d a t 9.1 GHz) a r e compared f o r s e v e r a l o r i e n t a t i o n s . The computer p r o g r a m u s e d f o r a l l s p e c t r a l c a l c u l a t i o n s i n t h i s paper was w r i t t e n by Mr. T. M. L e n h a r d t o f t h e U n i v e r s i t y o f I l l i n o i s and i s based on t h e f r e q u e n c y s h i f t p e r t u r b a t i o n t e c h ­ nique of reference ( 9 ) . I f t h e c r y s t a l s o f C r : A 1 ( u r e a ) 6 I 3 a r e s u b j e c t t o tempera­ t u r e dependent changes i n c r y s t a l and m o l e c u l a r s t r u c t u r e o f t h e type observed i n the cas p e r a t u r e o f most o f t h pected. A c o n t r a c t i o n o f t h e l a t t i c e ( a t l e a s t i n t h e cod i r e c t i o n ) w o u l d r e d u c e t h e m e t a l - m e t a l s e p a r a t i o n and have t h e e f f e c t o f i n c r e a s i n g b o t h d^ and i n a b s o l u t e v a l u e . The param­ e t e r d w i l l , o f c o u r s e , v a r y as l / r , w h i l e t h e r a t e o f change o f w o u l d depend on t h e m e c h a n i s m ( s ) o f t h e exchange, but i n any c a s e m i g h t r e a s o n a b l y be e x p e c t e d t o be somewhat g r e a t e r t h a n t h a t f o r d. A change i n t h e c o o r d i n a t i o n g e o m e t r y about t h e Cr a t o m w o u l d r e s u l t i n a c h a n g e i n J), but i t i s d i f f i c u l t t o make a priori p r e d i c t i o n o f the d i r e c t i o n or magnitude o f the change. E x p e r i m e n t a l l y , D, d, and J. a l l i n c r e a s e i n m a g n i t u d e w i t h decreasing temperature. A t a b o u t 90°K t h e p a r a m e t e r s g i v i n g t h e b e s t agreement between o b s e r v e d and c a l c u l a t e d s p e c t r a ( t h e f i t was a t l e a s t as good as t h a t a t room t e m p e r a t u r e ) w e r e g = 1.9752, D = - 5 U . 8 x l O - cm" , d = - 5 4 . 3 5 x l 0 " cm"', and J = 50. 1 3 x 1 0 " cm" . The v a l u e o f d, w h i c h was o b t a i n e d f r o m t h e s p a c i n g between d i a g n o s t i c l i n e s ]_ and 6 ( v i d e s u p r a ) , c o r r e ­ sponds t o an a p p a r e n t c o n t r a c t i o n o f t h e l a t t i c e i n t h e co­ d i r e c t i o n , or at l e a s t a r e d u c t i o n o f the Cr-Cr p a i r d i s t a n c e , by a b o u t 2$. T h i s i s about t w i c e t h e l a t t i c e c o n t r a c t i o n ob­ s e r v e d i n t h e v a n a d i u m c o m p l e x , but i s not a p h y s i c a l l y u n r e a ­ s o n a b l e amount. The f r a c t i o n a l i n c r e a s e i n i s about 1.6 t i m e s t h a t o b s e r v e d f o r id; t h a t i s , ,J a p p e a r s t o be v a r y i n g a p p r o x i ­ m a t e l y as l / r . W h i l e a pathway f o r d i r e c t e x c h a n g e e x i s t s and the l / r dependence does not seem u n r e a s o n a b l e f o r t h i s mecha­ nism at the observed i n t e r n u c l e a r d i s t a n c e , a super-exchange mechanism i s not c o n c l u s i v e l y r u l e d o u t . When t h e m a g n e t i c f i e l d d i r e c t i o n i s a p p r e c i a b l y away f r o m t h e z _ - a x i s , most o f t h e p a i r t r a n s i t i o n s w h i c h a r e c a l c u l a t e d t o have an o b s e r v a b l e i n t e n s i t y o c c u r a t f i e l d s s u c h t h a t t h e y a r e masked by t h e much more i n t e n s e i s o l a t e d - i o n l i n e s , and t h e agreement between t h e c a l c u l a t e d t r a n s i t i o n f i e l d s and t h o s e l i n e s w h i c h a r e o b s e r v e d i s not as good as f o r n e a r l y p a r a l l e l o r i e n t a t i o n s ( s e e T a b l e I V ) . I n a d d i t i o n , f o r t h e most d i l u t e 3

a n

4

4

1

4

1

5

5

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974. 3 G

6

3

Figure 5. Comparison of observed and calculated X-band epr spectra for coupled Cr(urea) -Cr(urea) * pairs. Heights of lines in the calculated stick spectrum represent rehtive intensities. The two outermost lines in the calculated spectrum are each actually two transitions calculated to occur at fields differing by less than 3 gauss.

62

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

T a b l e IV Observed

and C a l c u l a t e d Q-Band EPR S p e c t r a a t S e v e r a l

Orientations

f o r C o u p l e d Cr( u r e a )

θ = 0° (H // z) Calculated Field (gauss) 113651 11368 J

Observed Field (gauss)

Calculated Field (gauss) 114721 11483J

11367

13570 13635 13649

139601 13963 J

3

+

6

Pairs

θ = 30°

Observed C a l c u l a t e d Field Field (gauss) (gauss)

11480

Observed Field (gauss)

I 1767 11809

11500

11692 ( s h ) 11765 11837 12415 12476 12538 12790 12849 12910 13485 13538 13570 ( ? ) 13633 13660 ( s h ) 13793 13876

- C r ( urea)

θ = 15°

11447 11537 116791 11694 J 11758 11840 12417 12476 12538 12790 12852 12912 13488

3 +

6

a

11777J 11841 11924 12447 12501 12539 12805 12845 12892 13394 13477 135391 13540J 13815 13846

11765 11803 11843

12073J

11838 11930 12446 12501 12540 12802 12842 12888 13395 13460 Y

b

12155 12255 125251 12532J 12562 12817 12834 1 12840 J 13140

X

13225Ί

13805

13245 f 13287J 13429

12524 12563 12815 12831 13000 13120

13420

13958

S h o u l d e r s a r e o b s e r v e d on t h e i n t e n s e at a p p r o x i m a t e l y t h e s e f i e l d s .

isolated-ion transitions

P a i r t r a n s i t i o n s a t t h e s e f i e l d s w o u l d be o b s c u r e d by t h e i n ­ t e n s e i s o l a t e d - i o n t r a n s i t i o n i n t h e same r e g i o n .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6.

DAVIS

AND

Muitisite

BELFORD

Interactions

63

s a m p l e s weak l i n e s beyond t h o s e p r e d i c t e d by o u r model a r e ob­ served i n the r e g i o n o f the outermost l i n e s a s s o c i a t e d w i t h the i s o l a t e d - i o n , e v e n a t t h e p a r a l l e l o r i e n t a t i o n ( s e e F i g u r e s 4a and 5 ) . These d i s c r e p a n c i e s , w h i c h we do not f e e l i n d i c a t e any s e r i o u s d e f i c i e n c y i n t h e model, may a r i s e f r o m t h e terms w h i c h we have l e f t o u t o f o u r s p i n H a m i l t o n i a n . Another p o s s i b l e o r i ­ g i n f o r t h e ' e x t r a l i n e s w o u l d be t h e e x i s t e n c e o f two ( o r more) d i s t i n c t Cr( u r e a ) - C r ( u r e a ) s i t e s , c h a r a c t e r i z e d by d i f f e r ­ e n t I) v a l u e s . The e f f e c t o f s l i g h t l y d i f f e r e n t j) v a l u e s w o u l d be m i n i m a l on t h e d i a g n o s t i c p o r t i o n o f t h e s p e c t r u m , but i n t h e two o u t e r r e g i o n s w o u l d r e s u l t i n s e t s o f l i n e s d i s p l a c e d by 2 Δ ρ ^ β . As t h e c o n c e n t r a t i o n o f C r i s increased, the i n t e n s i t y o f the i s o l a t e d - i o n l i n e s r e l a t i v e t o the remaining f e a t u r e s o f t h e s p e c t r u m d e c r e a s e s , and a l l o f t h e l i n e s a r e b r o a d e n e d , r e ­ ducing r e s o l u t i o n (se that the 'extra' l i n e peared i n the spectru Figur spectru pur C r ( u r e a ) I ( F i g u r e 4c) i s d i f f e r e n t y e t , a l t h o u g h some g e n e r a l s i m i l a r i t i e s between i t and t h o s e o f t h e d i l u t e d s a m p l e s a r e evident. U n d o u b t e d l y t h e same i n t e r a c t i o n s a r e a t w o r k h e r e , but a t t h e l e a s t we c a n e x p e c t i t t o be n e c e s s a r y t o c o n s i d e r i n t e r a c t i o n s between e a c h C r i o n and b o t h o f i t s a x i a l n e a r e s t neighbors. 1

3 +

3 +

6

6

3 +

6

3

3 +

Additional

Studies of Interactions in Κ urea)5X3 L a t t i c e s

The v a r i o u s a d d i t i o n a l i n v e s t i g a t i o n s o f m a g n e t i c i n t e r a c ­ t i o n s i n hexaurea-metal ( I I I ) h a l i d e l a t t i c e s contemplated or c u r r e n t l y underway i n t h i s l a b o r a t o r y a r e c o n v e n i e n t l y d i v i d e d i n t o t h r e e c a t e g o r i e s by t h e c o m p o s i t i o n o f t h e m a t e r i a l s b e i n g studied. The f i r s t g r o u p c o n s i s t s o f s y s t e m s i n v o l v i n g a s i n g l e p a r a m a g n e t i c s p e c i e s , e i t h e r as a s u b s t i t u t i o n a l i m p u r i t y i n one o f t h e a l u m i n u m s a l t s o r as t h e p u r e M ( u r e a ) X s a l t . Here we w i s h t o d e t e r m i n e w h e t h e r c o u p l i n g between p a r a m a g n e t i c s i t e s o c c u r s and, i f s o , w h e t h e r a mechanism o f t h e t y p e e s t a b l i s h e d f o r t h e chromium s y s t e m a p p e a r s a p p r o p r i a t e . In t h e t i t a n i u m s y s t e m , t h e o n l y o t h e r one f o r w h i c h e x p e r i m e n t a l information is c u r r e n t l y a v a i l a b l e , t h e l a c k o f dependence o f t h e e p r s p e c t r u m on T i concentration suggests that i n t e r i o n i c coupling is m i n i ­ mal. A n a l y s i s i s c o m p l i c a t e d by t h e f a c t t h a t , e v e n f o r d i l u t e samples, the spectrum o f T i ( u r e a ) i s not t h a t a n t i c i p a t e d f o r a s i m p l e S, = 1/2 ( d ) i o n and has not been a d e q u a t e l y inter­ preted. I n t h e s e c o n d g r o u p a r e s y s t e m s i n w h i c h one o f t h e p a r a ­ m a g n e t i c i o n s i s doped i n t o t h e l a t t i c e o f a d i s s i m i l a r paramag­ n e t i c i o n . At t h e p r e s e n t o u r a t t e n t i o n has c e n t e r e d on C r : T i ( u r e a ) 0 X 3 and Cr:V( u r e a ) X . Because o f l i n e broadening due t o r a p i d r e l a x a t i o n , n e i t h e r T i ( u r e a ) ô n o r V ( u r e a ) 6 gives an o b s e r v a b l e e p r s p e c t r u m a t room t e m p e r a t u r e . For o u r 6

3

3

+

3 +

6

1

6

3

3+

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3 +

64

EXTENDED

INTERACTIONS

BETWEEN

METAL

3+

IONS

p u r p o s e s , s i t e s o c c u p i e d by C r ( u r e a ) ô i o n s as g u e s t s i n one o f t h e o t h e r l a t t i c e s may be c l a s s i f i e d a c c o r d i n g t o t h e a x i a l neighbors of a given C r ion. I f the c o n c e n t r a t i o n of C r is low, most w i l l have d i s s i m i l a r i o n s i n b o t h n e i g h b o r i n g a x i a l p o s i t i o n s , a few w i l l have one s i m i l a r and one d i s s i m i l a r n e i g h ­ b o r , w h i l e t h e number w i t h two s i m i l a r n e i g h b o r i n g i o n s w i l l be negligible. Depending on t h e s t r e n g t h o f t h e i n t e r i o n i c c o u ­ p l i n g , i f any, s p e c t r a l f e a t u r e s due t o i o n s i n t h e f i r s t t y p e o f s i t e p o t e n t i a l l y w o u l d p r o v i d e i n f o r m a t i o n on t h e p a r a m a g n e t i c ion o f the host l a t t i c e , s i n c e the Cr(urea)ô system i s a l r e a d y w e l l c h a r a c t e r i z e d . (Note, however, t h a t d i f f e r e n c e s i n l a t t i c e d i m e n s i o n s may r e s u l t i n d i f f e r i n g d e g r e e s o f d i s t o r t i o n o f t h e Cr(urea) i o n s o t h a t changes i n t h e s p e c t r u m f r o m t h e Cr:A1( u r e a ) 5 X 3 r e f e r e n c e w i l l p r o b a b l y be a c o m b i n a t i o n o f l a t ­ t i c e and s p e c i f i c i o n - c o u p l i n o f samples o f C r : T i ( u r e a ) f r o m Cr : A l ( u r e a ) I . The s t r o n g e s t f e a t u r e s o n c e a g a i n a r e c o n s i s t e n t w i t h the Hamiltonian (!). W h i l e t h e r e i s no d e t e c t a b l e change i n £, j) i s l a r g e r i n m a g n i t u d e t h a n o b s e r v e d i n t h e a l u ­ minum l a t t i c e . There are s e v e r a l p o s s i b l e e x p l a n a t i o n s o f the f a i l u r e to observe s i g n i f i c a n t T i - C r c o u p l i n g , but t h e most a t t r a c t i v e w o u l d seem t o be t h e a s s u m p t i o n t h a t t h e T i is re­ l a x i n g so r a p i d l y t h a t the C r i s not c o u p l e d t o i t , but s e e s o n l y an a v e r a g e d e n v i r o n m e n t w h i c h w o u l d p r o v i d e a v e r y s m a l l paramagnetic s h i f t . S i m i l a r l y , t h e s e c o n d a r y f e a t u r e s c a n be i n t e r p r e t e d on t h e b a s i s o f c o u p l e d C r - C r pairs. The u s e o f d i f f e r e n t i s o m o r p h o u s l a t t i c e s c o u p l e d w i t h measurements a t low temperature p r o v i d e s a range o f Cr-Cr s e p a r a t i o n s which s h o u l d h e l p t o f u r t h e r e l u c i d a t e t h e p r e c i s e n a t u r e o f t h e c o u p l i n g be­ tween t h e C r ( u r e a ) i o n s and, i n p a r t i c u l a r , t h e e x c h a n g e mechani sm. As y e t we have not p r e p a r e d any c r y s t a l s o f t h e t h i r d t y p e i n w h i c h one o f t h e Al(urea)ôX s a l t s i s doped s i m u l t a n e o u s l y w i t h two d i s s i m i l a r p a r a m a g n e t i c M( u r e a ) ions. The primary a i m h e r e w i l l be t o s t u d y c o u p l i n g i n d i s c r e t e M( u r e a ) W(urea) + pairs. I t i s a n t i c i p a t e d t h a t a l a r g e p a r t o f any s u c h i n v e s t i g a t i o n s w i l l have t o be c a r r i e d out a t l i q u i d h e l i u m temperature. I n summary, t h e i s o m o r p h o u s , i s o s t r u c t u r a l M ( u r e a ) X salts p r o v i d e an o p p o r t u n i t y f o r a v a r i e t y o f i n v e s t i g a t i o n s o f weak c o u p l i n g between p a r a m a g n e t i c i o n s i n t h e s o l i d s t a t e . Epr i n ­ v e s t i g a t i o n s o f t h e s e s y s t e m s not o n l y c a n p r o v i d e i n f o r m a t i o n on t h e c o u p l i n g i t s e l f , but i n d i r e c t l y , t h r o u g h t h e c o u p l i n g , i n f o r ­ m a t i o n about t h e i n d i v i d u a l i o n s i n v o l v e d c a n be o b t a i n e d w h i c h w o u l d not n o r m a l l y be a v a i l a b l e . 3 +

3 +

3+

3 +

6

6

3

3 +

3 +

3

+

3 +

3 +

3 +

3 +

6

3

3 +

6

3 +

6

6

5

6

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3

6.

DAVIS

AND BELFORD

Multisite

Interactions

65

Acknowledgements T h i s r e s e a r c h was s u p p o r t e d by t h e P e t r o l e u m R e s e a r c h t h e N a t i o n a l S c i e n c e F o u n d a t i o n , and t h e A d v a n c e d R e s e a r c h j e c t s Agency.

Fund, Pro­

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

D i n g l e , R., J . Chem. Phys. ( 1 9 6 9 ) 50, 545. L i n e k , Α., Siskova, J., a n d J e n s o v s k y , L., Acta Crystallogr. (1969) A25, 155S. D a v i s , P. H. a n d Wood, J . S., I n o r g . Chem. ( 1 9 7 0 ) 9, 1111. Figgis, Β. N., W a d l e y , L. G. Β., and Graham, J., Acta Crystallogr. ( 1 9 7 2 ) B28, 187. Figgis, Β. N. a n d 25, 2233. D a v i s , P. Η., Ph.D. Thesis, University of Illinois (1972), pp. 7-18. Figgis, Β. N. a n d W a d l e y , L. G. Β., J . Chem. Soc. Dalton (1973), 2182. S a n t a n g e l o , Μ., Atti a c a d . sci. lettere ed arti P a l e r m o , P a r t I ( 1 9 5 7 - 5 8 ) 1 8 , 123. Belford, R. L., D a v i s , P. Η., Belford, G. G., and L e n h a r d t , Τ. Μ., this volume.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

7 Polymeric, Mixed-Valence Transition Metal Compounds GILBERT

M.

THOMAS

J.

BROWN, MEYER,

ROBERT

and

W.

TOM

CALLAHAN,

RAY

EUGENE

C.

JOHNSON,

WEAVER

U n i v e r s i t y o f N o r t h C a r o l i n a , C h a p e l Hill, N . C . 27514

In our work we hav i n which metal atoms or ions are held i n c l o s e proximity by chemical l i n k a g e s . One o f our goals has been to e s t a b l i s h the chemical and p h y s i c a l p r o p e r t i e s which a r i s e from metal-metal i n t e r a c t i o n s i n such systems and to modify the p r o p e r t i e s i n a c o n t r o l l e d way by d i r e c t e d chemical s y n t h e s i s . Three d i f f e r e n t kinds o f systems have been studied which d i f f e r with regard to the nature o f the metal-metal i n t e r a c t i o n : 1. Compounds i n which there i s s t r o n g , d i r e c t metal-metal bonding. 2. Cases where there are strong metal-metal i n t e r a c t i o n s through b r i d g i n g ligands. 3. Cases where there are weak metal-metal i n t e r a c t i o n s through bridging l i g a n d s . In compounds c o n t a i n i n g strong metal-metal bonds, the e f f e c t o f the metal-metal bond(s) i s t o modify s t r o n g l y the chemical and e l e c t r o n i c p r o p e r t i e s o f the compounds when compared to the component monomers.(1) One o f the i n t r i g u i n g p r o p e r t i e s of such systems i s that if c e r t a i n bonding and/or s t r u c t u r a l features are p r e s e n t , they can e x i s t i n a v a r i e t y o f molecular oxidation states, e.g., [(π-C H )Fe(CO)] (2), {[(π-C H ) 5

Fe(CO)] (Ph P(CH ) PPh )} / / 2

2

2 3

[(π-C H )Fe(CO)]+4 5

5

2

2+

and

+

2+/+/0/4

5

5

5

(Ph is phenyl) (3). Ions l i k e

0

{[(π-C H )Fe(C0)] (Ph P(CH ) PPh )} 5

5

2

2

2 3

2

+

are

formally mixed-valence c a s e s , but evidence i s now being obtained which i n d i c a t e s that the metal atoms are s t r o n g l y coupled and that o x i d a t i o n - r e d u c t i o n processes involve d e l o c a l i z e d molecular orbitals. S t r o n g , C h e m i c a l l y - S i g n i f i c a n t I n t e r a c t i o n s Through a_ Bridging Ligand. μ-οχο-Bridged Complexes o f Ruthenium ( I I I ) . We have prepared several μ-οχο-bridged complexes o f ruthenium (III): [(AA)XRu-0-RuX(AA) ] *(AA i s 2 , 2 » - b i p y r i d i n e (bipy) o r 2

1,10-phenanthroline

2

(phen); X i s CI o r N 0 ) . The complexes 9

66

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

7.

BROWX

Polymeric

E TAL.

67

Compounds

have been c h a r a c t e r i z e d by elemental a n a l y s i s (as hexaflurophosphate s a l t s ) , s o l u t i o n c o n d u c t i v i t y measurements, e l e c t r o ­ chemical measurements, and spectrophotometric t i t r a t i o n s using Cr as reductant. Although x - r a y c r y s t a l l o g r a p h i c data i s not y e t a v a i l a b l e , i n the complexes the X groups are almost c e r t a i n l y c i s to the b r i d g i n g oxide ion(4·) and molecular models i n d i c a t e that d i r e c t , through-space Ru-Ru i n t e r a c t i o n s are probably not p o s s i b l e . The p r o p e r t i e s o f the μ-οχο-bridged dimers are unusual when compared to c l o s e l y r e l a t e d b i s ( 2 , 2 ' - b i p y r i d i n e ) complexes of ruthenium (II) and ruthenium ( I I I ) . From electrochemical studies i n a c e t o n i t r i l e , the system L(bipy) ClRu-0-RuCl(bipy) ] 2 +

2

2

+

2

a l s o e x i s t s as mixed-valence +3 (Ru(III)-Ru(IV)) and +1 ( R u ( I I ) Ru(111)) i o n s . The +1 i o n i s chemically unstable on time s c a l e s longer than th The e l e c t r o n i c spectr include h i g h l y intense bands i n the v i s i b l e : for [ ( b i p y ) 2

ClRu-0-RuCl(bipy) ] ,

x 668nm(e 17,000); f o r [ ( b i p y ) C ! R u

V

0.22 0.44

+

2 +

0.82

The EL. values are half-wave p o t e n t i a l s v s . the saturated sodiurfi c h l o r i d e calomel e l e c t r o d e i n 1:1 v/v dichloromethanea c e t o n i t r i l e a t 25+2°C. S o l u t i o n s c o n t a i n i n g mixed-valence ions such as ( F c - F c - F c ) and (Fc-Fc-Fc)2+ can be prepared by c o n t r o l l e d p o t e n t i a l electrolysis. For the mixed-valence biferrocenium i o n (Fc-Fc) , i t has been concluded that d i s c r e t e F e ( I I ) and F e ( I I I ) s i t e s e x i s t and that e l e c t r o n i c d e r e a l i z a t i o n between the two s i t e s i s small.(lO)Because o propertiesHBetween the biferrocenium i o n and the mixed-valence polyferrocene i o n s , i t appears t h a t the mixed-valence p o l y ferrocene ions a l s o contain weakly i n t e r a c t i n g but d i s c r e t e Fe(II) ana! F e ( I I I ) s i t e s . For the 1 , 1 ' - p o l y f e r r o c e n e compounds there are c h e m i c a l l y d i f f e r e n t s i t e s ( F c - and - F c - ) i n the polymeric c h a i n s . Upon o x i d a t i o n to the mixed-valence ions more than one o x i d a t i o n s t a t e isomer can e x i s t . The o x i d a t i o n s t a t e isomers d i f f e r with regard to the s i t e ( s ) o f o x i d a t i o n . For example, f o r the ion ( F c - F c - F c ) there are two e n e r g e t i c a l l y equivalent isomers — F c - F c - F c and F c - F c - F c — and one e n e r g e t i c a l l y nonequivalent isomer - - F c - F c - F c . From the e f f e c t of the f e r r o c e n y l group as a s u b s t i t u e n t , i t can be estimated that the isomers F c * - F c - F c and F c - F c - F c are of s i m i l a r energy. However, f o r other mixed-valence ions the d i f f e r e n c e i n f r e e energy between isomers can be s i g n i f i c a n t . From E% data and the f e r r o c e n y l s u b s t i t u e n t e f f e c t i t i s p o s s i b l e to estimate that f o r the r e a c t i o n +

+

+

+

+

+

+

Fc -Fc-Fc +

+

>

Fc -Fc -Fc +

+

AG * 2.8 kcal/mole. The p r o p e r t i e s of Intervalence T r a n s f e r (IT) bands are influenced by o x i d a t i o n s t a t e isomerism. IT bands f o r several of the mixed-valence ions are given i n Table I I . For some o f the ions the IT x i s s h i f t e d s i g n i f i c a n t l y from Xm^ f o r Fc -Fc. The band s h i f t s are expected when i t i s r e a l i z e d that the products o f l i g h t - i n d u c e d e l e c t r o n t r a n s f e r are not symmetric f o r the mixed-valence polyferrocene i o n s . In some cases the products of l i g h t - i n d u c e d e l e c t r o n t r a n s f e r are e n e r g e t i c a l l y unfavorable o x i d a t i o n s t a t e isomers, f o r example, m a x

x

+

F c

+

-Fc-Fc

+

—~—>

(Fcî F c - F c ) * +

As described by Hush(7)the IT t r a n s i t i o n energy i n such a case

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

a

I n 1:1

+

2

2

hv

hv

hv

hv

->

->

3

Transition

+

+

+

(Fc -Fc -Fc-Fc)*

+

(Fc -Fc -Fc)*

+

(Fc -Fc-Fc)*

+

(Fc-Fc )*

4

( v / v ) C H C l - C H C N , 0.1M i n (n-Bu) NPFg.

+

Fc -Fc

Fc-Fc

+

Fc -Fc

Fc -Fc

+

II

1790

1990

1900

nm

5590

max

1670

x

5990

5020

5260

kK

1720

1080

1560

760

ε

Intervalence T r a n s f e r Bands f o r Mixed-Valence 1 , 1 ' - P o l y f e r r o c e n e

Table Ions

7.

BROW

x

Polymeric

E T AL.

75

Compounds

w i l l i n c l u d e the d i f f e r e n c e i n ground s t a t e energies between the isomers F c - F c - F c and F c - F c - F c i n a d d i t i o n to the usual Franck-Condon energy b a r r i e r . An equation has been derived by Hush which r e l a t e s the energy of IT absorption to the rate of i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r i n mixed-valence compounds.(7jThe equation allows estimates to be made for processes l i i c e : +

Fc -Fc +

+

>Fc-Fc

+

+

k * 3x10

+

Fc -Fc-Fc—>Fc-Fc -Fc+ +

k * IxlO

+

10

1 0

sec"

1

sec"

1

The rate of e l e c t r o n hopping between f e r r o c e n y l and f e r r i cenium groups i s slower i n F c - F c - F c than i n F c - F c , mainly because o f the e n e r g e t i c a l l y d i f f e r e n t chemical s i t e s i n Fc -Fc-Fc . The r a t e d i f f e r e n c e s involved are s m a l l , but s i g n i f i c a n t , since the complexes, rates of i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r can be v a r i e d s y s t e m a t i c a l l y by changing the chemical environments of the c o n s t i t u e n t i o n s . +

+

+

+

+

Acknowledgements. Acknowledgements are made to the M a t e r i a l s Research the U n i v e r s i t y of North C a r o l i n a under grant number with DARPA and to the Army Research O f f i c e - Durham grant number DA-AR0-D-31-124-73-G104 f o r support of research.

Center of GH33632 under this

Literature Cited 1. 2. 3. 4. 5.

6. 7. 8. 9. 10.

Meyer, T . J., P r o g r . I n o r g . Chem., i n p r e s s . Ferguson, J. A. and Meyer, T . J., J. Amer. Chem. Soc., (1972), 94, 3409. Ferguson, J. Α . , and Meyer, T . J., Chem. Commun., (1971), 1544. Godwin, J. B. and Meyer, T . J., Inorg. Chem., (1971). 10, 471. The magnetic data was obtained i n c o l l a b o r a t i o n with Professor W. E . H a t f i e l d and h i s research group at the U n i v e r s i t y of North C a r o l i n a . Bleaney, B. and Bowers, K . , Proc. Royal Soc. (London), (1952), 214A, 451. Hush, N. S., Progr. Inorg. Chem., (1967), 8, 391. C r e u t z , C. and Taube, H . , J. Amer. Chem. Soc., (1973), 95, 1006. Adeyemi, S. Α . ; Johnson, E. C.; Miller, F. J.; and Meyer, T . J.; Inorg. Chem. (1973), 12, 2371. Cowan, D. O.; Le Vanda, C.; Park, J.; and Kaufman, F.; Accounts Chem. R e s . , (1973), 6, 1.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8 Exchange Interactions i n Nickel(II), Copper(II), and Cobalt(II) Dimers Bridged by Small Anions DAVID N. HENDRICKSON and D. MICHAEL DUGGAN University of Illinois, Urbana, Ill. 61801

Introduction In this paper some very recent work on "magnetic" exchange interactions in certain nickel(II) and copper(II) dimers will be summarized and some init­ ial findings on analogous cobalt(II) and manganese(II) compounds will be presented. The compounds to be discussed include the following: [M (tren) X ] (BPh )2 where, 2

2

2

4

M = Ni (II), Cu(II), Co(II), Mn(II) X = Ν ¯, OCN¯, SCN¯, SeCN¯, CN¯(Cu only) and, tren = 2,2,2"-triaminotriethylamine 3

One objective at the outset of this work was to keep the non-bridging ligand tren and the counterion tetraphenylborate constant while observing the effects on the exchange interaction of changing either the metal or the bridging ligand. The anticipation of obtaining similar dimer structures upon inter­ changing metals was not to be realized; however, interesting variations developed. It will be noticed that extended bridging groups have been selected to eliminate the consideration of direct metal-metal exchange interactions. In addition, the tetraphenyl­ borate counterion should provide some degree of shielding between dimer units. The phenomenon of magnetic exchange i s , of course, electronic in origin. A particular distinction is made in our work between the parameterization of the observed phenomenon (as per some effective spin Hamiltonian) and the interpretation of the effective 76 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8.

HENDRICKSON

AND

DUGGAN

Ni,

Cu, Co

77

Dimers

s p i n H a m i l t o n i a n p a r a m e t e r s f r o m t h e e l e c t r o n i c wavef u n c t i o n s of the system i n q u e s t i o n . Tnis l a t t e r pur­ s u i t i s the most i m p o r t a n t ; a t p r e s e n t i t i s o n l y p o s ­ s i b l e to provide q u a l i t a t i v e or pernaps semi-quantita­ t i v e i n t e r p r e t a t i o n s of observed trends i n parameters. nickel(II)Dimers T h e f o u r n i c k e l s y s t e m s a r e d i m e r i c w i t h two X~ a n i o n s e n d - t o - e n d b r i d g i n g s u c h t n a t t h e n i c k e l atoms are o c t a h e d r a l l y coordinated. Initially this was d e d u c e d f r o m c a r e f u l i n f r a r e d and e l e c t r o n i c absorp­ t i o n s p e c t r o s c o p y and X - r a y powder p a t t e r n work. For example, t h e powder p a t t e r n s o f [ N i - ( t r e n ) - N C O ) Λ (BPh ) and [Ni (tren) s i m i l a r and s i n c e t h e s t a b l i s h e d (1) t o b r i d g e i n a b i s - b i d e n t a t e f a s h i o n b e t w e e n two m e t a l c e n t e r s , t h e e n d - t o - e n d d i - b r i d g i n g n a t u r e o f t h e a z i d e c o m p o u n d i s i n d i c a t e d . A s we ex­ amined tue magnetic p r o p e r t i e s of t h i s s e r i e s o f n i c k e l d i m e r s we b e c a m e c o n c e r n e d w i t n t h e d e t a i l s o f the molecular s t r u c t u r e . The s i n g l e - c r y s t a l X - r a y s t r u c t u r e o f [Ni (tren) (NCOkl ( B P h J was d e t e r m i n e d (2) . Discrete cationic [Ni Ttren) NCO) J a n d a n i o n i c BPh " u n i t s a r e found, w h e r e t h e m e t a l s a r e b r i d g e d b y two c y a n a t e g r o u p s b o n d i n g i n a n e n d - t o - e n d mode. The structural c h a r a c t e r i s t i c s o f t h e c a t i o n a r e d e p i c t e d i n F i g u r e 1. A s a s i d e - p r o d u c t o f o u r e x c h a n g e w o r k we h a v e t h u s e s t a b l i s h e d t h e f i r s t c a s e o f an a u t h e n t i c a t e d o x y g e n bonded cyanate metal complex. E a c h n i c k e l atom i n the c a t i o n i s s i x c o o r d i n a t e with t r e n occupying four s i t e s (Ni-N=2.047(7), 2 . 0 5 4 ( 5 ) , 2 . 0 9 5 ( 7 ) , and 2.130(7) A) a n d t h e o t h e r two s i t e s o f t h e d i s t o r t e d octahedron a r e o c c u p i e d by t h e Ν and 0 atoms o f t h e b r i d g i n g c y a n a t e i o n s (Ni-N-2.018(7) and N i - 0 = 2 . 3 3 6 ( 5 ) A ) . The two c y a n a t e b r i d g e s a r e e s s e n t i a l l y c o - p l a n a r w h i l e t h e n i c k e l a t o m s l i e a b o v e a n d b e l o w t h e p l a n e b y ± 0.25A. I n r e s p e c t t o t h e e x c h a n g e work t h e r e a r e t h r e g germane points. F i r s t , the N i - N i d i s t a n c e of 5.385(1)A p r e ­ c l u d e s a d i r e c t N i - N i exchange i n t e r a c t i o n . Second, p a c k i n g d i a g r a m s show t h a t t h e l a r g e BPh-"" a n i o n s p r o ­ v i d e e f f e c t i v e s n i e l d i n g between d i m e r i c c a t i o n s . T h i r d , and m o s t i m p o r t a n t t o o u r d i s c u s s i o n , i t c a n be s e e n t h a t e a c h NCO i n the c a t i o n i s e s s e n t i a l l y b o n d i n g end-on a t t h e n i t r o g e n end ( $ N i N C = 1 5 5 . 0 ° ) and a l m o s t a t r i g h t a n g l e s a t the oxygen end ( £NiOC 117.1°). 4

2

2

2

2

2

2

2

2

β

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

78

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

lé

1

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In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8.

HENDRICKSON

AND

Ni,

DUGGAN

Cu,

Co

79

Dimers

B e c a u s e t h e r e was a v a i l a b l e a s t r u c t u r e o f a r e ­ lated end-to-end bridged di-p-thiocyanate n i c k e l dimer, i . e . , [ N i ( e n ) - ( N C S ) ~ ] I ~ (_3) t a n d o u r w o r k (4) on [Ni (trenf2(WCS) ](BPh-f had c o n v i n c e d us of their s i m i l a r i t y of bridge s t r u c t u r e , i n our o p i n i o n i t only remained to determine the s t r u c t u r e of [Ni (tren) ( W ^ ) 1 ( B P h - ) , w h i c h was s o l v e d v e r y r e c e n t l y by P i e r p o n t e t a l (_5) . Tn§ b a s i c s t r u c t u r a l f e a t u r e s o f tne [ N i ( t r e n ) ( N ) J c a t i o n are i l l u s t r a t e d i n F i g u r e z. The two e n d - t o - e n d b r i d g e d a z i d e g r o u p s a r e p a r a l l e l and t h e n i c k e l atoms a r e l o c a t e d ± 0.52A f r o m tiie a z i d e plane. Each a z i d e b r i d g e i s somewhat asym­ m e t r i c w i t h N i - N - N a n g l e s o f 1^5.3(7)° a n d lg3.3(6)° and Ni-N d i s t a n c e s o f 2.069(8)A and 2.195(7)A. The c o o r d i n a t i o n geometr mately octahedral We h a v e m e a s u r e d t h e m a g n e t i c s u s c e p t i b i l i t y o f t n e f o u r n i c k e l d i m e r s i n t h e r a n g e o f 4.2-283°K. In eacn case tne (molar) paramagnetic s u s c e p t i b i l i t y data were l e a s t - s q u a r e s f i t t o an e q u a t i o n s e t o u t by G i n s b e r g e t a l (β) : 2

2

2

2

2

2

2

2

2

2

3

2

ν χ

= (2NgV/3k) ίψζ^ΓΤΎΙ

Α

+

l-Îz'j'F*

1

+

N

a

In Not as pli fie ter

t n i s e q u a t i o n , Ν , β, a n d k n a v e t n e i r u s u a l m e a n i n g , i s tne temperature-independent paramagnetism (taken -200x10 c g s / m o l o f a i m e r ) , and a n d F" a r e com­ c a t e d functions of temperature, s i n g l e - i o n zerol d s p l i t t i n g D, a n d t n e i n t r a d i m e r e x c n a n g e p a r a m e ­ J. Tne e f f e c t i v e i n t e r d i m e r e x c n a n g e i s Ζ'J'. A s we r e p o r t e d e a r l i e r ( 7 ) , t h e c o m p o u n d [ N i (tren) (N^) J(BPn^) shows a r e l a t i v e l y s t r o n g antiferromagnetic exchange i n t e r a c t i o n w i t n a s u s c e p t i b i l i t y maximum a t ^10u°K. I n F i g u r e 3 we n a v e r e p r o d u c e d t n e s u s c e p t i o i l i t y ana e f f e c t i v e m o m e n t c u r v e s for [i>ii ( t r e n ) N C O ) J (BPn^) . In t n i s c a s e tae s u s c e p t i D i l i t y i n c r e a s e s with d e c r e a s i n g temperature u n t i l a maximum i s r e a c n e d a t 14°K, waereu^on χ decreases r a p i d l y to 4.2°K. Tne r a p i d decrease i s i n d i c a t i v e of a sample wnich i s r e l a t i v e l y f r e e of paramagnetic impurities. Tne s o l i d l i n e s i n F i g a r o 3 are leastsquares t n e o r e t i c a l l i n e s , f i t to tne auove equation. T h u s , q u a l i t a t i v e l y we s e e t n a t , a l t h o u g h t h e cyanate and a z i d e n i c k e l d i m e r s t r u c t u r e s a r e g r o s s l y s i m i l a r , t n e r e i s an a p p r e c i a b l e a t t e n u a t i o n i n a n t i f e r r o m a g n e t i c e x c n a n g e i n t e r a c t i o n i n g o i n g f r o m tne azide t o tne c y a n a t e c o m p o u n d . 2

2

2

2

2

2

2

2

An e v e n more s t r i k i n g n i c k e l b r i d g i n g groups are

cnange o c c u r s wnen tne c n a n g e d t o e i t h e r tne

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

80

EXTENDED

Figure 2. geometrical

INTERACTIONS

ORTEP parameters; and carbon and hydrogen

RETWEEN

atoms are not shown.

Figure 3. Experimental and calculated magnetic susceptibility data for [Ni (tren).(NCO) ](BPh, )j. Lines are least-squares fit theoretical lines; for fitting parameters, see text. :

J

l

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

METAL

IONS

8.

HENDRICKSON

AND

Cu, Co

Ni,

DUGGAN

Dimers

81

t n i o c y a n a t e (NCS ) o r s e l e n o c y a n a t e (NCSe ) g r o u p s . T u e e f f e c t i v e moment c u r v e s f o r t n e n i c K e l t n i o c y a n a t e a n d s e l e n o c y a n a t e d i m e r s a r e snown i n F i g u r e 4. liotn compounds Have f e r r o m a g n e t i c i n t e r a c t i o n s o f a p p r o x i ­ m a t e l y t i i e same m a g n i t u d e . For comparison purposes, t h e e f f e c t i v e moment c u r v e o f o n e a n a l o g o u s , s t r u c t u r ­ a l l y cnaracterized(£) , c i s - d i s u b s t i t u t e d t r e n n i c k e l c o m p l e x , i . e . , N i ( t r e n ) ( N C S ) , i s a l s o snown. In t h i s l a s t c a s e t h e r e i s a v e r y s m a l l d e c r e a s e i n t h e moment a t t h e l o w e s t t e m p e r a t u r e s and t n i s i s u n d o u o t e d l y due to s i n g l e - i o n z e r o - f i e l d s p l i t t i n g and p e r h a p s a l i t t l e intermolecular interaction. P a r a m e t e r i z a t i o n of the s u s c e p t i b i l i t y data f o r the four n i c k e l dimers g i v e s the f o l l o w i n g v a l u e s : 2

Compound NCO NCS*" Ncse"

J , cm" -35 -4.4 +2.4 +1.6

1

0.71 -0.16 -0.07 -0.02

-10.1 -0.45 -0.49 -0.76

2.282 2.255 3.347 2.181

I t i s i m p o r t a n t t o m e n t i o n t n a t G i n s b e r g e t a l (β) re­ p o r t e d a f e r r o m a g n e t i c exchange f o r [ N i ( e n ) ( N C S ) Λ I w h e r e t h e y f o u n d J=+4.5 cm" . W i t h t h e m a g n e t i c and s t r u c t u r a l d a t a i n hand i t i s now a p p r o p r i a t e t o a t t e m p t t o e x p l a i n t h e c h a n g e i n s i g n and magnitude o f the exchange p a r a m e t e r f o r t h e s e four n i c k e l dimers. I n o u r o p i n i o n i t i s t h e symmetry of the b r i d g i n g u n i t s t h a t i s the most i m p o r t a n t factor i n d e t e r m i n i n g t h e s i g n and t o a c e r t a i n d e g r e e tne magnitude o f the exchange parameter J . Assistance i n understanding t h e f o l l o w i n g e x p l a n a t i o n c a n be nad by r e f e r r i n g t o F i g u r e 5 w h e r e somewhat i d e a l i z e d (i.e., tne a z i d e system i s r e p r e s e n t e d as symmetric, b e c a u s e t h e f i g u r e was p r e p a r e d b e f o r e t h e s t r u c t u r a l d e t a i l s w e r e known) p l a n e p r o j e c t i o n s o f t h r e e o f t h e n i c k e l dimer b r i d g i n g u n i t s are given. The g e o m e t r y r e p r e ­ s e n t e d f o r the t h i o c y a n a t e c a s e i s t n a t drawn from the s t r u c t u r e f o r [ N i ( e n ) ( N C S ) ] I ( 3 ) , w h i c h we a r e c o n ­ v i n c e d a c c u r a t e l y approximates to t h a t p r e s e n t i n our compound. In the c a s e of the most symmetric system, the a z i d e dimer, the metals i o n s are bonding i n t o the same b r i d g e m o l e c u l a r o r b i t a l s . This leads to a p a i r ­ ing of e l e c t r o n s , that i s a net antiferromagnetic ex­ change. Changing the b r i d g e from a z i d e t o cyanate g i v e s a b r i d g i n g u n i t w h i c h i s no l o n g e r s y m m e t r i c and now t h e two n i c k e l a t o m s b o n d i n t o b r i d g e o r b i t a l s t h a t are not of n e c e s s i t y of equal c o n s t r u c t i o n with r e s p e c t t o t h e two m e t a l c e n t e r s . As a r e s u l t t h e r e 2

2

4

2

4

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

82

EXTENDED

INTERACTIONS

BETWEEN

METAL

4.00

3.50 > ( t r e n ) 6eCN} ](HR^ ^ 2

2

9

2 Niitrer)6CND

3.00

2

3.50 i>4 (tren) (SCN) ](BPh 5 2

2

2

4

2

3.00

300

100 200 TEMPERATURE (°K)

Figure 4. Experimental and calculated magnetic susceptibility data for [Ni (tren),(NCSe),](BPh,,)2 (top, left), Ni(tren)(NCS) (top, right), and [Ni^tren^NCS^fBPh^ (bottom). The lines are least-squares fit theoretical lines. 2

2

Nil

Nil

Ν

Ν

Figure 5. Diagramatic structures of di^-azide,

Ό

representation of di-fi-cyanate, and systems

the bridging di-fi-thiocyanate

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

8.

HENDRICKSON

AND

DUGGAN

N l , Cu,

Co

Dimers

83

a r e b o t h a n t i f e r r o m a g n e t i c and f e r r o m a g n e t i c pathways p o s s i b l e i n the d i - y - c y a n a t e system. The n e t e x c h a n g e i s weakly a n t i f e r r o m a g n e t i c . I t must be r e a l i z e d t h a t i t i s not p o s s i b l e to p r e d i c t from the X-ray s t r u c t u r e that [Ni (tren)^(NCO) ](BPh ) w o u l d be w e a k l y a n t i ­ f e r r o m a g n e t i c , however, i t appears p o s s i b l e t o r a t i o n ­ a l i z e t h e e x c h a n g e i n t h i s compound r e l a t i v e t o t h a t i n t h e a z i d e compound. I f we c o n t i n u e t h e a n a l y s i s , t n e n , i n t h e c a s e o f t n e t h i o c y a n a t e - and s e l e n o c y a n a t e - b r i d g e d d i m e r s t h e t r a n s i t i o n f r o m s t r o n g symmetry t o s t r o n g a n t i s y m m e t r y has r e a c n e d the p o i n t wnere the n i c k e l - b r i d g e b o n d i n g a t the s u l f u r o r s e l e n i u m atom i s c l o s e l y approacning 90°. B e c a u s e t h e b o n d i n g a t t h e n i t r o g e n atom i s s t i l l c l o s e to 180° and t h e n e t i n t e r a c t i o netic, vie d o n o t r e a l l y Know t h e d e t a i l s o f t n e geom­ e t r y o f t n e s e l e n o c y a n a t e o r i d g i n g u n i e , JJUC i t i s r e a s o n a b l e t o assume t n a t tne geometry i s c l o s e s t t o t n a t for tne t n i o c / a n a t e case. B e f o r e a c c e p t i n g t n i s a n g u l a r i t y o r syinmetry ex­ p l a n a t i o n of tne observed exchange e f f e c t s , t h e r e are t h r e e o t h e r c o n s i d e r a t i o n s w n i c n must be d i s c u s s e d : ' 1. Is the exchange propagated through a σ-bonding o r ττ-jjonding p a t h w a y ? , 2. D o e s t n e n o n - p l a n a r i t y o f t n e b r i d g i n g s y s t e m a f f e c t t h e symmetry a r g u m e n t s ? and, 3. vVnat r o l e m i g h t t n e isii-X (X=0 ,i\f ,S) d i s t a n c e p l a y ? Tne t n i r d q u e s t i o n i n e f f e c t a m o u n t s t o w n e t n e r o r not tne Ni-X o v e r l a p i n t e g r a l s v a r y g r e a t l y f o r tne d i s t a n c e s o f t\ii-u=2.34, i i i - a = 2 . o l , a n a n"i-W=2.02A. Tnese d i s t a n c e s are a r e s u l t of m i n i m i z a t i o n o f the energy of eacn system witn r e s p e c t t o o v e r l a p , o r o i t a l e n e r g y , and n u c l e a r r e p u l s i o n , anu as s u c h tne d i s t a n c e s o b t a i n e d a r e a t l e a s t p a r t i a l l y a d j u s t e d on t h e b a s i s of o v e r l a p . T h e o v e r l a p i n t e g r a l s c e r t a i n l y do n o t v a r y a s mucn a s t n e d i s t a n c e s w o u l d t e n d t o i n d i c a t e . I t c a n n o t be d e n i e d t h a t , i n c o m p a r i s o n t o t n e tfi-N Dond i n t n e a z i d e , t n e l o n g e r N i - 0 b o n d i n t n e cyanate may c o n t r i b u t e t o a l e s s e n i n g o f t h e e x c n a n g e i n t e r a c ­ tion. C e r t a i n l y t n e c n a n g e o f s i g n o f J down t n e s e r i e s i s not a r e f l e c t i o n of d i f f e r e n c e s i n Ni-X dis­ tances . Tne e f f e c t o f d i f f e r e n c e s i n p l a n a r i t y , as guaged b y t h e d i h e d r a l a n g l e φ b e t w e e n t h e iNîi-ΧΥΖ a n d X Y Z - N i ' p l a n e s , i s d i f f i c u l t t o judge. For tne t h i o c y a n a t e and c y a n a t e s y s t e m s φ i s c l o s e t o 0 ° , w h e r e a s , f o r t h e azide ψ i s 38.4°. T h e s e c n a n g e s seem t o p a r a l l e l a n expected i n c r e a s e i n " a l l e n e - l i k e " c h a r a c t e r o f the bridge unit. In r e f e r e n c e to p o s s i b l e σ-overlaps o f tne m e t a l d 2 _ 2 o r b i t a l s w i t n π-symmetry b r i d g e 2

2

v

4

2

v

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

84

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

o r b i t a l s , i t seems r e a s o n a b l e t h a t f o r Ο ° < φ < 9 0 ° , m o l e c ­ u l a r o r b i t a l s i n c o r p o r a t i n g i n a continuous bonding way b o t h m e t a l c h a r a c t e r a n d P and Ρ type π-orbitals on the b r i d g e would e x i s t . The 38.4° d i h e d r a l a n g l e of tne a z i d e dimer c o u l d , i n p a r t , l e a d t o a l a r g e r a n t i f e r r o m a g n e t i c exchange. An a n a l y s i s o f t n e d i h e ­ d r a l angle e f f e c t i s presented i n greater d e t a i l f o r two n i c k e l - a z i d e compounds i n a r e c e n t p a p e r (.5) a n d i t i s argued t h a t tne d i h e d r a l angle d i f f e r e n c e s are not i n the p r e s e n t case a major geometric factor. C o n s i d e r a t i o n of tne f i r s t q u e s t i o n r e v o l v e s around whether the excnange i n t e r a c t i o n i s o f a f i r s t o r d e r σ-exchange type o r a h i g n e r - o r d e r "promoted" e l e c t r o n π-exchange. The u n p a i r e d " n i c k e l d - e l e c t r o n s " are i n ά2 2 and d in construction. I m e t a l d - o r b i t a l s and t h e a p p r o p r i a t e s i g m a b r i d g e o r b i t a l s , then p o t e n t i a l l y v i a b l e σ-exchange pathways a r e p r e s e n t , a n d i t i s s u c h a m e c h a n i s m t h a t we b e l i e v e is present. In the p r o m o t i o n π-exchange mechanism one o f t h e u n p a i r e d n i c k e l e l e c t r o n s i s p r o m o t e d ( c o n ­ f i g u r a t i o n i n t e r a c t i o n w i t h an e x c i t e d s t a t e ) i n t o a π-type d - o r b i t a l (eg. , d ) and t h i s a l l o w s an o v e r l a p ( i e . , i n t e r a c t i o n ) v i a tne π-system o f the b r i d g e . The nature of say the ^ "P ( b r i d g e ) i n t e r a c t i o n w o u l d n o t be e x p e c t e d t o change s i g n i f i c a n t l y as a f u n c t i o n o f N i - o r i d g e a n g l e , and c e r t a i n l y n o t such as t o a f f e c t the s i g n o f the exchange i n t e r a c t i o n . Moreover, i n a v a l e n c e bond d e s c r i p t i o n (eg., sp h y b r i d s on t h e Ο and Ν a t o m s o f OCN" and sp h y b r i d i z a t i o n a t t h e c a r b o n ) , tne π-system o f the b r i d g e w o u l d i n v o l v e an orthogon­ a l i t y a t t h e c a r b o n atom. To t h e e x t e n t t h a t t h i s d e s c r i p t i o n i s a c c u r a t e , t h e e x c i t e d s t a t e π-exchange pathway i s i n e f f e c t i v e . To a c c o u n t q u a n t i t a t i v e l y f o r a l l f a c t o r s c o n ­ t r i b u t i n g t o tne observed exchange i n t e r a c t i o n s i s a t t h i s t i m e i m p o s s i b l e i n t h a t t o do s o w o u l d r e q u i r e a tnorougn u n d e r s t a n d i n g of the bonding i n v o l v e d . v

χ

Copper(II)

Dimers

We h a v e f o u n d t h a t t h e c o p p e r compounds p r e p a r e d under i d e n t i c a l c o n d i t i o n s are not i n n e r - s p h e r e bridged dimers. T h e c o p p e r d i m e r s a r e w h a t we shall c a l l "outer-sphere" dimers. The s i n g l e - c r y s t a l X - r a y s t r u c t u r e s o f t h e c y a n a t e and c y a n i d e c o p p e r compounds nave been d e t e r m i n e d . F i g u r e 6 g i v e s an ΟRTEΡ d r a w i n g of tne ççpper-cyanate dimer, i e . , [ C u ( t r e n ) ~ (NCO) ] . The r e m a r k a o l e f e a t u r e o f t h i s d i m e r i s t n a t t n e two h a l v e s o f t n e c y a n a t e d i m e r a r e b r i d g e d 2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8.

HENDRICKSOX

AND

DUGGAN

NU Cu, Co

85

Dimers

s o l e l y b y means o f two Ν - Η · ··ϋ n y d r o g e n - D o n d i n g c o n ­ t a c t s b e t w e e n a c y a n a t e o x y g e n and a t r e n n i t r o g e n on the second copper c e n t e r . The b r i d g i n g h y d r o g e n c o u l d n o t be s e e n , b u t i t s p o s i t i o n c a n b e c a l c u l a t e d t o be ^0.2Â f r o m t h e N-O v e c t o r . Eacn copper atom i n tne cation i s trigonal bi^yramidally coordinated witn tren o c c u p y i n g f o y r s i t e s [Cu-N=2.076(5), 2.090(5), 2.119(5) and 2.083(5)A] and w i t n an a x i a l c a r b o n - b o n d e d c y a n i d e [Cu-CN=1.967(7) a n d C - N - l . 1 2 7 ( 9 ) A ] . The t r i g o n a l b i p y r a m i d s a r e d i s t o r t e d f r o m p e r f e c t t h r e e - f o l d symmetry. F o l l o w i n g tne e l u c i d a t i o n o f the s t r u c t u r a l chara c t e r i s t i c s of these copper outer-spnere dimers, we nave s e t about the t a s k o f c o n s t r u c t i n g an extended s e r i e s o f sucn coppe c o n s i d e r t o be a s o l i o u t e r - s p h e r e e l e c t r o n t r a n s f e r as a s s i s t e d by v a r i o u s anions. P r z y s t a s a n d S u t i n (9) have s t u d i e d s e v e r a l o u t e r - s p h e r e redox r e a c t i o n s as tney a r e i n f l u e n c e d i n s o l u t i o n by added a n i o n s . Perhaps tne n y d r o g e n - b o n d i n g c o n t a c t t h a t i s p r e s e n t i n , f o r example, [ C u ( t r e n ) 2 ( N C O ) 1 ( B P n ) , i s s u f f i c i e n t to est a b l i s h an e l e c t r o n t r a n s f e r between the c o p p e r c e n t e r s . I f t h i s i s t h e c a s e a n d i f we c a n m e a s u r e t h e e x c h a n g e p a r a m e t e r J , t h e n we c o u l d s t u d y _ t h e e f f e c t s o f c h a n g i n g t h e b r i d g e ( i e . , CN~ f o r OCrt") o n t h e e x c h a n g e p a r a m e t e r J and t h e r e f o r e on the o u t e r - s p h e r e e l e c t r o n transfer rate. We h a v e v e r i f i e d t n a t t h i s same h y d r o g e n - b o n d i n g a s s o c i a t i o n i s f o u n d f o r a s e r i e s o f c o p p e r compounds. F i g u r e 7 shows a n ORTEP d r a w i n g o f t h e o u t e r - s p h e r e dimeric cation i n [Cu (tren) (CN) 1(BPn ) (10). A g a i n we h a v e e s s e n t i a l l y t r i g o n a l o i p y r a m i d a T copper c e n t e r s i n an o u t e r - s p h e r e a s s o c i a t i o n . Inspection of tiie d e t a i l s o f p a c k i n g t h e s e o u t e r - s p h e r e dimers w i t h BPh-~ a n i o n s shows t h a t t h e d i m e r s a r e r e a s o n a b l y i s o l a t e d by t h e v e r y l a r g e c o u n t e r i o n s . As m i g h t b e e x p e c t e d , e x c h a n g e i n t e r a c t i o n s a r e r e l a t i v e l y weak f o r these outer-sphere systems. F i g u r e 8 i l l u s t r a t e s the s u s c e p t i b i l i t y a n d e f f e c t i v e moment c u r v e s f o r t h e copper-cyanide dimer. A s c a n b e s e e n t h e r e i s no s i g n o f a n i n t e r a c t i o n u n t i l v e r y low t e m p e r a t u r e s . Howe v e r , i t i s q u i t e c l e a r t h a t t h e r e i s an i n t e r a c t i o n v i a t h e o u t e r - s p h e r e a s s o c i a t i o n i n t h i s compound. F i t t i n g the s u s c e p t i u i l i t y d a t a t o the usual^exchange e q u a t i o n f o r a c o p p e r d i m e r g i v e s J=-1.8 cm Because t h e c y a n a t e , t h i o c y a n a t e and s e l e n o c y a n a t e a n i o n s a r e m o r e e x t e n d e d , i t was a n t i c i p a t e d t h a t J would be s m a l l e r t h a n t n a t f o r t h e c y a n i d e . As s u c h i t b e c a m e c l e a r t o us t h a t we n e e d e d t o u s e e s r t o 2

2

2

4

2

2

2

4

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

86

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Figure 6. Molecufor structure of the "dimer" cation in form I of [Cu/tre^ifNCO)^ (BPh )t after refinement of atomic positions with isotropic thermal parameters. Hydrogen bonding h

Figure

7.

ORTEP

plotting

of [Cu,(tren),(CN) l e t transi­ t i o n s a r e e q u a l l y spaced from the Δ ^ = 1 t r a n s i t i o n , as e x p e c t e d - and t h e i r p o s i t i o n s g i v e a | j | v a l u e o f 0.09 cm"" . When t h e t e m p e r a t u r e o f [ C u ( t r e n ) (NCO) ] ( B P h . ) i s l o w e r e d t o 9 5 ° K , t a e two s i n g l e t - t r i p l e t f e a t u r e s move t o n i g n e r a n d l o w e r f i e l d p o s i t i o n s , r e s p e c t i v e l y , and at,95°K t h e v a l u e o f | j | i s c a l c u ­ l a t e d t o b e 0.16 cm" . vie n a v e , i n f a c t , m e a s u r e d t n e t e m p e r a t u r e d e p e n d e n c e o f J f o r t h i s compound a n d w i l l r e p o r t on t h i s i n a l a t e r paper (4). T h u s , w i t h a c o m b i n a t i o n o f e s r and v a r i a b l e t e m p e r a t u r e m a g n e t i c s u s c e p t i b i l i t y we h a v e d e t e r m i n e d tne J values f o r a s e r i e s of outer-sphere copper dimers: g

3

f

3

3

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

EXTENDED

88

INTERACTIONS

[Cu (tren) (X) J(uPn ) 2

2

2

4

X

METAL

IONS

2

J (cm

SeCN sasT (I) OCiNi ( I I ) QCi\I CN CI Br

BETWEEN

0.05 0.09 0.05 1.9 3.2 3.5

)

0.07 0.17 0.06

F o r t n e a z i d e and s e l e n o c y a n a t as y e t , been a b l e t c a s e s no s i g n s o f e x c h a n g e i n t e r a c t i o n a r e s e e n i n t h e s u s c e p t i b i l i t y t o 4 . 2 ° K n o r a r e t h e r e any weak s i n g l e t t r i p l e t esr transitions. V a r i o u s p h y s i c a l d a t a on t h e SeCN~ compound i n d i c a t e t h a t i t i s i s o s t r u c t u r a l w i t h the c y a n i d e and c y a n a t e . Tne c r y s t a l l o g r a p h y o f t h e c o p p e r a z i d e s y s t e m i s b e i n g p u r s u e d by P r o f . C o r t Pierpont. X - r a y p r e c e s s i o n w o r k o n t h e SCft~, C l ~ , a n d Br"" compounds i n d i c a t e s t n a t t h e y a r e a l s o o u t e r sphere copper dimers. T h e t h i o c y a n a t e compound shows s i n g l e t - t r i p l e t t r a n s i t i o n s i n i t s X-band e s r s p e c t r a , f r o m w h i c h we c a l c u l a t e d |j|=0.05 c m ~ a t 345°K and IJI=0.06 cm a t 95°K. T h e r e a r e two c r y s t a l l i n e f o r m s o f t i i e c o p p e r c y a n a t e compound; we h a v e o n l y d e t e r m i n e d the X-ray s t r u c t u r e o f the form l a b e l l e d I. Both forms exhibit singlet-triplet transitions. I t i s perhaps relevant to p o i n t out that [Ni (tren) (WCO) ](BPh ) , e v e n though i t i s composed o f i n n e r - s p h e r e d i m e r s , a l s o h a s two d i f f e r e n t f o r m s . Apparently, there are two w a y s o f p a c k i n g s u c n d i m e r s w i t h t h e t e t r a p h e n y l borate ion. T h e h a l i d e - b r i d g e d o u t e r - s p h e r e d i m e r s show t h e strongest interactions. The a n t i f e r r o m a g n e t i c exchange i n t e r a c t i o n s a r e l a r g e enough t o e a s i l y d i s c e r n w i t h m a g n e t i c s u s c e p t i b i l i t y t o 4.2°K. However, i t i s i n t e r e s t i n g a n d a t t h e same t i m e p u z z l i n g t h a t t h e b r o m i d e and c h l o r i d e b r i d g e s s u p p o r t a p p r o x i m a t e l y t h e same m a g n i t u d e o f i n t e r a c t i o n i f t h e s e i n t e r a c t i o n s are propagated through hydrogen bonds. The X - r a y c r y s t a l s t r u c t u r e o f t h e c h l o r i d e compound i s p r e s e n t l y b e i n g worked on. Work ( b o t h e x p e r i m e n t a l a n d t h e o r e t i c a l ) c o n t i n u e s on t h e s e c o p p e r o u t e r - s p h e r e s y s t e m s t o understand the r e l a t i o n s h i p between the " o u t e r - s p h e r e " e x c h a n g e s t u d i e s a n d t h e many r e p o r t i n g s o f " a n i o n a s s i s t a n c e " of outer-sphere s o l u t i o n redox processes. x

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4

2

8.

HENDRiCKSON

AND

N i , C u , Co

DUGGAN

89

Dimers

ο




Λ

2J

lo o>

: Λ Figure 9. Idealized energy level scheme and esr spectrum for two S = 1/2 systems interacting to show both zero-field splitting (D) and exchange coupling (J). Allowed transitions (AM = 1) are shown as large derivative feature in the spectrum whereas formally forbidden (ΔΜ = 2 and singlet to triplet state) transitions are shown at an in­ creased spectrometer gain (i.e., η times greater). 8

8

Figure

10.

Temperature dependence of all visible spectrum of [Cu^tren^lSlCO)*]

features in the X-band (BPh,,) , form I 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(9 GHz)

esr

90

EXTENDED

Cobalt(II)

and

Manganese(II)

INTERACTIONS

BETWEEN

METAL

IONS

Dimers

Schlenk tube techniques have been used t o p r e p a r e a s e r i e s o f h i g h - s p i n c o b a l t ( I I ) complexes w i t h the g e n e r a l c o m p o s i t i o n o f [ C o ( t r e n ) (X) «J ( B P h J . The e l e c t r o n i c a b s o r p t i o n and I n f r a r e a s p e c t r a o f t h e s e somewhat a i r - s e n s i t i v e c o b a l t ( I I ) c o m p l e x e s i n d i c a t e t h a t the complexes are f i v e - c o o r d i n a t e , probably tri­ g o n a l b i p y r a m i d a l (TBP). The m a g n e t i c s u s c e p t i b i l i t y c u r v e s i n d i c a t e an a t t e n u a t i o n o f m a g n e t i s m a t l o w temperatures. However, b e c a u s e z e r o - f i e l d i n t e r a c t i o n could be s u b s t a n t i a l i n s u c h a f i v e - c o o r d i n a t e C o ( I I ) s p e c i e s , i t i s n o t p o s s i b l e t o e a s i l y e x t r a c t the ex­ change parameter J from a f i t t i n g o f the d a t a . In agreement w i t h t h i statement i findin t h a t th magnetism f o r f i v e - c o o r d i n a t i n d i c a t e s a low-temperatur O u r a t t a c k on t h e c o b a l t ( I I ) s y s t e m s h a s b e e n one o f f i t t i n g the observed e l e c t r o n i c s p e c t r a t o l i g a n d f i e l d e q u a t i o n s f o r a t r i g o n a l b i p y r a m i d a l complex, f o l l o w e d by a t t e m p t s u s i n g the l i g a n d f i e l d p a r a m e t e r s to f i t the s u s c e p t i b i l i t y d a t a t o the equations f o r a m o n o m e r i c TBP c o b a l t ( I I ) c o m p l e x w i t h a reasonable spin-orbit interaction. P r e l i m i n a r y w o r k shows t h a t t h i s d o e s n o t work as w e l l as f i t t i n g t h e m a g n e t i c d a t a t o an e f f e c t i v e s p i n H a m i l t o n i a n f o r an S = 3 Co(II) dimer, i n c l u d i n g s i n g l e - i o n zero f i e l d s p l i t t i n g . The u l t i m a t e c h e c k w i l l be g a u g e d b y o u r s u c c e s s t o f i t t h e c o m p l i c a t e d l o w - t e m p e r a t u r e ( 4 . 2 - 9 0 ° K ) X- a n d Qb a n d e s r s p e c t r a we h a v e m e a s u r e d f o r t h e s e c o m p o u n d s . F i g u r e 11 shows t h r e e r e a s o n s ( X - b a n d a t ^ 15°K f o r A = Ν ~ , Β = 0CN~ a n d C = SCN~) why we a r e g o i n g t o have c o n s i d e r a b l e fun a n a l y z i n g these systems. Em­ p i r i c a l l y , we h a v e f o u n d an i n t e r e s t i n g t e m p e r a t u r e d e p e n d e n c e i n t h e s e e s r s p e c t r a a s c a n s e e i n F i g u r e 12 w h i c h s h o w s Q - b a n d s p e c t r a a t two t e m p e r a t u r e s f o r [ C o ( t r e n ) (NCO) 1 ( B P h ) . I t i s our a n t i c i p a t i o n a n d h o p e ( p e r h a p s u s i n g some o f t h e m a c h i n e r y s e t o u t i n an e a r l i e r symposium p a p e r by P r o f e s s o r R.L. B e l f o r d ) t h a t these e s r s p e c t r a h o l d the key t o a de­ termination of J . T h e c o b a l t c o m p l e x e s h a v e , a f t e r many a t t e m p t s , b e e n f o u n d t o be u n a v a i l a b l e i n t h e f o r m o f s i n g l e crystals. T h i s i s not the case w i t h the manganese(II) c o m p o u n d s w h e r e we h a v e o b t a i n e d c o l o r l e s s c r y s t a l s o f [ M n ( t r e n ) ( X ) ] (BPh.) , wheire X = NCO" and NCS~. X-ray c r y s t a l s t r u c t u r e work i s i n p r o g r e s s . These M n ( I I ) c o m p o u n d s a l s o show a n a t t e n u a t i o n i n m a g n e t i s m a t v e r y low t e m p e r a t u r e s . Large z e r o - f i e l d s p l i t t i n g s i n t h e e s r s p e c t r a and i n f r a r e d d a t a p o i n t t o the 2

2

2

1

2

2

2

2

2

4

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

HENDRiCKSON

A N D DUGGAN

τ

I 1

Figure 11. compounds,

1

Ni, Cu, Co

Dimers

1

1

Γ

I

I

ι

I

I

I

2

3

4 5 FIELD (kG)

I 6

I 7

8

L 9

X-band (—15°K) esr spectra for three high-spin Co(II) [Co (tren) Xi](B?h ) , where (A) X = N ~, (Β) X = OCN,and(C)X = SCN-. 2

t

h 2

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

92

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Figure 12. Q-band (35 GHz) esr spectra for [Co,(tren),(NCO),](BPh ) at two different tem­ peratures k t

Ο

'

Figure

'

5,000 FIELD (G)

'

13. Room-temperature X-band [Mn,(tren)t(NCO),](BPh0

'

10,000

esr spectrum

of

2

FIELD (G)

Figure

14.

Room-temperature

Q-band

esr spectrum

of

[Mn (tren) (~NCS) ](BPh,,) 2

i

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

t

i

8.

HENDRICKSON

A N D DUGGAN

Ni, Cil, Ci) DîmCfS

93

presence o f outer-sphere dimers. C o m p l i c a t e d and i r r e g u l a r X-band s p e c t r a such as t h a t f o r [Mn (tren) (NCO) ](BPh ) d e p i c t e d i n F i g u r e 13 a r e c l e a r i n d i c a t o r s t h a t we a r e d e a l i n g w i t h d i m e r s . T h e r e a r e a c o n s i d e r a b l e number o f f e a t u r e s i n t h i s X-band spectrum, a p p a r e n t l y encompassing the whole 10,000 G a u s s r a n g e . The Q-band s p e c t r u m o f t h e t h i o c y a n a t e compound i n F i g u r e 14 i s s o m e w h a t s i m i l a r t o the Q-band s p e c t r u m o f t h e c y a n a t e and as s u c h i t shows t h a t t h e r e i s a n a p p r e c i a b l e c h a n g e i n a p p e a r ance upon c h a n g i n g microwave e n e r g i e s . 2

2

2

4

2

Literature Cited 1. 2. 3.

4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14.

Curtis, N.F., M c C o r m i c k J. Chem. Soc. D u g g a n , D.M. a n d Hendrickson, D.N., I n o r g . Chem., in press; J. Chem. Soc., Chem. Commun., 4 1 1 ( 1 9 7 3 ) . Shvelashvili, A.E., Porai-Koshits, M.A., a n d Antsyshkina, A.S., J. Struct. Chem. ( U S S R ) , (1969), 10, 5 5 2 . D u g g a n , D.M. a n d Hendrickson, D.N., Inorg. Chem., submitted for publication. Pierpont, C.G., Hendrickson, D.N., D u g g a n , D.M., Wagner, F., a n d Barefield, E.K., Inorg. Chem., submitted for publication. Ginsberg, A.P. Martin, R.L., Brookes, R.W., a n d S h e r w o o d , R.C., Inorg. Chem., (1972), 1 1 , 2884. D u g g a n , D.M. a n d Hendrickson, D.N., I n o r g . Chem., (1973), 12, 2422. C r a d w i c k , P.D. a n d Hall, D., Acta Cryst., (1970), B26, 1384 . Przystas, T.J. a n d Sutin, Ν., J. Amer. Chem. Soc., (1973), 95, 5545. D u g g a n , D.M. a n d Hendrickson, D.N., Inorg. Chem., in press. J a m e s , P.G. a n d Luckhurst, G.R., M o l . P h y s . , (1970), 18, 1 4 1 . M e r e d i t h , D.J. a n d Gill, J.C., Phys. Lett., (1967), 25A, 429. Birgeneau, R.J., Hutchings, M.T., a n d wolf, W.P., Phys. Rev. Letters, ( 1 9 6 6 ) , 1 7 , 308. Jeter, D.Y. a n d Hatfield, W.E., I n o r g . C h i m . Acta, (1972), 6, 440.

Ac^nowledgjnj^ri^. from

National

We

are grateful

Institutes

f o rp a r t i a l

funding

o f H e a l t h g r a n t HL13652.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

9 Structural and Magnetic Properties of Chromium(III) Dimers DEREK J. HODGSON U n i v e r s i t y of N o r t h C a r o l i n a , C h a p e l Hill, N.C. 27514

Introduction A number of s t r u c t u r a l i n v e s t i g a t i o n s in our l a b o r a t o r y and elsewhere have demonstrated that complexes of stoichiometry [Cu(L)OH]22+, where L is a bidentate l i g a n d , c o n t a i n a dimeric u n i t i n which two copper(II) centers are bridged by two hydroxo groups (1-5), and much of our recent research has been d i r e c t e d towards the c o r r e l a t i o n of the s t r u c t u r a l and magnetic p r o p e r t i e s of dimers of t h i s type (6,7). We have r e c e n t l y extended these s t u d i e s to chromium(III) complexes of the type [Cr(L) OH]2n+, where L i s again a bidentate l i g a n d , and i n t h i s paper I d e s c r i b e the r e s u l t s of our work i n t h i s area. The s t r u c t u r e s with which we are concerned are molecules or ions of the type shown i n f i g u r e 1, i n which we have two chromium ( I I I ) centers which are bridged by two hydroxo groups i n a planar array; the remaining c o o r d i n a t i o n s i t e s of the chromium octahedron are occupied by the bidentate l i g a n d s . For a symmetric bidentate l i g a n d , there are two p o s s i b l e geometries f o r t h i s dimer: if, i n f i g u r e 1, atoms AN(1) and AN(10), BN(1) and BN(10), e t c . form the c h e l a t e r i n g s , the dimer l a c k s an i n v e r s i o n center but has approximately D symmetry, but i f , f o r example, the c h e l a t i o n at C r ( l ) i s changed to AN(1)-BN(10) and BN(1)-AN(10) while that at Cr(2) i s unchanged the dimer has an i n v e r s i o n center and approximates C symmetry. Each of these geometries i s found. I t should be noted, however, that i n both of these geometries we maintain a planar b r i d g i n g u n i t and octahedral c o o r d i n a t i o n at the metal. Sinn (8) and G l i c k (9) have noted the i n f l u e n c e of the geometry at copper on the magnetic i n t e r a c t i o n s i n copper dimers, but here we are able to keep the geometry at the metal center approximately constant. 2

2

2h

The magnetic p r o p e r t i e s of these systems have been examined by my colleague, Professor W.E. H a t f i e l d . The Van Vleck equation f o r exchange coupled C r ( I I I ) ions (S = 3/2,3/2) can be w r i t t e n (10) as

94 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

9.

Chromium(IIl)

HODGSON

2

2

Ng $ kT

Dimers

2 exp(2J/kT) + 10 exy(6J/kT) + 28 exy(12J/kT) 1+3 exp(2J/kT) + 5 exp(6J/kT) + 7 exp(12JA!T)

95

(1)

where J i s the exchange coupling constant and -2J represents the energy d i f f e r e n c e between the s i n g l e t ground s t a t e and the t r i p l e t f i r s t e x c i t e d s t a t e . As a r e s u l t of the presence of a manif o l d of r e l a t i v e l y low-lying paramagnetic e x c i t e d s t a t e s , however, i t has been suggested (11) that the Van Vleck expression should be modified by the i n c l u s i o n of b i q u a d r a t i c exchange.This gives r i s e to the Hamiltonian

H

Q

=

-2J(S S )-é(S V

2

)

(2)

and the expanded expressio m

X

m

NgH

2

χ

kT

(3)

2 exp[(2eT-6.5j)/feff}f 10 exp[ (6J-13.5.1)/kT] + 28 exp[ (12e7-9.7)/kT] 1.0+3 exp[(2J-6.5j)/kT] + 5 exp [ (6J-13.5j) /kT]+7 exp [ (12J-9j)/kT] In t h i s modified form of the Van Vleck equation, the energy se­ p a r a t i o n between the s i n g l e t ground s t a t e and t r i p l e t f i r s t e x c i t e d s t a t e , ΔΕ, i s -2J + 6.5j. Since i t i s our aim to c o r r e l a t e s t r u c t u r a l and magnetic p r o p e r t i e s , t h i s d i s c u s s i o n deals only with complexes whose s t r u c t u r e s have been p r e c i s e l y determined and does not include the l a r g e number of complexes (12-16) f o r which only magnetic data are a v a i l a b l e . G l y c i n a t o Complex The f i r s t s t r u c t u r e which was determined was that of the g l y c i n a t o complex [Cr(gly)2OH]2 > which c r y s t a l l i z e s i n the mono­ c l i n i c space group Ρ2χ/η with two dimers Jn a c e l l of dimensions a = 5.691(3), b= 16.920(9), a - 7.900(4) A, and 3 - 79.90(3)° (17,18). With only two dimers i n the c e l l , i t i s apparent that there must be an i n v e r s i o n center i n the middle of the dimer, and that the molecule must be of the approximately type; an examination of the s t r u c t u r e , which i s shown i n f i g u r e 2, v e r i ­ f i e s t h i s conclusion. The Cr-Cr and 0-0 separations i n the b r i d g i n g u n i t are 2.974(2) and 2.575(6) A, r e s p e c t i v e l y , and the s i m i l a r i t y of the two independent b r i d g i n g Cr-0 bond lengths of 1.966(4) and 1.968(4) A demonstrates that the b r i d g i n g i n t h i s u n i t i s symmetric. The s t r u c t u r a l parameter of greatest i n t e r e s t i s the value of the Cr-0-Cr b r i d g i n g angle, φ, and i n t h i s case i t i s 98.2(2)° (18).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

96

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Figure 1. Coordination about chromium(III) centers in a typical dihydroxo-bridged dimer. O(l) and G(2) are the oxygen atoms of the hydroxo bridges. Chelate rings are formed by joining AN(1) to AN(10), BN(1) to BN(10) etc. Data are for the [Cr(phen) OH], cation in [Cr(phen),OU],l, · 411,0. y

r

4

Inorganic Chemistry

Figure

2.

View

of the [Cr(gh/)^OH] atoms omitted

j molecule (18)

with

hydrogen

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

9.

Chromium(III)

HODGSON

Dimers

97

The low temperature magnetic s u s c e p t i b i l i t y data f o r [Cr( g l y ) O H ] are shown i n f i g u r e 3, i n which the dashed l i n e r e p r e ­ sents the best l e a s t - s q u a r e s f i t to the unmodified form of the Van V l e c k expression (equation (1)) while the s o l i d l i n e represents the best l e a s t - s q u a r e s f i t to equation (3). I t i s evident t h a t , i n t h i s case, the observed s u s c e p t i b i l i t y data are much more r e a d i l y approximated by the s o l i d l i n e , i.e. the i n c l u s i o n of b i q u a d r a t i c exchange i s s i g n i f i c a n t i n t h i s case. The magnetic s u s c e p t i b i l i t y of [ C r ( g l y ) O H ] maximizes near 20°K. The l e a s t - s q u a r e s f i t t i n g process leads to values of 2J = -7.4 cm~l and j = 0.04 cm~l, or ΔΕ = -10.0 cm~l. These values are i n good agreement with the value of 2J p r e d i c t e d by Earnshaw and Lewis (12) on the b a s i s of high temperature s u s c e p t i b i l i t y data. 2

2

2

2

Phenanthroline Complexe The second complex whose s t r u c t u r e was determined was the 1,10-phenanthroline complex [ C r ( p h e n ) 0 H ] C l i * 6 H 0 . T h i s complex c r y s t a l l i z e s i n the t r i c l i n i c space group PI with two dimers i n a c e l l of dimensions a « 14.056(7), b = 11.296(6), ο = 18.990(9) A, α = 87.15(3), 3 = 107.63(2), and γ = 74.68(3)° (19). With two d i ­ mers i n P I , no c r y s t a l l o g r a p h i c symmetry i s imposed on the system, and t h i s s t r u c t u r e i s an e i g h t y atom problem, not counting the hydrogen atoms! The s t r u c t u r e of the c a t i o n i s shown i n f i g u r e 4, and i t i s apparent that t h i s i o n c l o s e l y approximates Z? symmetry; there i s no i n v e r s i o n center, but there are three approximate two­ f o l d axes. I f the dimer attempted to adopt the approximately C h geometry found i n the g l y c i n a t o complex, there would be very se­ vere proton-proton i n t e r a c t i o n s across the dimer, e.g. between the phenanthroline group l a b e l e d G2 and that l a b e l e d G3. Hence, f o r the bulky phenanthroline l i g a n d , only the Z? geometry i s s t e r i cally feasible. The c o o r d i n a t i o n about the chromium(III) atoms i s shown i n f i g u r e 5. The Cr-Cr s e p a r a t i o n of 3.008(3) A i s a l i t t l e l a r g e r than that i n the g l y c i n a t o complex, and t h i s change i s due to an increase of approximately 4.5° i n the value of the b r i d g i n g angle, φ. Thus, i n t h i s phenanthroline complex the average value of φ i s 102.7°, while i n the g l y c i n a t o complex φ i s 98.2° (vide supra). The low temperature magnetic s u s c e p t i b i l i t y of [ C r ( p h e n ) OH] C l i · 6H 0 e x h i b i t s a maximum near 110°K, and the best l e a s t squares f i t to equation (3) gives a value of ΔΕ of approximately -55 cnT^ (20). S i n g l e c r y s t a l epr examinations of t h i s complex are c u r r e n t l y nearing completion. The magnetic data f o r t h i s c h l o r i d e s a l t of the phenanthro­ l i n e complex are of c o n s i d e r a b l e i n t e r e s t s i n c e , on the b a s i s of high temperature measurements, Earnshaw and Lewis (12) have c a l ­ c u l a t e d that the corresponding i o d i d e s a l t has a ΔΕ of a p p r o x i ­ mately -14 cm~l. Hence, i f our contention that the magnetic pro­ p e r t i e s of di-hydroxo-bridged dimers are p r i n c i p a l l y determined by the geometry of the bridge were c o r r e c t , i t appeared that the geo­ metry of the i o d i d e s a l t must be c o n s i d e r a b l y d i f f e r e n t from 2

2

+

2

c

2

2

2

2

2

+

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

98

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

04

Inorganic Chemistry

Figure 3. Temperature variation of the magnetic susceptibility of [Cr(gly) OH] . Dashed line represents the best jit to equation (1); solid line represents the best fit to equation (3) (see text) (18). É

Â

Acta Crystaltographica

Figure 4. View the cation in [Cr(phen) OH] Ch, • 6H 0 (19) 2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

of

9.

Chromium(III)

HODGSON

Dimers

99

that of the c h l o r i d e . Hence, we a l s o examined the s t r u c t u r e of the i o d i d e s a l t , [ C r ( p h e n ) 0 H ] I i f 4 H 0 . The i o d i d e s a l t a l s o c r y s t a l l i z e s i n the t r i c l i n i c space group PI with two dimeric u n i t s i n a c e l l of dimensions a = 11.464(12), b = 9.893(11), ο = 22.757(25) Â, α = 90.06(2), 3 = 93.04(2), and γ = 82.82(2)° (21). The s t r u c t u r e of the c a t i o n , which i s shown i n f i g u r e 6, i s very s i m i l a r to that found i n the c h l o r i d e s a l t , with the bulky phenanthroline l i g a n d s again f o r c ­ ing the dimer to adopt the roughly £> geometry. The inner coor­ d i n a t i o n sphere f o r t h i s dimer i s shown i n f i g u r e 1$ and a com­ p a r i s o n of t h i s f i g u r e with f i g u r e 5 demonstrates that the [Cr(phen) 0H] u n i t s i n these two s a l t s are s t r u c t u r a l l y sub­ s t a n t i a l l y s i m i l a r (21). Thus, f o r examgle, the Cr-Cr s e p a r a t i o n and Cr-O-Cr b r i d g i n g angle of 2.986(4) A and 102.2(2)°, r e s p e c t ­ i v e l y , i n the i o d i d e s a l from the values (19) o analog. T h i s s t r u c t u r a l r e s u l t i s , c l e a r l y j i n c o n s i s t e n t with the magnetic p r o p e r t i e s reported by Earnshaw and Lewis (12), and so we have reexamined the magnetic s u s c e p t i b i l i t y of the i o d i d e s a l t (22). The low temperature s u s c e p t i b i l i t y data are shown i n f i g u r e 7, i n which the s o l i d l i n e represents the best f i t to the Van Vleck equation modified by the i n c l u s i o n of b i q u a d r a t i c exchange. The magnetic s u s c e p t i b i l i t y of [Cr (phen) 0H] Ii «4H 0 i s seen to maxmize near 110°K, and the l e a s t - s q u a r e s f i t to equation (3) y i e l d s 2J = -43.8 cm" , j = +1.5 cm , and ΔΕ - -53.6 cm" (22). These values are very s i m i l a r to those obtained (20) f o r the c h l o r i d e but are s u b s t a n t i a l l y d i f f e r e n t from the value of ΔΕ = -14 cm"" reported by Earnshaw and Lewis (12). Moreover, the sim­ i l a r i t y between these r e s u l t s and those f o r the c h l o r i d e s a l t i s c o n s i s t e n t with the s i m i l a r i t y of the two s t r u c t u r e s noted above. I t i s , however, noteworthy that while the magnetic pro­ p e r t i e s of the corresponding n i t r a t e s a l t , [Cr (phen) 0H] (N03)i* · 7H 0, with values of 2J - -42.2 cm" , J = 0.0 cm" , and ΔΕ -42.2 cm" (23) are s u b s t a n t i a l l y s i m i l a r to those of the c h l o ide and i o d i d e s a l t , the bromide s a l t , [Cr(phen^ OH^ Br^ ·8Η2θ, apparently undergoes a weaker i n t e r a c t i o n with values of 2J = -29.2 cm" , J = 0.5 cm" , and ΔΕ - -32.5 cm" (23). I t would appear, t h e r e f o r e , that the c a t i o n i n the bromide s a l t may i n ­ deed be s t r u c t u r a l l y d i f f e r e n t from that i n the c h l o r i d e and i o d i d e cases, but no s t r u c t u r a l data are a v a i l a b l e to confirm or deny t h i s hypothesis. e

2

2

2

2

i++

2

2

2

1

2

+

2

-1

1

1

2

1

2

1

2

1

1

1

1

Oxalato Complex The f i n a l s t r u c t u r e of t h i s type, which has r e c e n t l y been completed i n our l a b o r a t o r i e s , i s that of the oxalato complex Na [ C r ( 0 X ) 0 H ] * 6 H 0 . T h i s m a t e r i a l c r y s t a l l i z e s i n the mono­ c l i n i c space group P 2 j / c with four dimers i n a c e l l of dimensions a = 19.530(12),b = 9.860(7), ο - 12.657(10) A, and 3 - 106.93(4)° 2

2

2

Q

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

100

EXTENDED

Figure

6.

View

of the cation

INTERACTIONS

in [Cr(phen),OII]J,,

BETWEEN

METAL

4HjO

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

9.

HODGSON

Chromium(III)

101

Dimers

While no c r y s t a l l o g r a p h i c symmetry i s r e q u i r e d f o r four dimers i n t h i s c e l l , i t t r a n s p i r e s that each dimer s i t s on a c r y s t a l l o g r a p ­ h i c i n v e r s i o n center so that there are, i n e f f e c t , two separate independent " h a l f - d i m e r s " i n the c e l l r a t h e r than one independent dimer. Hence, of course, the geometry of the anion must be of the ^2h y P r a t h e r than the D type. The s t r u c t u r e of the anion i s shown i n f i g u r e 8, the bond lengths and angles given being the average of the values obtained f o r the two independent halves; the agreement between these two independent measurements i s e x c e l l e n t (24). The values of the Cr-Cr s e p a r a t i o n of 3.000 A and the Cr-0Cr angle of 99.6(3)° are intermediate between those f o r the g l y ­ c i n a t o and phenanthroline complexes. Hence, while the only magnet­ i c data a v a i l a b l e at present are the room temperature values (y= 3.43 μΒ) obtained on the t e t r a h y d r a t e (13), i t i s evident that the value of ΔΕ f o r t h i s comple there i s a simple c o r r e l a t i o t

e

2

Interpretation The s t r u c t u r a l and magnetic data described above are c o r r e l ­ ated i n f i g u r e 9, i n which the s i n g l e t - t r i p l e t s p l i t t i n g ΔΕ i s p l o t t e d against the b r i d g i n g angle φ; a s i m i l a r p l o t f o r the analogous copper(II) dimers [ C u ( L ) 0 H ] i s a l s o included i n figure 9 examination of the chromium data suggests that the oxalato complex Na^[Cr(0X) 0H] *6H 0, which has a φ of 99.6° (vide supra), should have a ΔΕ of approximately -25 cm . F i g u r e 9 i s noteworthy f o r two separate reasons: f i r s t l y , because f o r a given metal there i s apparently an almost l i n e a r c o r r e l a t i o n between ΔΕ and φ, and secondly because the slope of the ΔΕ v s . φ p l o t f o r the chromium(III) complexes i s c o n s i d e r a b l y smaller than that f o r the copper(II) complexes. Each of these f e a t u r e s i s r e a d i l y explained i n terms of simple bonding theory. 2 +

2

#

2

2

2

-1

The c o r r e l a t i o n between ΔΕ and φ. The c o r r e l a t i o n noted can be explained i n terms of valence bond theory and the p r i n c i ­ p l e s of super exchange(25).If the o r b i t a l s used by the b r i d g i n g oxygen atoms are pure ρ o r b i t a l s , the bond angle i s expected to be 90° and the ground s t a t e i s p r e d i c t e d to be a t r i p l e t (i.e. ΔΕ > 0); i f the o r b i t a l s are purely s , the ground s t a t e i s pre­ d i c t e d to be a s i n g l e t (i.e. ΔΕ < 0). Hence, s i n c e an increased value of the b r i d g i n g angle i m p l i e s greater s character i n the b r i d g i n g o r b i t a l s , we would expect a decrease i n ΔΕ as the b r i d ­ ging angle i s increased from 90°. For the s i x copper and three chromium cases which have been studied i n d e t a i l , t h i s trend i s observed (_2). T h i s c o r r e l a t i o n can a l s o be expressed i n terms of molecular o r b i t a l theory. The M-O-M-O r i n g i s of approximate Z? symmetry i n these molecules, with the x-axis defined as the Cu-Cu d i r e c t ­ ion and y - a x i s p a r a l l e l to the 0-0 v e c t o r (26).Neglecting oxygen s o r b i t a l s , the eight σ-orbitals i n t h i s system transform i n D h 2h

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

102

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

X 006 |-

120 TEMPERATURE,

Figure

7.

ϊ·0 Κ

Temperature variation of the magnetic susceptibility 4H 0. Solid line represents the best fit to equation 2

of [Cr(phen) OH] (3) (see text). É

Figure 8. View of the anion in Na [Cr(ox) OH] · 6H 0. Data are the average values of the two crystallographically independent dimers. r

2

2

2

Oi

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

I

g

5

9.

HODGSON

Chromium(III)

Dimers

103

-500 h Figure 9. M-O-M bridging angle, φ (abscissa) v s . the singlet-triplet splitting energy, A E (ordinate) for dihydroxo-bridgecl complexes of Cu (steeper line) and Cr (flatter line) B2u

Big / A g ^ J

- dxy \

\\ V \

/

w w \\ w

/

\/

'dx^y metal ion

2

Px

\

\

/

\ \

/

\

11

\ \ \

/

// / /

liqand σ orbitals

/

/

/ /

B2u/ /

Agi - t molecular

orbitals

Figure 10. Molecular orbital diagram for the σ-orbitals in the CrO-Cr ring, assuming D symmetry and a Cr-O-Cr angle of 90° 2ll

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

104

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Γ \

/

/

*

\B2u

/-

/ / \

Ν \

~^ 3u\\ B

—\ d x y

W W

y

^d 2- 2 \\ x

metal

ion

•i b a s i s

set

y

>w

D

J

-> -> · S.xS. + S.' J 1

Γ..*

1

=

1 J

-> S. J

where D^. i s the antisymmetric vector coupling constant and r ^ . i s the a A i s o t r o p i c coupling tensor. For o r b i t a l s i n g l e t single** ions undergoing exchange, Moriya (2.) has estimated that D

^(Ag/g)J

and 2

—(Ag/g) J ij where Ag = |g-2|. For many copper compounds Ag i s approximately 108

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

n

10.

HATFIELD

Superexchange

109

Interactions

0. 1, so | D | ~ 5 cm" f o r J = 100 cm~l. There i s no l i m i t on the magnitude of D and Γ f o r o r b i t a l l y degenerate s i n g l e i o n s t a t e s undergoing exchange, and a suggestion has been made that t h i s may be an a p p r o p r i a t e approach f o r the r a t i o n a l i z a t i o n of the magnetic p r o p e r t i e s of the tetramers [Cu^OXi+L^ ]. (3) These pro­ blems w i l l be discussed here. 1

1. Di-y-Hydroxo-bridged

Copper(II) Complexes

I t has been known f o r some time that copper (II) forms com­ plexes of the type [CuL0H]2X2 > where L i s a bidentate amine and X~ i s an a p p r o p r i a t e counterion. (4-8) These complex ions may be described as two planar or t e t r a g o n a l pyramidal u n i t s sharing an edge which i s defined by two b r i d g i n g hydroxo oxygen atoms. The magnetic p r o p e r t i e s o i n t e r a c t i o n s which d i f f e the s t r u c t u r a l and magnetic data f o r s i x of these compounds i t has been p o s s i b l e to i d e n t i f y some of the f a c t o r s which i n f l u e n c e the exchange i n t e r a c t i o n s , and the r e s u l t s of that study w i l l be reviewed here. The chemical and s t r u c t u r a l f e a t u r e s which w i l l be examined i n c l u d e 1) the nature of the c h e l a t i n g amine. 2) the copper-oxygen (hydroxo) bond d i s t a n c e . 3) the nature of any out-of-plane c o o r d i n a t i o n . 4) the geometry of the b a s a l plane. 5) the s i n g l e i o n ground s t a t e . 6) hydrogen bonding by the hydroxo-bridge hydrogen atom. 7) the Cu-Cu s e p a r a t i o n . 8) the Cu-O-Cu bridge angle. Magnetic parameters have been obtained from analyses of EPR s p e c t r a , and from the temperature v a r i a t i o n of the magnetic sus­ c e p t i b i l i t y . The l a t t e r i s c h a r a c t e r i s t i c of exchange coupled copper(II) p a i r s and the s i n g l e t - t r i p l e t s p l i t t i n g s have been determined by f i t t i n g the data to the Van Vleck equation X

m

-

{ l + ~ exp(-2J/kT)}'

1

+ Να

(2)

where the symbols have t h e i r usual meaning, Να i s the temperature independent paramagnetism, and the equation as w r i t t e n gives the s u s c e p t i b i l i t y per copper i o n . In some cases Τ has been replaced by (T-9) to account f o r interdimer i n t e r a c t i o n s . A. S t r u c t u r a l and Magnetic

Data.

f

1. Di-y-hydroxobis[N,N,N ^ ' - t e t r a m e t h y l e t h y l e n e d i a m i n e copper(II)] bromide. The f i r s t compound of t h i s type to be c h a r a c t e r i z e d by an X-ray c r y s t a l s t r u c t u r e determination was di-y-hydroxobis[N,N,N jN'-tetramethylethylenediaminecopper(II)] bromide, [Cu(tmen)0H]2Br .(9) The s t r u c t u r e of the formula u n i t viewed along the b-axis i s shown i n F i g u r e 1 along with some of f

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

110

EXTENDED

Figure

1.

Structure

INTERACTIONS

BETWEEN

of [Cu(tmen)OH] * viewed along the b axis from Ref. 9) 2 2

METAL

(adapted

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

10.

HATFIELD

Superexchange

111

Interactions

the important s t r u c t u r a l parameters. The s t r u c t u r a l data are a l s o c o l l e c t e d f o r a l l compounds of t h i s type i n Table I. The Cu-N bond d i s t a n c e of 2.030 A, the Cu-0 bond d i s t a n c e of 1.902 A, and the N-Cu-N angle of 86.7° are a l l normal f o r s u b s t i t u t e d e t h y l enediamine complexes. The c o o r d i n a t i o n about the copper i s square planar with the n i t r o g e n atoms being 0.14 A out of the plane of the CU2O2 unit.The bromide i o n i s involved i n hydrogen bonding with the hydroxo-bridge, s i n c e O-Br d i s t a n c e of 3.366 A i s com­ parable to the O-Br d i s t a n c e s of 3.39 and 3.37 A, r e s p e c t i v e l y , i n the hydrogen bonded systems M n B ^ ^ ^ O and CoBr *2H20. The i n f r a r e d spectrum a l s o i n d i c a t e s the presence of hydrogen bond­ ing i n that there i s a strong OH s t r e t c h i n g band at 3410 cm" , a value which i s approximately 200 cm" lower than that of a f r e e OH group.(10-12) The magnetic s u s c e p t i b i l i t 77-300°K both on a powdere the a,b, and c c r y s t a l l o g r a p h i c axes. (13) The data were f i t t e d to the Van Vleck equation (2) y i e l d i n g the magnetic parameters 2J = -509 cm" , g = 2.0, and Να = ~150xl0"6 cgs u n i t s . Within the p r e c i s i o n of the experimental measurement the c r y s t a l sus­ c e p t i b i l i t i e s were i s o t r o p i c . e

o

2

1

1

1

2. D i - y - h y d r o x o b i s t N ^ N * ,Ν'-tetraethylethylenediamine copper(II)] p e r c h l o r a t e . As a part of an i n v e s t i g a t i o n of the thermochromic p r o p e r t i e s of N - a l k y l s u b s t i t u t e d ethylenediamine complexes of c o p p e r ( I I ) , H a t f i e l d , P i p e r , and Klabunde(6) reported, i n 1963, the temperature v a r i a t i o n of the magnetic s u s c e p t i b i l i t i e s of the Ν,Ν,Ν ,Ν'-tetraethylethylenediamine (teen) and NjN-diethyl-N'-methylethylenediamine complexes of the general formula [Cu(diamine)OH] (CIO1J2. The data f o r the l a t t e r com­ pound were f i t t e d to Equation (2) f o r the determination of the magnetic parameters, while 2J f o r the teen compound was d e t e r ­ mined from the expression 2J = -1.11 T where T i s the temperature at which the magnetic s u s c e p t i b i l i t y a t t a i n s the max­ imum v a l u e , and the constant has u n i t s of cm" deg" . The s t r u c t u r e of [Cu(teen)0H] (C10i )2 has been completed only r e c e n t l y . ( 1 4 ) The s t r u c t u r e c o n s i s t s of [ C u ( t e e n ) 0 H ] 2 u n i t s and d i s c r e t e ClOi*" anions. (While t h i s geometry at the copper(II) i o n i s comparable to the s i t u a t i o n described above(9) f o r [Cu(tmen)0H]2Br2, i t i s i n marked contrast to the geometry of the s t r u c t u r e s of other compounds i n t h i s s e r i e s with oxyanions, v i d e post.) The best l e a s t squares plane of the N Cu0 CuN2 u n i t c a l l s a t t e n t i o n to the s l i g h t d i s t o r t i o n i n the molecule, the oxygen and n i t r o g e n atoms are approximately 0.15 Â out of the plane. The CU2O2 u n i t i s planar owing to the i n v e r s i o n center. The s t r u c t u r a l f e a t u r e s , which are given i n Table I, are very s i m i l a r to those determined f o r the tmen compound with the s i g n i f i c a n t exception being the decrease i n Cu-O-Cu angle from 104.1° i n [Cu(tmen)OH]2Br2 to 103.0° i n [ C u ( t e e n ) 0 H ] ( C l O ^ ) . Apparently there i s a hydrogen bonding i n t e r a c t i o n between 1

2

m a x

m a x

1

2

1

+

2+

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

4

2

4

3

pyridine nitrogen

*

4

2

2

80.6(1)° 95.6(1)

2

2

97° 97°

96.3(5)° 98.8(3)° 93.0(5)° 99.5(2)°

[Cu(bipy)OH] (N0 )

2

81°

4

100.4(1)°

87.8(2)° 103.0(2)°

[Cu(bipy)0H] S0 -5H 0

2

[Cu(EAEP)0H] (C10 )

2

β-[Cu(DMAEP)0H] (C10 )

2

[Cu(teen)0H] (C10 )

2

[Cu(tmen)0H] Br

Cu-O-Cu

ο

in-plane

ο

1.99 2.00 2.00 2.02

4

2.379(2)

2

2.917(5) -130

2.847

+172

2.21(S0 ") 2.893(2) +48 2.24(H 0)

1.981(8)* 2.562(10) 1.998(10) 2.618(9) 2.001(8)* 2.053(10)

1.920(2) 1.998(2) 1.923(1) 2.000(2)

1.92 1.94 1.95 1.95

1.895(7) 1.913(7) 1.927(2) 1.930(8)

2.935(1) -201

-509

2.978(2) -410

x

-

2

-i cm

-

ο Cu-Cu,A 3.000

out-ofplane Cu-0,A

1.900(3) 2.003(3)* 2.721(4) 1.919(3) 2.066(3)

1.899(4) 2.013(5) 1.907(4) 2.024(15)

Cu-0,A Cu-N,A 86.7(8)° 104.08(.17)° 1.902(3) 2.030(10)

N-Cu-N

S t r u c t u r a l and Magnetic P r o p e r t i e s of [Cu(diamine)OH]

Table I

29,30

25,26,27, 28

17,20,21

15,17

6,14

8,9,13

References

10.

HATFIELD

Superexchange

Interactions

113

the hydroxo group and the p e r c h l o r a t e i o n , s i n c e the average Cl-0 bond digtance f o r the three oxygens which are not i n v o l v e d i s 1.389(5) A, while the Cl-0 bond d i s t a n c e f o r the oxygen which i s probably hydrogen bonded i s 1.434(5) Â . 3. 3-Di-y-hydroxobis[2-(2-dimethylaminoethyl)pyridinecopper( I I ) ] p e r c h l o r a t e . Two forms of the compound [Cu(DMAEP)0H]2" ( C l O i ^ can be i s o l a t e d . (15,16) The 3-form i s obtained, essent i a l l y uncontaminated with the α-form, by mixing equimolar q u a n t i t i e s of copper(II) p e r c h l o r a t e hexahydrate and DMAEP i n ethanol/ether while both forms may be found i f methanol/ether i s used. U h l i g and co-workers (17) had reported only one form of t h i s compound i n t h e i r study of the c o o r d i n a t i o n chemistry of Ns u b s t i t u t e d 2-(2-aminoethyl)pyridine, and from a comparison of the magnetic p r o p e r t i e they had the t r i c l i n i two expected hydroxo b r i d g e s , two p e r c h l o r a t e bridges and i t w i l l not be considered f u r t h e r here.(16) The 3-form has two hydroxo bridges (15) and, as shown i n F i g u r e 2, the copper ions are i n a t e t r a g o n a l pyramidal environment with oxygen atoms from p e r c h l o r ­ ate ions occupying the a x i a l p o s i t i o n s . The N2CUO2CUN2 u n i t i s e s s e n t i a l l y planar with the l a r g e s t d e v i a t i o n from the best l e a s t squares plane being 0.09 A. An unusual f e a t u r e s obtains here i n that the copper ions are not d i s p l a c e d out of the plane toward the a x i a l l i g a n d as has been observed i n many t e t r a g o n a l pyramidal copper(II) complexes. Presumably t h i s absence of d i s ­ placement i s a r e f l e c t i o n of the weak nature of the p e r c h l o r a t e c o o r d i n a t i o n . The bond d i s t a n c e s and angles are comparable to those observed i n other s i m i l a r complexes. The s t r u c t u r a l para­ meters p e r t i n e n t to t h i s d i s c u s s i o n are l i s t e d i n Table I where i t may be seen that the Cu-pyridine n i t r o g e n bond d i s t a n c e i s somewhat shorter than the Cu-amine n i t r o g e n bond. The Cu-O-Cu angle i s 100.4(1).° Hydrogen bonding i n v o l v i n g the hydroxo b r i d g i n g group i s i n d i c a t e d by the oxygen-oxygen s e p a r a t i o n of 2.993 A, which i s l e s s than twice the van der Waals r a d i u s of oxygen as given by Bondi (3.02 A) (18) but s l i g h t l y greater than the corresponding value given by P a u l i n g (2.80 Â ) . (19) That the bonding i s weak i s evident from the s t r o n g , sharp 0-H s t r e t c h i n g band at 3580 cm~l, a value which i s only 40 cm" l e s s than the value u s u a l l y asc r i b e d to f r e e hydroxyl groups. The temperature v a r i a t i o n of the magnetic s u s c e p t i b i l i t y of 3-[Cu(DMAEP)0H] (C10 ) from 50-300°K i s shown i n F i g u r e 3.(15) There i s a very broad maximum i n the magnetic s u s c e p t i b i l i t y at about 175°K, and the data may be f i t t e d to expression f o r ex-_^ change coupled p a i r s of copper(II) ions y i e l d i n g 2J = -195 cm and g = 2.00, where the c r i t e r i o n f o r the best f i t was the minimization of the f u n c t i o n 1

2

A

B F

l+

2

= Z[{ (exptl).X

X

(calcd) }T ] i

2

i

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS

114

BETWEEN M E T A L IONS

3h

01 50

I

1 100

1

1 150

1

J — 200

1

1

250

300

β

Τ ( Κ) Inorganic Chemistry

Figure 3. Susceptibility of fi-[Cu(DMAEP)OH] (ClO, )2 per Cu atom as a function of temperature. Solid line represents values calculated from equation (2) with g = 2.03 and 2] = 201 cm (15). 2

t

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchonge

115

Interactions

4

For t h i s set of parameters A = 2 . 1 x l 0 ~ . Since t h i s best f i t g-value i s somewhat lower than the average g-value (2.03) found from the EPR spectrum taken at 77°K, a second c a l c u l a t i o n was made i n which the g-value was held constant and only 2J was v a r i e d . The somewhat poorer f i t ( A B F " 5.7x10"^) gave 2J = -201 cm"l. In view of these r e s u l t s a value of = 2.03 i s probably accurate w i t h i n 2% and 2J - -200110cm" . B F

1

4. Du-y-hydroxobis[2-(2-ethylaminoethyl)pyridinecopper(II)]p e r c h l o r a t e . The s t r u c t u r e of the complex [Cu(EAEP)0H]2~ (010^)2 and c o o r d i n a t i o n geometry around copper i s very s i m i l a r to that described above f o r [Cu(DMAEP)OH] (CIO1J2 · The p e r t i n e n t s t r u c t u r a l d e t a i l s are tabulated i n Table I. In t h i s compound the copper ions are d i s p l a c e d approximately 0.12 A from the best least-squares b a s a l plan and the two b r i d g i n g oxyge the copper-oxygen(perchlorate) a x i a l i n t e r n u c l e a r separations are s i g n i f i c a n t l y shorter i n [Cu(EAEP)0H]2(CIO1J2 than i n the c o r r e s ponding complex [Cu(DMAEP)0H]2 (C10i*)2 where the copper ions are not d i s p l a c e d from the b a s a l plane. The two b a s a l planes are n e a r l y coplanar, with the angle between them being 1.4°. The average of the two Cu-O-Cu angles i s 99.2(3)° and the Cu-Cu s e p a r a t i o n i s 2.917(5) Â . The s t r u c t u r a l data i n d i c a t e that any hydrogen bonding i n v o l v i n g the b r i d g i n g hydroxo hydrogen atom i s very weak. The short oxygen(bridge)-oxygen(perchlorate) separations are 2.89 and 2.95 A, values which are l e s s than twice the van der Waals r a d i u s of oxygen as given by Bondi but greater than that given by P a u l i n g . The i n f r a r e d spectra shows a strong, sharp band at 3580 cm"" . The magnetic s u s c e p t i b i l i t y data (21) f o r [Cu(EAEP)0H] (010^)2 show a broad maximum at ~ 1 2 0 ° K and when the data are f i t t e d to Equation (2) the parameters 2J = -130cm~ and g = 2.04 result. 2

c

1

2

1

5. D i - y - h y d r o x o b i s [ 2 , 2 * - b i p y r i d i n e c o p p e r ( I I ) ] s u l f a t e pentahydrate. The complexes of the general formula [Cu(diamine)OH]2X "nH20 which are formed by 1,10-phenanthroline and 2 , 2 ' - b i p y r i d i n e are of c o n s i d e r a b l e i n t e r e s t s i n c e a v a r i e t y of counter ions may be used and the magnetic p r o p e r t i e s depend on the i d e n t i t y of the counter ion.(22-25) The f i r s t compound of t h i s type to be c h a r a c t e r i z e d f u l l y was [Cu(bipy)0H] S0i · 5H 0 (25-28). The s t r u c t u r a l d e t a i l s necessary f o r t h i s d i s c u s s i o n are l i s t e d i n Table I. In t h i s compound the copper ions are found i n d i s t o r t e d t e t r a g o n a l pyramidal environments with a water molecule c o o r d i n ated to one copper and the s u l f a t e i o n coordinated to the other coppeç. The copper ions are d i s p l a c e d approximately 0.18 and 0.23 A from the b a s a l planes formed by the n i t r o g e n atoms and the b r i d g i n g oxygens. I t i s i n t e r e s t i n g to note that the greater 2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

+

2

116

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

displacement of 0.23 Â occurs i n the p o r t i o n of the molecule i n which the s u l f a t e i o n i s coordinated to copper and that that Cu-0 ( a x i a l ) bond d i s t a n c e i s the shorter of the two. The d i h e d r a l angle between the b a s a l planes i s 7.9°. There i s extensive hydrogen bonding with an 0(hydroxo)-0(sulfate) separation of 2.77 A. The magnetic s u s c e p t i b i l i t y of t h i s compound has been measured as a f u n c t i o n of temperature over the range 4.2-300°K, (26-28) and the χ vs.Τ p l o t , Figure 4, c l e a r l y i n d i c a t e s a d e v i a t i o n at low temperature from the Curie-Weiss behavior e x h i ­ b i t e d at higher temperatures. The measured s u s c e p t i b i l i t i e s deviate i n the manner expect f o r an exchange coupled p a i r of copper ions with a p o s i t i v e 2J value i n d i c a t i n g a ferromagnetic i n t e r a c t i o n and a t r i p l e t ground s t a t e with a low l y i n g s i n g l e t s t a t e . The best l e a s t square f i t of the data y i e l d 2J = +48(±10) cm"" and g = 2.2. The presence of th spectrum shown i n Figure 5. The spectrum may be described using the spin Hamiltonian - 1

1

H , = g J |BH S s

z

where the resonance \(z>

f

2

z

+ gj3(S^ H + s J H ) + D(S ' -2/3) y

f i e l d s are

= (g| | 3 )

- 1

^ ν -D| 1

H ( z ) = (g| j B ) " ^

+D)

2

1

Η ( χ , ) = (g^3)" [hv(hv - D ) ] χ

z

1 / 2

Υ

1

H (x,y) = (gjB)" [hv(hv + D ) ]

1 / 2

2

H ^ f o r b ) = ( 2 g | |Β)"\ν H ( f o r b ) = (2gj3) 2

-12 2 2 1/2 ( h V -D ) A

Z

± / Z

Here the s u b s c r i p t 1 designates the low f i e l d t r a n s i t i o n s and the s u b s c r i p t 2 designates the high f i e l d t r a n s i t i o n s , while the ΔΜ = 2 t r a n s i t i o n s are l a b e l e d forbidden. 3

1

6. D i - y - h y d r o x o b i s [ 2 , 2 - b i p y r i d i n e c o p p e r ( I I ) ] n i t r a t e . The s t r u c t u r e of [Cu(bipy)0H]2(N03) i s very s i m i l a r to that of the s u l f a t e s a l t with the d i f f e r e n c e being that n i t r a t e s are coor­ dinated i n the a x i a l p o s i t i o n s of the t e t r a g o n a l pyramids. (29) The copper(II) ions are d i s p l a c e d from the b a s a l planes toward the oxygen atom of the coordinated n i t r a t e where the Cu-0 ( a x i a l ) bond i s r e l a t i v e l y s h o r t , being 2.379(2) Â. S t r u c t u r a l d e t a i l s are l i s t e d i n Table I . The magnetic s u s c e p t i b i l i t y has been measured as a f u n c t i o n of temperature i n the range 1.6-300°K and can be f i t to Equation (2) y i e l d i n g 2J = 172 cm"! and g = 2.10. (30) The estimated 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchonge

Interactions

117

20

40

60

80

300

Temperature (°K) Inorganic Chemistry

Figure 4. Inverse of magnetic susceptibility per cop­ per atom of [(hipy)Cu(OH) Cu(bipy)]SO · 511*0 as a function of temperature. Solid line represents valuescalculated from the Van Vleck equation; dashed line is extrapolation of Curie-Weiss law. Observed values are black dots. Data above 80K from Ref. 26 (28). 2

Figure

5.

X-band EPR spectrum of [Cu(bipy)OH] of/,Ο at 77°K, 0-10.0 kG

lf

,SO, ·

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

118

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

standard d e v i a t i o n on t h i s 2J value i s very l a r g e s i n c e the equa­ t i o n i s very i n s e n s i t i v e to v a r i a t i o n s i n l a r g e p o s i t i v e 2Jvalues. B. C o r r e l a t i o n of S t r u c t u r a l and Magnetic P r o p e r t i e s . 1. The nature of the c h e l a t i n g amine. Inspection of the data i n Table I r e v e a l s that the two complexes with the aromatic d i a ­ mine, 2 , 2 - b i p y r i d i n e , have t r i p l e t ground s t a t e s , the two com­ plexes with a l i p h a t i c diamines, tmen and teen, have s i n g l e t ground s t a t e s with l a r g e | 2 j | values, and that the two complexes with the mixed a l i p h a t i c / a r o m a t i c diamines, DMAEP and EAEP, have s i n g l e t ground s t a t e s with intermediate |2j| v a l u e s . There i s no c o r r e l a t i o n with the N-Cu-N angle, s i n c e these angles increase i n the order aromatic < a l i p h a t i c < mixed. Furthermore, Casey (25) has shown that [Cu(bipy)OH] and [ C u ( b i p y ) 0 H ] C l - 3 H values of -6, -34, and -39 cm. , r e s p e c t i v e l y . Since the N-Cu-N angle i s expected to be f a i r l y constant f o r a l l of the 2,2 b i p y r i d i n e compounds, and s i n c e there i s no apparent c o r r e l a t i o n of t h i s angle with 2J, then i t i s reasonable to conclude that changes i n t h i s angle are of secondary importance .However, the chemical nature of the bidentate amine i s probably important s i n c e as of yet there are no known overlaps of 2J values between the groups of compounds formed by the aromatic, mixed aromatic/ a l i p h a t i c , and a l i p h a t i c diamines. f

2

2

f

2. The copper-oxygen(hydroxo) bond d i s t a n c e . The copperb r i d g i n g oxygen bond d i s t a n c e s are a l l comparable and f a l l i n the range 1.9 to 1.95 A with the average bond d i s t a n c e being 1.915 A. For a l l p r a c t i c a l purposes the copper-oxygen bond d i s t a n c e i s constant i n t h i s s e r i e s of compounds. 3. The nature of any out-of-plane c o o r d i n a t i o n . The two com­ plexes with the a l i p h a t i c diamines, [Cu(tmen)OH] Br and [Cu(teen)OH] (ClO^) , are formed by two planar u n i t s sharing an edge. The c l o s e s t out-of-plane contacts to copper i n [Cu(tmen)O H ] B r are to bromide ions which are centered over the f i v e member c h e l a t e r i n g , these d i s t a n c e s are 4.778 and 4.933 Â , and are considered to be too long even foç semi-coordination. There are no out-of-plane atoms w i t h i n 4.0 A of copper(II) i n [Cu(teen)0H] (C10i ) . (31) The other four compounds are composed of t e t r a g o n a l pyramidal u n i t s sharing an edge with a p i c a l l i g a n d s on opposite s i d e s of the j o i n e d b a s a l planes. There i s an important trend here; the out-of-plane copper-oxygen bond d i s t a n c e s decrease with an i n c r e a s e i n the displacement of the copper i o n from the b a s a l plane toward the a p i c a l l i g a n d , v i z . , 2

2

2

2

2

2

2

+

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

Superexchonge

HATFIELD

compound

Cu-0,

3-[Cu(DMAEP)OH] (C10 ) 2

4

[Cu(EAEP)OH] (C10 ) 2

4

[Cu(bipy)OH] (N0 ) 2

3

4

ο

ο

displacement, A

A

none

2.721(4)

2

2

2

2.618(9) 2.562(10) 2.379(2)

0.11 0.13 0.16

2

0.23 0.18

2.21(SO, ") 2.24(H 0)

[Cu(bipy)OH] S0 -5H 0 2

119

Interactions

2

2

There i s a general c o r r e l a t i o n of J with the copper-out-of-plane l i g a n d d i s t a n c e . Except f o r [Cu(bipy)0H] S0i 5H 0, the exchange coupling constant become d i s t a n c e decreases. e

2

+

2

4. The geometry of the b a s a l planes. To a good approximation the bases of the t e t r a g o n a l pyramids are a l l planar with d e v i a t i o n s f r o m the best l e a s t squares plane being on the order of 0.15 A or l e s s . The two b a s a l planes [or the c o o r d i n a t i o n planes i n the case of [Cu(tmen)0H] Br and [Cu(teen)0H] (C10i ) ]are f r e q u e n t l y coplanar, with the l a r g e s t d e v i a t i o n from c o p l a n a r i t y being a 7.9° d i h e d r a l angle between these planes i n [ C u ( b i p y ) 0 H ] SO^'5^0. Consequently, except f o r t h i s l a t t e r compound, the geo­ metry of the b a s a l plane remains constant throughout the s e r i e s . o

2

2

2

+

2

2

5. The s i n g l e i o n ground s t a t e s . I t i s very w e l l e s t a b l i s h e d that the unpaired e l e c t r o n i s i n the σ* o r b i t a l f o r square planar copper(II) complexes,(32) Although s p i n - o r b i t c o u p l i n g and the low symmetry c r y s t a l f i e l d components permit the mixing i n of other s t a t e s , to a good approximation the exchange mechanisms can be given i n terms of t h i s e l e c t r o n i c c o n f i g u r a t i o n . Although the displacement of the copper(II) i o n from the b a s a l plane toward the a x i a l l i g a n d i n the t e t r a g o n a l pyramidal complexes complicates even f u r t h e r the nature of the s i n g l e i o n ground s t a t e , i t w i l l be assumed that the unpaired e l e c t r o n i s i n the d >cr*, o r b i t a l i n these complexes. This assumption i s a d m i t t e d l y ïess tenable f o r [Cu(bipy)0H] S0i '5H 0 and [ C u ( b i p y ) 0 H ] ( N 0 ) where the d i s p l a c e ments are considerable and the a x i a l bond d i s t a n c e s are rather short. P r e c i s e d e s c r i p t i o n s of the ground s t a t e s must await the completion of d e t a i l e d EPR i n v e s t i g a t i o n s which are p r e s e n t l y underway. (33) 2

2

X

2

t

2

2

3

2

6. Hydrogen bonding by the h y d r o x o - b r i d g e h y d r o g e n atoms.The nature of the hydrogen bonding spans a wide range i n these compounds. A s u i t a b l e working hypothesis would suggest that the e l e c t r o n d e n s i t y on the b r i d g i n g oxygen atom should increase with the s t r e n g t h of the hydrogen bond and that t h i s v a r i a t i o n i n e l e c t r o n d e n s i t y should have a s i g n i f i c a n t e f f e c t on the exchange

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

120

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

coupling. I f e i t h e r the 0···Χ i n t e r n u c l e a r separation or Δν(ΟΗ) (the d e v i a t i o n from the " f r e e " hydroxyl s t r e t c h i n g energy) are taken as gauges of the hydrogen bond, then i t i s c l e a r from the data i n Table I I that there i s no simple c o r r e l a t i o n between e i t h e r of these parameters and the exchange coupling constant. Thus i t i s reasonable to conclude that hydrogen bonding i s of secondary importance i n determining the nature of the exchange coupling. 7. The copper-copper s e p a r a t i o n . There i s an i n t e r e s t i n g c o r r e l a t i o n between the Cu-Cu separation and the s i n g l e t - t r i p l e t s p l i t t i n g . As the Cu-Cu s e p a r a t i o n increases from 2.847 A i n [Cu(bipy)0H] (N0 ) to 3.000 Â i n [Cu(tmen)0H] Br ,_Jhe s i n g l e t t r i p l e t s p l i t t i n g changes from +172 cm" to -509 cm . I f the data f o r [Cu(bipy)0H] S0^·5H 0 omitted s i n c th b a s a l planes i n the compoun through the f i v e a v a i l a b l and^2J has a slope of -4545 c n r V A and an i n t e r c e p t of 13,130 cm 2

3

2

2

2

1

8. The copper-oxygen-copper bridge angle. Since the copDeroxygen (bridge) bond d i s t a n c e s are n e a r l y constant at 1.915 A, and s i n c e the C u 0 u n i t s are a l l n e a r l y planar there i s a s i m i l a r s t r i k i n g c o r r e l a t i o n between the 2J values and the Cu-0Cu bridge angle. T h i s l i n e a r c o r r e l a t i o n ( i n the range 95.6° < φ < 104.1°) i s i l l u s t r a t e d i n Figure 6, where the slope i s -79.5 cm" deg" and the i n t e r c e p t i s 7790 cm" . 2

1

2

1

1

C. R a t i o n a l i z a t i o n i n Terms of a Molecular O r b i t a l Model. The c o r r e l a t i o n s which have been observed can be understood i n terms of a simple molecular o r b i t a l model, which i s most appro­ p r i a t e f o r the two joined-planar compounds, but which i s s t i l l a good approximation f o r the j o i n e d t e t r a g o n a l pyramidal compounds. (34) I t i s necessary to consider the two oxygen o r b i t a l s , p , Py, and the two copper in-plane o r b i t a l s d _ , d . I f the f o l l o w ­ ing coordinate system i n D i s adopted x

2

v 2

x v

2 n

01

Cui X

i t can be seen that the o r b i t a l s transform as A

d

2

x -y

2

+

d

2

x -y

2

Yl " Ύ2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

2

4

2

4

ο

" f r e e " hydroxyl

4

3

2

2

3620

2.877

2

2

-

2.89

2.993

3.00

3.366

X, A

[Cu(bipy)OH] (N0 )

3580

0

2.77

2

3580

1

-

4

2

3410

v(O-H),cnf

[Cu(bipy)OH] S0 '5H 0

2

[Cu(EAEP)OH] (C10 )

2

β-[Cu(DMAEP)] (C10 )

2

[Cu(teen)OH] (C10 )

[Cu(tmen)OH] Br

Compound

3

2

0N0 "

3

oso "

3

ocio ""

3

ocio "*

ocio "

Br"

X

Hydrogen Bonding Parameters f o r the B r i d g i n g Hydroxo Group.

Table I I

2

2

ο Bondi (3.02) Pauling (2.80)

MnBr ·2Η 0(3.39 A) CoBr -2H 0(3.37 A) ?

Comments

122

EXTENDED

B, lg

:

d x

B„ : 2u

l

d

"

xy

Yl

B

0

3u

+ d

xy

x

INTERACTIONS

BETWEEN

METAL

IONS

xy

2

- d

xy

+ Ύ2

: d 2 2 x -y z

^ 2

z

Χχ + x

2

2

where x^ and y^ designate the oxygen ρ c a l l y these symmetry r e l a t i o n s h i p

χ

and p

v

orbitals.

Symboli­

67

J

3u

Inspection of these w i l l show that the A and B^ molecular o r b i t a l s w i l l have i d e n t i c a l energies as w i l l ^ t h e B j ^ a n d u

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

Superexchange

HATFIELD

200 -

\

*

Interactions

123

bipy NO,

100 ^bipy

S0

4

0

-100 τ

-

IAEP

ε ^

X.

-200

-300

-

-400

-

-500

-

-DM ΑΙ Ρ

\o

t

e

e

n

Ν.

1 95

Figure

$

1

6*. Correlation

1

! 97

of singlet-triplet

I 99

1

splittings

1 101

1—

i 103

—

with the Ctt-O-Cu

tmen

λ „

1 105

bridge

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

angle

124

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

o r b i t a l s . With t h i s information the molecular o r b i t a l diagram f o r t h i s fourteen e l e c t r o n system may be constructed; i t i s given i n Figure 7a. As the Cu-O-Cu angle increases the overlap of the A and B combinations increase r e l a t i v e to that of the B3 and 2u a t i o n s . At a s u f f i c i e n t l y l a r g e enough angle i t may be a n t i c i p a t e d that the separation between the B2 * and B i g * mole­ c u l a r o r b i t a l s would exceed the p a i r i n g energy and the s i n g l e t s t a t e with c o n f i g u r a t i o n ( b 2 * ) would r e s u l t as the ground s t a t e . The t r i p l e t s t a t e from the c o n f i g u r a t i o n ( b * ) ( b i * ) would be the low l y i n g paramagnetic s t a t e . g

l g

B

U

c o m D i n

U

2

u

2 u

II.Di-y-Chloro-bridged

g

Copper(II) Complexes

The p r o p e r t i e s of four d i - y - c h l o r o - b r i d g e d copper(II) com­ plexes w i l l be describe [Co(en)3] [Cu2Cl8]Cl2'2H copper(II) ions are i n d i s t o r t e d t r i g o n a l bipyramids which share an equator-to-apex edge, (35-40) and (Class II) [Cu(2-methyl2

p y r i d i n e ) 2 C l 2 l 2 and [Cu(dimethylglyoxime)Cl2]2»where the coordina­ t i o n about copper i s d i s t o r t e d t e t r a g o n a l pyramidal and the d i meric s t r u c t u r e i s formed by the sharing of the base-to-apex edge. (41-44) Unlike the hydroxo-bridged copper(II) complexes described i n Section I where the Cu-0 (bridge) bond lengths are constant, the chloro-bridged dimers have g r e a t l y d i f f e r e n t Cu-Cl (bridge) d i s t a n c e s . I t w i l l be shown here that the s i n g l e t t r i p l e t s p l i t ­ t i n g s are dependent on the b r i d g i n g bond lengths as w e l l as the Cu-Cl-Cu b r i d g i n g angle. A. S t r u c t u r a l and Magnetic Data. 1. T r i s ( e t h y l e n e d i a m i n e ) c o b a l t ( I I I ) D i - u - c h l o r o b i s [ t r i c h l o r o c u p r a t e ( I I ) ] D i c h l o r i d e Dihydrate. The unusual spec­ t r a l p r o p e r t i e s (45) of the compound Co(en)3CUCI5Ή2Ο, as f o r ­ mulated by Kurnakowin 1898, l e d to an X-ray c r y s t a l s t r u c t u r e examination (35,36) which revealed that the new and unusual [C^Cls] *"" ion was present. The compound c r y s t a l l i z e s i n the orthorhombic space group Pbca with four molecules i n a u n i t c e l l of dimensions a = 13.560(9), b = 14.569(9), and c = 17.885(12) A. The b r i d g i n g Cu-Cl distances are 2.325(5) and 2.703(5) A, the Cu-Cu separa­ t i o n i s 3.722(5) A, and the angle at the bridge i s 95.2(1)°. The exchange i n t e r a c t i o n (37) i n [ C u ( e n ) 3 ] [ C u C l 8 ] C l 2 ' 2 H 0 has been p r e c i s e l y c h a r a c t e r i z e d by s i n g l e c r y s t a l magnetic s u s c e p t i b i l i t y measurements. (38) Data were c o l l e c t e d i n the temperature range 4.2-80°K on a c r y s t a l ( 5 . 2 x 2 . 9 x 2 . 5 mm) with the magnetic f i e l d a p p l i e d along the c r y s t a l l o g r a p h i c axes. The data are given i n Figure 8, where i t may be seen that the s u s c e p t i b i l i t y maximizes at 12.9°K, and the usual g - f a c t o r a n i s o tropy i s observed ( x > x > χ ^ ) . The data may be f i t t e d to 1

2

c

2

a

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

10.

HATFIELD

Superexchange

Interactions

125

», *

2u

a.

4 -

\

\

\

d 2 χ

-v

\

2

W

>

Λ-

\ X \ \ \

W e l e c t rons metal

molecular

ion orbitals

ligand

orbitals

orbitals

b.

/

/

/

/

Λ

*

Η—-χ. κ *

/

/

Figure 7. (a) Molecular orbital diagram for the in-plane orbitals in the CuO, ring, D symmetry, 90° Cu-O-Cu angle, (b) Antibonding molecular orbitals for Cu-O-Cu angle different from 90° (adapted from Ref. 34). 2ll

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

126

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Equation (2) y i e l d i n g J / k = -10.7±.2° and g = 2.07±.02; J / k = -10.8±.2° and g = 2.03±.02; and J / k = -10.6±.2° and g = 2.18±.02. The data f i t the t h e o r e t i c a l curve very w e l l . For example, of the 44 data p o i n t s c o l l e c t e d with the f i e l d a p p l i e d along the c - a x i s , only f i v e deviated from the c a l c u l a t e d values by more than 1.5% while 34 of the p o i n t s d i f f e r e d by l e s s than 1%. No s i g n i f i c a n t improvement of the f i t s were observed when interdimer i n t e r a c t i o n s were included. Thus w i t h i n experimental e r r o r J = J = J = J , and = (1/3)(g +g +g ) = 2.09, a value which i s i n e x c e l l e n t agreement with the g value obtained from powder data c o l l e c t e d i n the temperature range 130-235°K. Here g was c a l c u l a t e d from the C u r i e constant using the formula a

a

b

a

b

b

c

c

c

a

g

2

b

c

2

= 3kC/N3 S(S+l)

The exchange i n t e r a c t i o i s almost c o l i n e a r with the copper(II)-copper(II) v e c t o r , and the exchange i n t e r a c t i o n observed along the a and b-axes, which are almost perpendicular to the copper(II)-copper(II) v e c t o r , are equal i n magnitude. A l s o , there i s no s i g n i f i c a n t long range interdimer exchange present i n the system. This r e s u l t i s not unexpected i n view of the l a r g e interdimer separation. The l a r g e c o p p e r ( I l ) - c o p p e r ( I I ) separation of 3.722 A precludes any through-space i n t e r a c t i o n s s i n c e no s i g n i f i c a n t o r b i t a l overlap can occur over t h i s d i s t a n c e , and d i p o l e - d i p o l e i n t e r a c t i o n s could not produce a s p l i t t i n g of the observed magnitude. Hence, i t seems reasonable to conclude that the i n t e r a c t i o n occurs v i a superexchange through the c h l o r i d e b r i d g e s . 0

2. D i - y - c h l o r o b i s [ d i c h l o r o ( g u a n i n i u m ) c o p p e r ( I I ) ] d i h y d r a t e . In 1970 Carrabine and Sundaralingam (39a) reported the s t r u c t u r e of d i - y - c h l o r o b i s [ d i c h l o r o ( g u a n i n i u m ) c o p p e r ( I I ) ] dihydrate, where the guaninium l i g a n d i s the c a t i o n formed by monoprotonating guanine, one of the bases bonded to the sugar residues i n the backbone of d i o x y r i b o n u c l e i c a c i d (DNA). The same authors presented a more complete s t r u c t u r a l a n a l y s i s the f o l l o w i n g year, (39b) and the s t r u c t u r e was then confirmed by Declercq, Debbaudt, and Van Meerssche (39c) i n an independent i n v e s t i g a t i o n . Both research groups reported the s t r u c t u r e to be that of a dimer c o n s i s t i n g of chloro-bridged, t r i g o n a l - b i p y r a m i d a l l y coordinated copper(II) ions, as shown i n Figure 9. The monoprotonation was shown to occur at the imidazole n i t r o g e n , N(7), of the purine r i n g system, and binding to the copper i o n , at N£9). The b r i n g ­ ing Cu-Cl distances were determined to be 2.447 A and 2.288 A, with a Cu-Cl-Cu b r i d g i n g angle of 98° and a Cu-Cu separation of 3.575 A. The temperature v a r i a t i o n of the inverse s u s c e p t i b i l i t y ( c a l c u l a t e d per copper(II) ion) of a powdered sample i n the temperature region 1.6 to 255°K i s represented by the black data p o i n t s i n Figure 10. (40) The maximum i n the curve occuring at 0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

Superexchange

HATFIELD

Figure

9.

Structure

of

Interactions

[Cu(guaninium)Cl. ]j {

(adapted

from

Ref.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

39a)

128

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

approximately 15°Κ i s probably due t o a small percentage of mono­ mer i c impurity which d i d not a f f e c t the percentage composition of the elemental a n a l y s i s . To c o r r e c t f o r t h i s impurity the data p o i n t s from 1 . 6 t o 11.2°K were f i t to the Curie-Weiss law χ = C/(T - Θ) Values of 7.86 χ 10 and -3.22° were obtained f o r the constants C and θ, r e s p e c t i v e l y . In t h i s temperature r e g i o n the s u s c e p t i ­ b i l i t y of the dimer i s n e g l i g i b l y small (vide p o s t ) . A l l the data p o i n t s were then c o r r e c t e d f o r the c o n t r i b u t i o n of the impurity to the observed s u s c e p t i b i l i t y , and the c o r r e c t e d p o i n t s a r e a l s o p l o t t e d i n F i g u r e 10 as the u n - f i l l e d c i r c l e s . The impurity was estimated to be present to the extent of 1% i n the f o l l o w i n g way: The s u s c e p t i b i l i t y f o d havin molecula weight equal t o one-hal the expression χ - N3 y /3kT 2

2

1/2 where μ = g$[S(S + 1) ] , a t a s e l e c t e d temperature, and the c a l c u l a t e d s u s c e p t i b i l i t y was compared with the experimental sus­ c e p t i b i l i t y a t that temperature. The s o l i d l i n e i n F i g u r e 10 i s the best f i t of the c o r r e c t e d data to Equation ( 2 ) , which y i e l d s the parameters 2 J « - 8 2 . 6 ± 1 . 0 cm" and g - 2.12±0.02. 3. D i - y - c h l o r o b i s [ c h l o r o ( d i m e t h y l g l y o x i m e ) c o p p e r ( I I ) ] . The s t r u c t u r e (43) of [Cu(DMG)Cl2l2 i s shown schematically i n Figure 11 with p e r t i n a n t molecular dimensions i n d i c a t e d thereon. The copper atom i s f i v e - c o o r d i n a t e d i n a square-based pyramidal a r ­ rangement c o n s i s t i n g of a n e a r l y square planar arrangement of two n i t r o g e n atoms and two t i g h t l y bound c h l o r i n e atoms with a c h l o r i n e atom from an adjacent u n i t i n the a p i c a l p o s i t i o n . The magnetic data (44) can be described by the s i n g l e t t r i p l e t equation ( 2 ) with 2 J = 6 . 3 cm" , g = 2 . 0 6 , and θ = - 1 . 7 ° . A d d i t i o n a l and convincing evidence f o r the t r i p l e t ground s t a t e i s provided by the magnetization s t u d i e s . The magnetization curves f o r S' = 1/2 and S' » 1 were evaluated from the B r i l l o u i n function (46) 1

1

1

τ>ίγ\ - ^ S ' * „ .. B(X) , coth n#

2 s

2S'+1

1 χ - 2s' v

c o t h

2S*

where X • (H/T)(S g3/k) and S i s the e f f e c t i v e s p i n . To account f o r interdimer i n t e r a c t i o n s the f i e l d H was s e t equal t o the e x t e r n a l f i e l d p l u s a molecular f i e l d , H , where i t was assumed that Hm was p r o p o r t i o n a l to the magnetization. Thus, 1 ^ = N N3

where = gS'B(X) and = (3ke )/[Ng 3 S (S +1)].(47) A best f i t ( l e a s t squares) of the experimental data was used to s e l e c t a value of -1.2 f o r Θ thereby i n d i c a t i n g an a n t i f e r r o f

f

m

f

2

2

f

f

W

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

Superexchange

HATFIELD

129

Interactions

TEMPERATURE Inorganic Chemistry

Figure 10. Temperature vs. inverse susceptibility for the complex [(guaninium)CuClg]* · 2H 0 (38). Experimental points, · ; experimental points corrected for monomeric impurity, Q; Van Vleck equation best fit, . 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

130

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IONS

1

magnetic interdimer i n t e r a c t i o n . The Θ term i n the magnetization s t u d i e s i s very s i m i l a r to the θ term i n Equation 2, but i n view of the approximate nature of the theory, i d e n t i c a l values f o r these interdimer i n t e r a c t i o n parameters are not expected. 4. D i - p - c h l o r o b i s [ c h l o r o b i s ( 2 - m e t h y l p y r i d i n e ) c o p p e r ( I I ) ] . In recent years many copper(II) complexes of the type CUL2X2» where X i s c h l o r i d e or bromide and L i s p y r i d i n e or s u b s t i t u t e d p y r i ­ dine, have been prepared and c h a r a c t e r i z e d . (48) These complexes are mainly polymeric, having s i x c o o r d i n a t i o n about the copper ion with h a l i d e l i g a n d s from adjacent molecules occupying the o u t of-plane c o o r d i n a t i o n p o s i t i o n s . However, the complexes of 2methylpyridine were found to have p r o p e r t i e s somewhat d i f f e r e n t to those found f o r the analogous p y r i d i n e complexes, (49)and i t was p o r t u l a t e d that th s t e r i c hindrance to th quently, Duckworth and Stephenson (41) determined that such was the case f o r d i c h l o r o b i s ( 2 - m e t h y l p y r i d i n e ) c o p p e r ( I I ) . The coor­ d i n a t i o n about copper i n t h i s complex i s t e t r a g o n a l pyramidal with the f i f t h p o s i t i o n (out-of-plane) occupied by a c h l o r i d e l i g a n d from an adjacent planar moiety, and the s i x t h p o s i t i o n i s e f f e c t i v e l y blocked by the methyl groups of the p y r i d i n e l i g a n d s . The s t r u c t u r a l d e t a i l s are summarized i n Table I I I . As shown i n F i g u r e 12, the magnetic s u s c e p t i b i l i t y (42) data f o r Cu(2-methylpyridine)2CI2 obey the Curie-Weiss law, χ = C / ( T - 9 ) , i n the range 295°K to approximately 30°K. For the c h l o r o complex. the C u r i e constant C = 0.394, θ = 1°K. and y f f 2.828C ' i s 1.78 B.M. However, a t the low temperature l i m i t i t i s apparent that the Curie-Weiss law f a i l s . There i s a d i s t i n c t minimum i n the χ"~^ versus Τ p l o t a t approximately 7°K. The data obey the Van V l e c k equation (2) f o r m a g n e t i c a l l y coupled p a i r s of copper ions y i e l d i n g g = 2.15 and -2J = 7.4 cm"^. =

e

1

2

B. C o r r e l a t i o n of S t r u c t u r a l and Magnetic P r o p e r t i e s . I t i s of i n t e r e s t to compare the magnetic parameters and s t r u c t u r a l data f o r the s t r u c t u r a l l y - and m a g n e t i c a l l y - c h a r a c t e r i z e d c h l o r o bridged b i m e t a l l i c copper(II) complexes discussed here. These data a r e compiled i n Table I I I . Both the guaninium complex and the [Cu2Cl3]^""anion are made up of t r i g o n a l bipyramids sharing equatorial-to-apex edges, w h i l e the other two complexes l i s t e d i n the t a b l e are square-based pyramids sharing base-to-apex edges. In the t r i g o n a l - b i p y r a m i d a l complexes i t i s l i k e l y that the un­ p a i r e d e l e c t r o n s a r e i n the d o r b i t a l s of the copper(II) ions i n the ground s t a t e , whereas i n the square-pyramidal complexes i t i s l i k e l y that they a r e i n the d _ 2 o r b i t a l s . A q u a n t i t a t i v e comparison of the exchange energies of the four complexes cannot be based on s t r u c t u r a l data because of the d i f f e r e n t o r b i t a l s involved i n superexchange, but a q u a l i t a t i v e comparison i s pos­ sible. The s t r u c t u r a l parameters of the guaninium complex and the z 2

x 2

y

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

2

4-

2

2

1

DMG = dime thy lgloxime

2-mepy = 2-methylpyridine

2

[(DMG)CuCl ]

2

2

anion

[(2-mepy) CuCl ]

2

[Cu Clg] 2.26

2.24

101.4

88

squarepyramidal squarepyramidal

+6.3

2.70

2.45

43,44

41,42 3.37

2.70

35,36, 37,38

39,40

REF.

2.33

2.29

Cu-Cl BOND Cu-Cl BOND in-plane out-of-plane (A) (A)

-7.4

95.2

98

Cu-Cl-Cu ANGLE (degrees)

trigonalbipyramidal

trigonalbipyramidal

STRUCTURE

-14.6

-82.6

2

[(guaninium)CuCl ] ·2H 0

3

2J (cm )

COMPLEX X

Table I I I . Magnetic and s t r u c t u r a l data f o r c h l o r o - b r i d g e d copper(II) dimers.

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IONS

[ C i r c l e ] ^ " a n i o n are compared by superimposition i n Figure 13 with the s o l i d l i n e representing the guaninium complex. The smaller s i n g l e t - t r i p l e t s p l i t t i n g f o r the [ C ^ C l e l anion i n comparison to the guaninium complex^accompanies an increase i n the Cu-Cl-Cu bond angle from 95° to 98° and a decrease i n the b r i d g i n g bond lengths. Although i t has been demonstrated that the b r i d g i n g angle i s important i n determining the s i g n and magnitude of the s p l i t t i n g parameter 2J i n the s e r i e s of hydroxobridged copper(II) complexes i t i s u n l i k e l y t h i s e f f e c t can be presented as the s o l e explanation i n t h i s comparison because the b r i d g i n g bond lengths of these two chloro-bridged species are q u i t e d i f f e r e n t , ranging between 2.3 and 2.7 A, whereas they arg n e a r l y constant i n the hydroxo-bridged species at 1.90 to 1.95 A. (See Table I ) . The complexes [ ( 2 - m e t h y l p y r i d i n e g l y o x i m e ) C u C l 2 l 2 have d i f f e r e n t exchange coupling mechanism from that described above. In comparing these two square-pyramidal complexes i t should be noted that there i s a change i n ground s t a t e m u l t i p l i c i t y . The b r i d g i n g bond length i n [(2-mepy)2CUCI2]2 0.67 A longer than the comparable bond i n [(DMG)CuCl2]2 and angle at the b r i d ging c h l o r i d e i s 13.4° l a r g e r i n the 2-methylpyridine complex than i n the dimethylglyoxime complex. I t i s , t h e r e f o r e , not p o s s i b l e to a t t r i b u t e the change i n the exchange coupling constant only to bridge angle changes. C l e a r l y a number of a d d i t i o n a l chloro-bridged copper(II) dimers of both s t r u c t u r a l forms must be studied before the bridge-angle e f f e c t on 2J can be separated from the b r i d g i n g bond-length e f f e c t . i s

t n e

I I I . The Polymeric Compound [Cu(pyrazine)(N0q)?] Chains.

n

and

Related

The magnetic p r o p e r t i e s of the 1:1 copper(II) n i t r a t e pyrazine complex, [ C u i C i ^ H ^ ) ( ^ 3 ) 2 ] , r e f l e c t an exchange coupl i n g between the copper ions although as shown i n Figure 14 the copper(II) ions are separated by 6.712 A.(50) While exchange coupling across bidentate h e t e r o c y c l i c amine l i g a n d s had been suggested p r e v i o u s l y , (51) the p r e l i m i n a r y i n v e s t i g a t i o n (52) provided the f i r s t demonstration of an antiferromagnetic i n t e r a c t i o n i n a system which has been c h a r a c t e r i z e d by s t r u c t u r a l s t u d i e s , magnetic measurements c o l l e c t e d over a wide temperature range and EPR measurements. The c r y s t a l s t r u c t u r e of t h i s compound r e v e a l s a chemical chain p a r a l l e l to the a a x i s i n the orthorhombic c r y s t a l . Copper atoms are bridged along t h i s a x i s by the aromatic h e t e r o c y c l i c bidentate amine, pyrazine. Coordinated oxygen atoms from the n i t r a t e ions complete the h i g h l y d i s t o r t e d octahedron around each copper atom. The magnetic s u s c e p t i b i l i t y data c o l l e c t e d using a powdered n

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchange

133

Interactions

800h

0

10

30

50

100 200 3 0 0

T e m p e r a t u r e , °K Figure 12. Temperature variation imental inverse susceptibility of pyridine) Clt~\ t

of the exper­ [Cu(2-methyl-

g

Ν (CI)

Inorganic Chemistry

Figure 13. Comparison of the structural parameters for ninium)CuCl ] · 2H 0 and [Cu Cl ywith the solid line senting the guaninium complex (40) 3 2

£

â

8

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

[(guarepre-

134

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INTERACTIONS

BETWEEN

METAL

IONS

sample were d e s c r i b e d (52) by the I s i n g model f o r l i n e a r a n t i ferromagnetic i n t e r a c t i o n s i n chains using equations (3a,3b) which was developed by F i s h e r . (53) The equations are 2 2 XJL - ^ f j — [tanh ( φ Η φ sech (^)] (3a) 2

2

and 2 2 XII - ^ f i J —

exp

(2J/kT)

(3b)

where 1 s

^

2

3 XII +3

(4)

XI

The parameters whic data are = 2.22 an to be compared with the EPR r e s u l t s of Kokosζka and Reimann, (54) who reported g = 2.295, g = 2.054, and g = 2.070 ( = 2.133). In an attempt to determine more p r e c i s e l y the magnetic model which i s a p p r o p r i a t e f o r the d e s c r i p t i o n of the p r o p e r t i e s of t h i s compound and to c l a r i f y the exchange pathway, measurements have been performed on s i n g l e c r y s t a l s of t h i s m a t e r i a l between 1.7° and 60°K. (55) Reasonably l a r g e needle-shaped s i n g l e c r y s t a l s of Cu(pyrazine)(N03)2 were grown by slow-evaporation of an aqueous 1:1 s o l u t i o n of copper (II) n i t r a t e and pyrazine (Ci+Hi^). Because of the c r y s t a l morphology s e v e r a l of the l a r g e s t c r y s t a l s were s e l e c t e d and c a r e f u l l y o r i e n t e d under a microscope so that a l l a axes of the c r y s t a l s were c o l l i n e a r . The s u s c e p t i b i l i t i e s from 1.7° to 60°K both p a r a l l e l and perpendicular to the & a x i s are shown i n F i g u r e 15. A c l e a r maximum i s observed at approximately 6.8°K i n each d i r e c t i o n . These measurements r e v e a l t y p i c a l i s o ­ l a t e d antiferromagnetic l i n e a r chain behavior down to the lowest temperature achieved i n t h i s experiment. The I s i n g chain equations can not be used to f i t these data. Bonner and F i s h e r (56) have performed machine c a l c u l a t i o n s on f i n i t e Heisenberg r i n g s and have been able to estimate the l i m i t i n g behavior f o r an i n f i n i t e r i n g . Applying these r e s u l t s to the experimental data i n both d i r e c t i o n s J/k equals -5.30 (±0.05) °K where J i s as d e f i n e d by the term -2JS-j/Sj i n the Hamiltonian described by Bonner and F i s h e r . The g value g i v i n g the best f i t to theory f o r the perpendicular d i r e c t i o n was 2.10 (±0.01) and 2.03 (±0.01) f o r the p a r a l l e l d i r e c t i o n . As can be seen i n F i g u r e 15, the f i t f o r both d i r e c t i o n s down to 1.7°K i s i n q u i t e good agree­ ment with experiment. Commonly "4+2" t e t r a g o n a l l y d i s t o r t e d copper complexes e x h i b i t two small g values ( g ) and one l a r g e g value (S, J)· (57) I f i t i s assumed that the smallest g value l i e s along the a^ a x i s , corresponding to the s h o r t e s t bond d i s t a n c e , then one of the other small g values and the l a r g e s t g value l i e i n the plane perpendicular to t h i s a x i s . The s u s c e p t i b i l i t y perpendicular z

x

y

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchange

135

Interactions

Journal of the American Chemical Society

Figure

14.

Structure

(52)

of [CufC H N XN0 ) ']n 4

h

i

8

i

Tempera tu re *K

Journal of Chemical Physics

Figure 15. Best fit of the corrected magnetic susceptibility to the one-dimensional Heisenberg model with T/k = -5.30° for both directions and g == 2.03 and gj_ — 2.10 (55) n

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

136

EXTENDED

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BETWEEN

METAL

IONS

to the a a x i s should then r e f l e c t a g value intermediate between the two extremes detected by the EPR measurements i f one assumes a random o r i e n t a t i o n of the b and c axes. T h i s i s c o n s i s t e n t with the experimental o b s e r v a t i o n . The pathway of the exchange i n t e r a c t i o n must now be i d e n t i ­ f i e d . N i t r a t e ions bridge copper(II) ions i n Cu(N0 ) *2.5H 0 forming a crooked c h a i n . Even though t h i s chain i s present the magnetic s u s c e p t i b i l i t y , which d i s p l a y s a rounded maximum at 3.2 °K, has been adequately d e s c r i b e d as a r i s i n g from antiferromagn e t i c a l l y exchange coupled p a i r s (58,59) with the predominant mode of exchange thought to occur between the chemical chains.(60) The s h o r t e s t Cu-Cu s e p a r a t i o n (^4.7 A) i s between copper atoms i n the crooked c h a i n . Much weaker a n t i f err omagne t i c interdimer ex­ change e f f e c t s were i n c l u d e d to improve the f i t at low tempera­ t u r e s . As shown i n F i g u r 16 possibilit fo thi nitrat b r i d g e e x i s t s i n the bCu-Cu s e p a r a t i o n i s 5. i t i e s dimeric s u s c e p t i b i l i t y behavior was explored i n Cu(pyrazine)(Νθ3)2· When the dimer s u s c e p t i b i l i t y equation was used to f i t the data a value of J/k » -5.4°K r e s u l t s when the g v a l u e was r e s t r i c t e d to the range 2.00 to 2.20. However, the c a l c u l a t e d s u s c e p t i b i l i t i e s are c o n s i s t e n t l y lower than the experimental values with the d i f f e r e n c e i n c r e a s i n g as the temperature de­ creases. 3

2

2

Although an exchange pathway through the n i t r a t e i o n cannot be completely r u l e d out i n Cu(pyrazine)(N03)2> the c l e a r l i n e a r c h a i n behavior i n t h i s compound as compared to the p a i r i n t e r ­ a c t i o n i n Cu(N03)2·2.5H20 argues s t r o n g l y a g a i n s t t h i s pathway f o r exchange. The r e s u l t s with the s u b s t i t u t e d p y r i d i n e complexes to be d e s c r i b e d below o f f e r s convincing evidence against t h i s pathway, however, before that can be presented the p o s t u l a t e d mechanism of the exchange i n t e r a c t i o n through the pyrazine bridge w i l l be d e s c r i b e d . The p y r a z i n e r i n g i s p e r p e n d i c u l a r to the plane i n which the b i s ( n i t r a t o ) c o p p e r ( I I ) u n i t s l i e s , and the unpaired e l e c t r o n , i n the s i n g l e i o n approximation, i s i n the d 2 . 2 o r b i t a l . Now i t develops that the pyrazine r i n g i s canted wïth respect to the xy plane such that the highest energy occup i e d molecular o r b i t a l , the B^ π o r b i t a l , has the proper symme­ t r y to overlap with the d 2 „ 2 o r b i t a l . I t i s suggested here that the exchange i s propagated i n t h i s manner. To t e s t t h i s mechanism complexes have been prepared with s u b s t i t u t e d pyrazines with the expectation that the exchange c o u p l i n g would be a f f e c t e d by the e l e c t r o n i c nature of the s u b s t i t u e n t on the p y r a z i n e r i n g . Ex­ change by means of the a l t e r n a t e pathway through the n i t r a t e " b r i d g e " would be dependent on the s t e r i c p r o p e r t i e s of the sub­ s t i t u e n t and only s e c o n d a r i l y dependent on the e l e c t r o n i c nature. y

x

The experimental data which have been c o l l e c t e d (61,62) are tabulated i n Table IV. The s i m i l a r i t y of the s p e c t r a an3"tne g values can be taken as good evidence that the s t r u c t u r e s of the complexes are the same as that of the pyrazine complex. As can be

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

HATFIELD

Superexchange

Interactions

c

Journal of Chemical Physics

Figure 16. A view of Cu(pyrazineXNO$) in the be plane which illustrates the weak nitrate bridge between copper atoms. For clarity only copper atoms lying at (Oil) have been included (55). 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

138

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METAL

IONS

seen i n Table IV there i s a s t r i k i n g c o r r e l a t i o n of the l i g a n d π«-ΗΓ* t r a n s i t i o n with 2J.There i s a l s o a c o r r e l a t i o n between 2J Table IV S p e c t r a l and Magnetic Data f o r [Cu(R-pyrazine) ( N C O J ligand

LIGAND

ΤΓ—TT* , cm

quinoxaline

42,300

pyrazine

38,460

methyl pyrazine

37,90

chloro pyrazine

33,700

^ 2J,cm" -9.0

1

2.15

complex d-d,cm" π -KJ*cm~ 18,500 29,000

-7.2

2.133

17,860

34,600

-2.8

2.153

17,800

36,400

and the charge t r a n s f e r band i n the complex which i s most l i k e l y 7r(ligand)-*J*(metal). Since there i s such a good c o r r e l a t i o n of 2J with the e l e c t r o n i c p r o p e r t i e s of the l i g a n d , and none at a l l with the s i z e of the s u b s t i t u e n t (which should separate the chains thereby a f f e c t i n g exchange through the n i t r a t e i o n ) , i t seems c o n c l u s i v e that the exchange pathway i s v i a the pyrazine bridge. IV. Acknowledgements T h i s research has been supported by the N a t i o n a l Science Foundation through grant number GP-22887 and by the M a t e r i a l s Research Center of the U n i v e r s i t y of North C a r o l i n a through grant number GH-33632 from the N a t i o n a l Science Foundation. I am g r a t e f u l f o r t h i s c o n t i n u i n g support. T h i s research e f f o r t has b e n e f i t e d g r e a t l y from c o l l a b o r a t i o n with P r o f e s s o r D.J. Hodgson and from the work of s e v e r a l i n d u s t r i o u s graduate students and research a s s o c i a t e s , many of whom are named i n the r e f e r e n c e s . T h e i r c o n t r i b u t i o n to t h i s program has been i n v a l u a b l e . V. L i t e r a t u r e C i t e d 1. For reviews see a. R.L. M a r t i n in New Pathways in Inorganic Chemistry,Edited by E.A.V. Ebsworth, A.G. Maddock, and A.G. Sharpe, Cambridge U n i v e r s i t y Press, 1968. b. E. Sinn, Coordn. Chem. Rev., 5., 313 (1970). c. G.F. Kokoszka and G. Gordon i n T r a n s i t i o n Metal Chemistry V o l . 5, E d i t e d by R.L. C a r l i n , Marcel Dekker, Inc., New York, 1969. d. J.B. Goodenough,Magnetism and the Chemical Bond, I n t e r s c i e n c e P u b l i s h e r s , Inc., New York, 1963.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchange

Interactions

139

2. T. Moriya, Phys. Rev., 120, 91 (1960); T. Moriya in Magnetism, T r e a t i s e Modern Theory of Matter, 1, 85 (1963). 3. M.E. L i n e s , A.P. Ginsberg, R.L. M a r t i n , and R.C. Sherwood, J. Chem. Phys., 57, 1 (1972). 4. P. Pfeiffer and H. G l a s e r , J. Prakt. Chem., 151, 134 (1938). 5. F.G. Mann and H.R. Watson, J. Chem. Soc., 2772 (1958). 6. W.E. H a t f i e l d , T.S. P i p e r , and U. Klabunde, Inorg. Chem., 2, 629 (1963). 7. D.W. Meek and S.A. Ehrhardt, Inorg. Chem., 4, 584 (1965). 8. J.R. Wasson, T.P. M i t c h e l l , and W.H. Bernard, J. Inorg. Nucl. Chem., 30, 2865 (1968). 9. T.P. M i t c h e l l , W.H. Bernard, and J.R. Wasson, Acta C r y s t . , B26, 2096 (1970). 10. J.R. F e r r a r o and W.R. Walker, Inorg. Chem., 4, 1382 (1965). 11. W.R. McWhinnie, J. 12. J.C.D. Brand and G Organic Chemistry, Oldbourne Press, London (1965). 13. B.J. Cole and W.H. Brumage, J. Chem. Phys., 53, 4718 (1970). 14. E.D. Estes, W.E. H a t f i e l d , and D.J. Hodgson, Inorg. Chem., i n press. 15. D.L. Lewis, K.T. McGregor, W.E. H a t f i e l d , and D.J. Hodgson, Inorg. Chem., 13, 1013 (1974). 16. D.L. Lewis, W.E. H a t f i e l d , and D.J. Hodgson, Inorg. Chem., 13, 147 (1974). 17. P. Krähmer, M. Maaser, K. S t a i g e r , and E. U h l i g , Z. Anorg. Allgem. Chem., 354, 242 (1967). 18. A. Bondi, J. Phys. Chem., 68, 441 (1964). 19. L. P a u l i n g , The Nature of the Chemical Bond, 3rd ed., C o r n e l l U n i v e r s i t y Press, Ithaca, New York (1960). 20. D.L. Lewis, W.E. H a t f i e l d , and D.J. Hodgson, Inorg. Chem., 11, 2216 (1972). 21. D.Y. J e t e r , D.L. Lewis, J.C. Hempel, D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Chem., 11, 1958 (1972). 22. R.L. Gustafson and A.E. M a r t e l l , J. Amer. Chem. Soc., 81, 525, (1959). 23. D.D. P e r r i n and V.S. Sharma, J. Inorg. Nucl. Chem., 28, 1271, (1966). 24. C.M. H a r r i s , E. Sinn, W.R. Walker, and P.R. Woolliams, Aust. J . Chem., 21, 631 (1968). 25. A.T. Casey, Aust. J. Chem., 25, 2311 (1972). 26. A.T. Casey, B.F. Hoskins, and F.D. W h i l l a n s , Chem. Commun., 904 (1970). 27. J.A. Barnes, W.E. H a t f i e l d , and D.J. Hodgson, Chem. Commun., 1593 (1970). 28. J.A. Barnes, D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Chem., 11, 144 (1972). 29. R.J. Majeste and E.A. Meyers, J . Phys. Chem., 74, 3497 (1970). 30. K.T. McGregor, N.T. Watkins, D.L. Lewis, R.F. Drake, D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Nucl. Chem. L e t t e r s , 9, 423 (1973).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

140

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

31. E.D. Estes, p r i v a t e communication. 32. See, f o r example, C. Chow, K. Chang, and R.D. W i l l e t t , J . Chem. Phys., 59, 2629 (1973). 33. K.T. McGregor and W.E. H a t f i e l d , to be p u b l i s h e d . 34. D.J. Hodgson, Progress in Inorganic Chemistry, E d i t e d by S.J. L i p p a r d , W i l e y - I n t e r s c i e n c e , New York, in p r e s s . 35. D.J. Hodgson, P.K. Hale, J.A. Barnes, and W.E. H a t f i e l d , Chem. Comm., 786 (1970). 36. D.J. Hodgson, P.K. Hale, and W.E. H a t f i e l d , Inorg. Chem., 10, 1061 (1971). 37. J.A. Barnes, D.J. Hodgson, and W.E. H a t f i e l d , Chem. Phys. Letters, 7, 374 (1970). 38. K.T. McGregor, D.B. Losee, D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Chem., 13, 756 (1974). 39. a. J.A. Carrabine 92, 369 (1970);b. Molec. Biol., 61, 287 (1971); c. J.P. D e c l e r c q , M. Debbaudt, and M. Van Meerssche, B u l l . Soc. Chim. Belges, 80, 527 (1971). 40. R.F. Drake, V.H. Crawford, N.W. Laney, and W.E. H a t f i e l d , Inorg. Chem.,13, 1246 (1974). 41. V.F. Duckworth and N.C. Stephenson, A c t a C r y s t a l l o g r . , Sect. B, 25, 1795 (1969). 42. D.Y. J e t e r , D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Chim. Acta, 5, 257 (1971). 43. D.H. Svedung, Acta Chem. Scand., 23, 2865 (1969). 44. N.T. Watkins, E.E. Dixon, V.H. Crawford, K.T. McGregor, and W.E. H a t f i e l d , Chem. Commun., 133 (1973). 45. W.E. H a t f i e l d and T.S. P i p e r , unpublished o b s e r v a t i o n s . 46. J.S. Smart, E f f e c t i v e F i e l d Theories of Magnetism, W.B. Saunders Co., P h i l a d e l p h i a , 1966. 47. For example, see J.A. Bertrand, A.P. Ginsberg, R.I. Kaplan, G.E. Kirkwood, R.L. M a r t i n , and R.C. Sherwood, Inorg. Chem., 10, 240 (1971). 48. W.E. H a t f i e l d and R. Whyman, T r a n s i t i o n Metal Chemistry, E d i t e d by R.L. C a r b i n , Marcel Dekker, Inc., New York, V o l . 5, p. 53ff (1969). 49. D.P. Graddon, R. Schulz, E.C. Watton, and D.G. Weeden, Nature, 198, 1299 (1963). 50. A. Santoro, A.D. M i g h e l l , and C.W. Reimann, A c t a C r y s t . , B26, 979 (1970). 51. See f o r example, D.E. Billing, A.E. U n d e r h i l l , D.M. Adams, and D.M. M o r r i s , J. Chem. Soc. (A), 902 (1966); M.J.M. Campbell, R. Grzeskowiak, and F.B. T a y l o r , ibid., 19 (1970). 52. J.F. Villa and W.E. H a t f i e l d , J . Amer. Chem. Soc., 93, 4081 (1971). 53. M.E. F i s h e r , J. Math Phys., 4, 124 (1963). 54. G.F. Kokoszka and C.W. Reimann,J. Inorg. Nucl. Chem., 32, 3229 (1970). 55. D.B. Losee, H.W. Richardson, and W.E. H a t f i e l d , J . Chem. Phys., 59, 3600 (1973).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchange

Interactions

141

56. J. Bonner and M. F i s h e r , Phys. Rev. Sect. A, 135, 640 (1964). 57. E. König Magnetic P r o p e r t i e s of T r a n s i t i o n Metal Compounds., S p r i n g e r - V e r l a g , B e r l i n , 1966. 58. L. Berger, S. F r i e d b e r g , and J. Schriempf, Phys. Rev., 132, 1057 (1963). 59. B. Myers, L. Berger, and S. F r i e d b e r g , J. A p p l . Phys., 40, 1149 (1969). 60. J. Bonner, S. F r i e d b e r g , H. Kobayashi, and B. Myers, Proc. 12th I n t e r n a t i o n a l Conf. on Low Temp. P h y s i c s , Kyoto (1970). 61. H.W . Richardson and W.E. H a t f i e l d , t o be p u b l i s h e d . 62. H.W. Richardson, W.E. H a t f i e l d , H.J. S t o k l o s a , and J.R. Wasson, Inorg. Chem., 12, 2051 (1973).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

11 Electronic and Magnetic Properties o f Linear Chain Complexes Derived from Biscyclopentadienyl Titanium(III) and o f the Infinite

RMX

Linear

3

Chain Complexes D.

SEKUTOWSKI,

R.

JUNGST,

and G .

D.

STUCKY

M a t e r i a l s Research L a b o r a t o r y , U n i v e r s i t y o f Illinois, U r b a n a , Ill. 61801

Introduction T h e f o c u s o f this p a p e r will b e o n the results of experimental studies of the structural and electronic properties o f two types o f linear chain systems containing m e t a l atoms:

and

The results we h a v e obtained for the biscyclopentadienyl c o m p l e x e s , w h i c h are recent a n d of a substantially m o r e preliminary nature, will be described first. A part o f o u r earlier work on the infinite linear tribridged chain systems will t h e n be described partly by way of introduction to the following two p a p e r s by Holt and McPherson on their studies of these materials. Biscyclopentadienyl

Titanium

Derivatives

T h e r e d u c t i o n c h e m i s t r y o f T i ( l V ) h a l i d e s was f i r s t e x t e n s i v e l y s t u d i e d as p a r t o f t h e development o f Z i e g l e r - N a t t a c a t a l y s t s i n t h e l a t e 1950 s. I t was recognized t h a t an important feature o f t i t a n i u m ( i l l ) chemistry i s t h e a b i l i t y of T i ( l l l ) t o a c t as a Lewis 1

142

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

11.

SEKUTOwsKi E T A L .

Linear

Chain

143

Complexes

a c i d , a n d one c a n o b t a i n a v a r i e t y o f c o m p l e x e s d e p e n d ­ i n g upon t h e s o l v e n t , r e d u c i n g agent and a n i o n . For example, t h e r e d u c t i o n o f C p T i C l w i t h Zn p r o c e e d s ac­ cording to: 2

2Cp TiCl

+

2

ZnCl

2 C

} or

2Cp TiCl 2

+

2

2

H

i

e

[Cp TiCl] ZnCl 2

2

2

Zn DME

[Cp TiCl] 2

+

2

ZnCl

[Cp Ti(DME)] [Zn Cl ] 2

2

2

2

6

[Cp TiCl] 2

+ 2

DM T h e 2:1 c o m p l e x was f i r s t r e p o r t e d b y S a l z m a n n i n 1968 (JL). T h e 1:1 complex has not been p r e v i o u s l y r e ­ p o r t e d a n d was o b t a i n e d b y a d d i t i o n o f d i m e t h o x y e t h a n e t o a s o l u t i o n o f t h e 2:1 complex. The s t r u c t u r a l c h e m i s t r y o f t h e s e mixed m e t a l T i ( l l l ) compounds i s i n t e r e s t i n g . The m o l e c u l a r s t r u c ­ t u r e o f t h e 1:1 z i n c c o m p l e x a s p r e p a r e d w i t h d i m e t h ­ o x y e t h a n e was r e c e n t l y d e t e r m i n e d b y u s a n d i s shown i n f i g u r e 1. I t contains [Cp Ti(DME)] c a t i o n s and the f i r s t example o f t h e Z n C l " a n i o n , w h i c h i s i s o e l e c tronic with Ga Cl . I t w i l l be i m p o r t a n t t o o u r l a t e r d i s c u s s i o n t o h a v e some i d e a o f w h a t one m i g h t e x p e c t f o r the i n t e r m o l e c u l a r magnetic i n t e r a c t i o n s of b i s cyclopentadienyl d systems. We h a v e e x a m i n e d t h e mag­ netic s u s c e p t i b i l i t y of [Cp Ti(DME)] [Zn Cl ]·C H to τ.5°Κ and found t h a t t h e i n t e r m o l e c u l a r exchange i s l e s s t h a n 1 cm"" . The m o l e c u l a r s t r u c t u r e o f t h e dibenzene s o l v a t e o f t h e 2:1 compound was d e t e r m i n e d b y u s a n d i n d e ­ p e n d e n t l y b y V o n k (_2). O u r c o o r d i n a t e s a r e shown i n f i g u r e 2. The p a r a m a g n e t i c m o l e c u l e s a r e s e p a r a t e d from each o t h e r i n t h e l a t t i c e by benzene m o l e c u l e s . T h e Z n - C l - T i a n g l e i s 90° w i t h i n e x p e r i m e n t a l e r r o r , w h i l e t h e C l - T i - C l a n g l e i s 8 l . 9 ( l ) . The T i - Z n - T i a n g l e i s 173.2(l)° and t h e T i - T i d i s t a n c e i s 6 820(5)S. An obvious q u e s t i o n o f i n t e r e s t i s whether or not t h e r e i s any e v i d e n c e o f exchange c o u p l i n g between t h e two d m e t a l atoms w h i c h a r e s e p a r a t e d by t h e diamagn e t i c z i n c atom. T h e r o t a t i o n o f t h e two C p T i g r o u p s t h r o u g h 90° w i t h r e s p e c t t o e a c h o t h e r s u g g e s t s t h a t any exchange pathway t h r o u g h t h e z i n c atom w h i c h i n ­ volves zinc o r b i t a l s which are orthogonal with respect t o a 90° r o t a t i o n a b o u t t h e l o n g o r t h o g o n a l a x i s o f t h e 2

2

2

2

6

6

1

2

2

2

6

e

2

1

#

1

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6

144

EXTENDED

Figure

2.

INTERACTIONS

Molecular structure of [Cp TiCÏ] ZnCl Solvent molecules not shown. É

É

t

BETWEEN

METAL

· 2C H . R

6

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

11.

SEKUTOwsKi E T A L .

Linear

Chain

145

Complexes

m o l e c u l e must be f e r r o m a g n e t i c . Thus, exchange t h r o u g h ρ , ρ or s p h y b r i d o r b i t a l s on t h e z i n c atom would re­ sult i n ferromagnetic coupling. T o o u r k n o w l e d g e , t h e r e a r e no k n o w n e x a m p l e s o f l i n e a r o r n e a r l y l i n e a r t r i n u c l e a r m e t a l l i c systems i n w h i c h 1,3 m a g n e t i c e x c h a n g e h a s b e e n d e t e c t e d t h r o u g h a d i a m a g n e t i c m e t a l atom. S i n n (3) has r e p o r t e d on t h e m a g n e t i c i n t e r a c t i o n s o f a number o f t r i n u c l e a r s y s t e m s o f t h e t y p e shown b e l o w i n w h i c h t h e a p p r o x i m a t e a r ­ rangement o f t h e m e t a l atoms i s a n i s o s c e l e s t r i a n g l e 3

f

w h e r e M = Cu a n d M i s a v a r i e t y of metals including Mg. No 1,3 m a g n e t i c c o u p l i n g was d e t e c t e d , a l t h o u g h i t s h o u l d be p o i n t e d o u t t h a t t h e m a g n e t i c d a t a were measured o n l y t o l i q u i d n i t r o g e n t e m p e r a t u r e and t h e a b s e n c e o f m a g n e t i c e x c h a n g e was b a s e d o n t h e m a g n i ­ t u d e o f t h e W e i s s c o n s t a n t , Θ. S t u d i e s o f the tempera­ t u r e dependence o f t h e magnetic s u s c e p t i b i l i t y o f Ni (acac) h a v e shown t h a t a n a n t i f e r r o m a g n e t i c ex­ change between t h e t e r m i n a l n i c k e l atoms v i a t h e p a r a ­ m a g n e t i c n i c k e l ( I I ) c e n t r a l atom i s n e c e s s a r y t o f i t the experimental data Q). T h e 2:1 t i t a n i u m - z i n c c o m p l e x e s d o i n f a c t e x ­ h i b i t a n t i f e r r o m a g n e t i c b e h a v i o r a s shown b y t h e mag­ n e t i c s u s c e p t i b i l i t y d a t a ( f i g u r e 3) f o r t h e compound 3

6

[Cp TiBr] ZnBr *2C H 2

2

These d a t a were f i t by the s i n g l e t - t r i p l e t system:

X

m

= |^(τ?θ) t

1

+

x

/

3

2

Van

e x

P

6

6

Vleck

expression f o r a

(-2JAT)]'

1

The d a t a were measured a t a m a g n e t i c f i e l d g a u s s w i t h a Zeeman e n e r g y o f ^ 1 cm" . 1

+ Να o f 10 Κ In Table

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1

146

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

2.00 r

Figure 3. Temperature depend­ ence of molar susceptibility and effective moment per titanium (BM) for lCp TiBr] ZnBr, · 2C H . Diamagnetic correction of —483 X lOr c.g.s. was applied. Theoretical curves cakuhted using: J = —15.66 cm , g = 1.94, θ 1.37°K, Να = 260 Χ lOr c.g.s. g

t

6

6

6

1

6

300

MO DIAGRAMS* FOR M(h -C H ) CI 5

5

0

ΠΊ4ρ

Tv4p

Figure 4. Molecular orbital di­ agrams for d° and d biscyclopentadienyl systems 1

Ti

d

v

V

cr

* ARROWS DESIGNATE NUMBER OF ELECTRONS IN HOMO

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5

2

2

11.

SEKUTOwsKi E T A L .

Linear

Chain

Complexes

147

are the r e s u l t s of a s i m i l a r a n a l y s i s f o r the c h l o r i d e and f o r t h e d i m e r i c s p e c i e s ; [ C p T i X ] , X = CI, B r as r e p o r t e d b y C o u t t s , W a i l e s and M a r t i n (5). Martin (6) 2

Table

2

1 x

J(cm" ) [Cp TiCl] ZnCl -2C H

e

[Cp TiBr] ZnBr «2C H

6

2

2

2

2

[Cp TiBr]

2

2

2

2

e

6

-15.66

•125

a

From r e f e r e n c e

-8.93

J

l

J

B r

=

1.75

/ J ,C I =

I.60

*

/

J

C l

-78

a

[Cp TiCl]

a.

2

2

5

has p o i n t e d out t h a t r a n s f e r i n t e g r a l and hence t h e exchange c o u p l i n g s h o u l d become l a r g e r a s t h e e l e c t r o n e g a t i v i t y o f t h e a n i o n d e c r e a s e s a n d t h i s a p p e a r s t o be t h e c a s e f o r t h e t r i m e t a l l i c systems as w e l l . However, as i n d i c a t e d below, t h i s s i m i l a r i t y may v e r y w e l l be f o r t u i t o u s a n d , i n f a c t , t h e r e i s e v i d e n c e t h a t t h e exchange mechanisms i n t h e b i n u c l e a r and t r i n u c l e a r systems a r e q u i t e different. F o r t u n a t e l y , t h e r e i s some e x p e r i m e n t a l a n d t h e o r e c t i c a l information a v a i l a b l e concerning the e l e c t r o n i c d i s t r i b u t i o n about t h e m e t a l atom i n b i s c y c l o p e n t a dienyl d systems w h i c h l e a d s t o a p l a u s i b l e mechanism f o r the observed antiferromagnetic coupling. D a h l and P e t e r s e n ( 7 ) 5 ( 8 ) have r e c e n t l y d e m o n s t r a t e d by s i n g l e c r y s t a l e s r e x p e r i m e n t s and p h o t o e l e c t r o n spectroscopy that the unpaired e l e c t r o n i n the d system, C p V C l , i s p r i m a r i l y i n a molecular o r b i t a l of a symmetry w h i c h i s 3-5 ev below t h e l o w e s t u n o c c u p i e d molecular o r b i t a l ( f i g u r e 4). The u n p a i r e d e l e c t r o n d e n s i t y i s p r i m a r i l y i n a metal o r b i t a l which i s perpendicular t o t h e t w o f o l d symmetry a x i s o f C p V C l and i n t h e p l a n e o f t h e VC1 group. A s m a l l e r amount o f u n p a i r e d d e n s i t y i s i n a d o r b i t a l which l i e s along the b i s e c t o r of t h e C l - V - C l g r o u p ( T a b l e 2). T h e same o r d e r i n g o f l e v e l s was o b t a i n e d t h e o r e t i c a l l y f o r C p T i C l , a n d i t seems l i k e l y t h a t a s i m i l a r e l e c t r o n i c d i s t r i b u t i o n about t h e t i t a n i u m atom w i l l p r e v a i l i n [ C p T i X ] Z n X (X = C I , B r ) . I n f i g u r e 5* two a n t i f e r r o m a g n e t i c e x c h a n g e p a t h ways a r e shown w h i c h a r e c o n s i s t e n t w i t h t h e s y m m e t r y o f t h e m o l e c u l e and w i t h t h e t h e o r e t i c a l and e x p e r i m e n t a l r e s u l t s o f D a h l and P e t e r s e n . B o t h a r e o f symmetry b i n the p o i n t group D . A d o r b i t a l can a l s o 1

1

2

2

x

2

2

2

2

2

2

2

2

h

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974. 3.3

67.β

Cp

4.9

69.5

M

% Orbital

(a

y

22.3

z

0.3

0.4

ei (Humo) (Homo)

4.4

2

6.1

e

L. D a h l and J . P e t e r s e n P r i v a t e Communication

0.2

0.3

8-2

symmetry)

0.8

p

x

1-6.1

Cl

2

0.9

Character

Table

SEKUTOWSKi E T A L .

Linear

Chain

Complexes

x

Figure

5.

Exchange pathways for tallic Ti(III) complexes

trime-

0.40 r 0.35 h

ooo '

I

1

OO

'

600

'

'

I

1200

I

180.0

1

1

1

240.0

1

300.0

Degrees Kelvin Figure 6. Experimental data (A) and theoretical curve for tem­ perature dependence of molar susceptibility of [Cp TiCl] BeCk ' 2C H . Parameters used in curve generation: J = —6.89 cm' , g = 1.91, θ = 1.87°K, Να = 260 Χ ΙΟ c.g.s. t

6

1

2

G

6

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

150

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

be u s e d r a t h e r t h a n a ρ o r b i t a l g i v i n g a m o l e c u l a r orbital of a symmetry ( D . ) . We n o t e i n p a s s i n g t h a t the combination a t t h e bottom o f t h e f i g u r e which i n ­ v o l v e s t h e more h i g h l y p o p u l a t e d d o r b i t a l (d i n the C n o t a t i o n o f Dahl and Petersen) i s only p o s s i b l e i f a n s o r d o r b i t a l i s a v a i l a b l e on t h e c e n t r a l atom. I n order t o determine i ft h epresence o f a d o r b i t a l i s e s s e n t i a l f o r t h e presence o f magnetic exchange i n t h i s s y s t e m , t h e compound [ C p T i C l ] B e C l * 2 C H was p r e p a r e d and r e c r y s t a l l i z e d from a benzene s o l u t i o n . I t i s isos t r u c t u r a l w i t h t h e z i n c compounds a n d h a s a t e m p e r a ­ t u r e d e p e n d e n t m a g n e t i c s u s c e p t i b i l i t y , w h i c h i s shown in^figure_6 The value o f t h e coupling constant (-6.89 cm" ) i s comparable t o t h a t o f t h e z i n c c h l o ­ r i d e compound. I o r b i t a l s on the c e n t r a 1,3 e x c h a n g e . A s l i g h t l y d i f f e r e n t t y p e o f one d i m e n s i o n a l c h a i n system i s o b t a i n e d w i t h l a r g e r metal i o n s i n t h e central position. With Mn(ll) i ntetrahydrofuran ( T H F ) , t h e s t r u c t u r e , a s s h o w n i n f i g u r e 7, c o n t a i n s a s i x c o o r d i n a t e c e n t r a l m e t a l atom. The spin state cor­ r e l a t i o n diagram f o r a d - d - d s y s t e m ( f i g u r e 8) s u g ­ g e s t s two p o s s i b l e ground s t a t e s f o r < 1* i.e., forJ < J . Here, S represents the t o t a l s p i n s t a t e o f t h e s y s t e m a n d S* i s t h e s p i n s t a t e o b ­ t a i n e d b y c o u p l i n g t h e 1,3 a t o m s . J has been abbrevi­ a t e d as J . T h e (3/2, l ) ground s t a t e w i l l be o b t a i n e d i n t h e above range f o r a l l J < 0, w h i l e t h e (7/2,1) s t a t e w i l l be l o w e s t i n e n e r g y f o r J > 0. T h e e x ­ perimental μ curve f o r [ C p T i C l ] M n C l ( T H F ) i s shown i n f i g u r e 9 . The value o f M extrapolates t o ^ ~ 3 . 8 BM a t 0 ° K w h i c h i s w i t h i n e x p e r i m e n t a l e r r o r o f t h e s p i n o n l y v a l u e o f 3 . 8 7 BM f o r t h r e e u n p a i r e d e l e c ­ t r o n s and a q u a r t e t ground s t a t e , i . e . , J must be n e g a t i v e w i t h a n t i f e r r o m a g n e t i c c o u p l i n g between t h e t i t a n i u m a n d manganese atoms. I t was p o s s i b l e t o o b ­ t a i n an approximate value f o r J o f -o c m " b y f i t t i n g the experimental data, but J c o u l d n o t be determined w i t h any a c c u r a c y s i n c e o n l y s m a l l changes i n t h e c a l c u l a t e d magnetic s u s c e p t i b i l i t i e s over a narrow temperature range r e s u l t e d from l a r g e changes i n J . Zero f i e l d s p l i t t i n g a t t h e d i o n has been neglected i n t h e above c o n s i d e r a t i o n s . P o l y c r y s t a l l i n e e s r m e a s u r e m e n t s h a v e b e e n made a n d i n d i c a t e a z e r o f i e l d s p l i t t i n g o f 0.1 c m " f o r t h i s m a t e r i a l . Further studies o f c^-c^-d coupling arei n progress. I n a d d i t i o n t o t h eb r i d g i n g groups and t h e c e n t r a l m e t a l atom, a n o t h e r p a r a m e t e r w h i c h c a n i n f l u e n c e t h e exchange c o u p l i n g i n b i s c y c l o p e n t a d i e n y l d complexes x

2

2

2

2

2

2

e

6

#

1

1

5

1

IJ13/J12I

f

1

3

1

2

1

f

f

1

2

1

2

2

2

2

2

2

e f f

1

2

1

1

1

2

3

1

5

1

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3

SEKUTOwsKi E T A L .

Figure

7.

Molecular

Linear

Chain

Complexes

structure of [Cp TiCl] MnCh(THF) . soids shown at 50% probability level. t

g

2

Thermal

ellip-

Figure 8. Spin state correlation diagram for (P-cP-d systems. S' is total spin state, S* is spin state obtained by coupling only the terminal paramagnetic centers, J is the exchange integral between terminal and central metals, and ] is exchange integral between terminal metals. 1

u

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

152

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

i s the c y c l o p e n t a d i e n y l r i n g . The m a g n e t i c s u s c e p t i b i l i t y data f o r [ C p T i C l ] shows a s m a l l a n o m a l y j u s t above the t r a n s i t i o n temperature which M a r t i n suggests may be due t o t h r e e d i m e n s i o n a l c o o p e r a t i v e i n t e r a c t i o n s (5)*(9). The most l i k e l y mechanism f o r t h e t h r e e dimensional c o o p e r a t i v e i n t e r a c t i o n would i n v o l v e the hindered r o t a t i o n of the c y c l o p e n t a d i e n y l groups. Do c o o p e r a t i v e p h e n o m e n a o f t h i s n a t u r e h a v e a n y e f f e c t on m a g n e t i c exchange i n t h i s m o l e c u l e o r i n t h e t r i nuclear molecules? When t h e m o n o - m e t h y l s u b s t i t u t e d derivative, [(MeCp) TiCl] i s e x a m i n e d , one f i n d s t h e r e s u l t s shown i n f i g u r e 10 f o r t h e m a g n e t i c s u s c e p t i bility. T h e r e i s more t h a n a t w o f o l d i n c r e a s e i n t h e v a l u e of J t o ^166 cm"" . No s t r u c t u r a l d a t a h a v e b e e n reported for [ C p T i C l ] however a model b a s e d on t h e structural results s t r o n g l y suggests tha be c o u p l e d v i a i n t r a m o l e c u l a r i n t e r a c t i o n s b e t w e e n c y c l o p e n t a d i e n y l r i n g s o n t h e same t i t a n i u m a t o m a n d b e t w e e n c y c l o p e n t a d i e n y l r i n g s on t h e d i f f e r e n t t i t a n i u m atoms w i t h i n a d i m e r . We w o u l d s u g g e s t t h a t t h e mean T i - T i d i s t a n c e w i t h i n a d i m e r f o r t h e v a r i o u s rotameric isomers which are obtained a t h i g h temperat u r e w i l l p r o b a b l y be g r e a t e r t h a n t h a t f o r t h e g r o u n d s t a t e low t e m p e r a t u r e r o t a m e r i c i s o m e r . I n t h e compound [ ( M e C p ) T i C l ] , t h e m e t h y l group " l o c k s i n " t h e ground s t a t e isomer, i . e . , r a i s e s the p o t e n t i a l b a r r i e r to r o t a t i o n . T h i s h a s some i n t e r e s t i n g i m p l i c a t i o n s . I t s u g g e s t s , f o r example, t h a t the o r d e r o f Br I ^ C1 y M a r t i n (5) f o r [ C p T i X ] c o m p l e x e s i s aue t o t h e a n o m a l o u s b e h a v i o r o f X = C I and not X = I , i . e . , t h e i n c r e a s i n g m a g n e t i c exchange does not f o l l o w d e c r e a s i n g e l e c t r o n e g a t i v i t y as noted above, but i n s t e a d f o l l o w s a decrease i n the T i - T i distance. The p r e d o m i n a n t mechanism f o r exchange w o u l d t h e n be a d i r e c t m e t a l - m e t a l i n t e r a c t i o n . T h i s i s cons i s t e n t with the unpaired s p i n d e n s i t y d i s t r i b u t i o n f o u n d by D a h l , w h i c h , however, does n o t e x c l u d e superexchange v i a the b r i d g i n g l i g a n d s . A d d i t i o n a l magnetic s u s c e p t i b i l i t y , h e a t c a p a c i t y , and s t r u c t u r a l s t u d i e s a r e now i n p r o g r e s s a t t h e U n i v e r s i t y o f I l l i n o i s i n o r d e r t o more c l e a r l y d e f i n e t h e a b o v e a n d r e l a t e d s y s tems. I f t h e a b o v e m o d e l i s v a l i d , one w o u l d n o t e x p e c t a s i g n i f i c a n t d i f f e r e n c e i n the magnetic exchange upon methyl s u b s t i t u t i o n i n the t r i n u c l e a r [Cp TiCl] ZnCl compounds. The m a g n e t i c s u s c e p t i b i l i t y data f o r [ ( M e C p ) T i C l ] Z n C l i s shown i n f i g u r e 12 a n d g i v e s a v a l u e o f J o f -7.3 cm"" w h i c h i s comparable to that of the unsubstituted m a t e r i a l . 2

2

2

2

1

2

2

J

>

J

J

o

2

b

s

e

r

v

e

d

b

2

2

2

2

2

2

2

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

11.

SEKUTowsKi E T A L .

Linear

Chain

Complexes

Figure 9. Temperature dependence of effective moment (BM) per trimer of [Cp TiCl] MnCl (THF) . Diamagnetic correction of —416 X 10~ c.g.s. was applied. 2

2

2

2

6

002 h QOO « QO

ι

I

6O0

'

1

I

I

120.0

1800

I

I

24O0

1

1 3000

°K Figure 10. Temperature dependence of molar susceptibility of [(MeCp) TiCl] . Theoretical curve calculated using J = —166 cm , θ = 0.87°Κ,g = 2.06, Να = 260 Χ ΙΟ c.g.s. 2

2

1

Figure 11. Intramolecular of cyclopentadienyl rings in complexes

6

interactions [Cp TiX] 2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

154

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

F i n a l l y , we would l i k e t o note t h a t numerous o t h e r c o m b i n a t i o n s o f m e t a l i o n s ( e . g . , z i r c o n i u m and vanadium) and b r i d g i n g group ( e . g . , - N R j -PR * -OR, -SR) a r e p o s s i b l e i n t h i s s e r i e s and a s y s t e m a t i c ex­ p e r i m e n t a l s u r v e y o f t h e exchange i n t h e s e compounds can be e x p e c t e d t o p r o v i d e , a t t h e v e r y l e a s t , a n em­ p i r i c a l u n d e r s t a n d i n g and c l a s s i f i c a t i o n o f t h e e l e c ­ t r o n i c c o u p l i n g o f t r i a t o m i c m e t a l systems. 2

RMX

3

2

Complexes

Compounds o f t h e t y p e RMX (R = u n i p o s i t i v e c a t i o n , M = divalent t r a n s i t i o n metal c a t i o n , X = halo­ gen) form a l a r g e c l a s s o f one d i m e n s i o n a l svstems w i t h the general s t r u c t u r dimensional propertie s t r a t e d by l o w t e m p e r a t u r e nmr and n e u t r o n d i f f r a c t i o n s t u d i e s ( l l ) . The f a c t t h a t one i s n o t d e a l i n g w i t h a c o m p l e t e l y i s o l a t e d one d i m e n s i o n a l c h a i n i s e v i d e n t i n t h a t t h e R group c a n be used t o "tune" t h e e l e c t r o n i c p r o p e r t i e s o f the t r a n s i t i o n metal as i l l u s t r a t e d i n T a b l e 3. Thus, one f i n d s t h a t t h e r e i s a change o f 0.141 i n t h e n i c k e l - n i c k e l d i s t a n c e g o i n g from R = ( C H ) N t o R = Κ . The c o r r e s p o n d i n g change i n 10 Dq i s 750 cm" and t h e C u r i e - W e i s s c o n s t a n t changes from 0 t o -112°K ( R b N i C l ) . There a r e few, i f any, systems i n w h i c h t h e f i r s t c o o r d i n a t i o n sphere o f t h e m e t a l atom c a n be s o s y s t e m a t i c a l l y and s u b t l y v a r i e d . An i m p o r t a n t q u e s t i o n w h i c h has become o b v i o u s t o many p e o p l e d u r i n g t h e p a s t y e a r i s , "Can one p r e d i c t i f a one d i m e n s i o n a l s t r u c t u r e s u c h a s t h a t shown i n f i g u r e 13 w i l l be o b t a i n e d ? " We have examined t h i s q u e s t i o n from a s t r i c t l y e m p i r i c a l p o i n t o f view w i t h t h e r e s u l t s shown i n f i g u r e 14 and T a b l e 4. There a r e a t l e a s t f i v e d i f f e r e n t configurât i o n s adopted b y RMX compounds ( f i g u r e 1 4 ) : p r i m i t i v e c u b i c , h e x a g o n a l , and v a r i o u s c o m b i n a t i o n s o f h e x a g o n a l (one d i m e n s i o n a l ) and p r i m i t i v e c u b i c packed s t r u c ­ t u r e s . The p u r e l y h e x a g o n a l c o n f i g u r a t i o n i s t h e 2L, one d i m e n s i o n a l s t r u c t u r e d i s c u s s e d p r e v i o u s l y . 2L r e f e r s t o the c r y s t a l l o g r a p h i c repeat d i s t a n c e i n t h e l i n e a r c h a i n d i r e c t i o n . The r a t i o o f h e x a g o n a l ( l i n e a r ) t o c u b i c p a c k i n g i n c r e a s e s f r o m 6 L t o 4L t o 9L. Remembering t h i s , t h e n , a n e x a m i n a t i o n o f known RMX s t r u c t u r e s r e v e a l s t h a t t h e h e x a g o n a l 1-d con­ f i g u r a t i o n i s f a v o r e d by: ( l ) I n c r e a s e d CFSE ( c r y s t a l f i e l d s t a b i l i z a t i o n energies) 2) L a r g e r X groups 3/ L a r g e r R groups ( t o a p o i n t ; ( C 5 H 5 ) N 3

3

4

1

3

3

3

+

4

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

SEKUTOwsKi E T A L .

Linear

Chain

Complexes

0.32 ρ 0.28 0.24 ^ χ

0.20 I

S 0.16 25 §" 0.121

C 3O

CO

0.08 004 0.00J

OO

600

1200

1800

Degrees Kelvin Figure 12. Temperature dependence of molar susceptibility of [(MeCp) TiCl] ZnCl . Theoretical curve calculated using J = -7.26cm , g = 1.92, θ = 0.69°Κ, Να = 260 Χ ΙΟ c.g.s. B

g

a

1

6

Figure 13. One-dimensional structure RMX compounds. R = (CH ),,N\ M = and X = Cl (10). 3

S

of Ni

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4

3

3

CsNiBr

RbNiBr

3

(CH ) NNiBr

3

3

RbNiCl

KNiCl

3

4

CsNiCl

3

3

(CH ) NNiCl

3

3.10(1)

3.12(1)

11,300 11,300

6600

11,600

6500

6300

7100

2.91(1)

3.17(1)

(14),(15)

-112

12,400

7000

2.9^(1) 12,300

(14),(15)

-76

12,600

6800

2.96(1)

-156

(14),(15)

-101 ( 1 4 ) , ( 1 5 )

0 (13)

13,450

6350

C u r i e Weiss Θ(°Κ)

3.054(5)

- 1

15B'(cm )

-1

10Dq(cm )

Ni-Ni distance Ά

Table 3

11.

SEKUTOwsKi E T A L .

Linear

Chain

Table

Complexes

4

Summary o f S t r u c t u r e s o f C s M X

3

Compounds

F

Cl

Br

I

(1.36)

(1.81)

(1.95)

(2.16)

—

2L (16)

2L

(17)

2L (18)

V(0.87)

—

2L (19)

2L (20)

2L (18)

Cr(0.84)

—

2L (21)

2L (22)

2L (18)

Mg(0.65)

a

Mn(0.8o)

6L ( 2 3

Fe(0.76)

6L

(25) 2L (26)

Co(0.78)

9L

(21) 2L (28)

Ni(0.78)

2L (2g) 2L (30)

2L (31) 4L (22)

9L

(24)

Cu(0.69)

—

2L (32)

Cd(0.97)

—

6L (32)

2

(20)

2L (20)

—

—

—

2L (18)

Summary o f S t r u c t u r e s o f RMC1 Compounds 3

(Me) N

Cs

Rb

K

(2.60)

(1.69)

(1.48)

(1.33)

4

Vo(0.87) Cr(0.84) Mn(0.80)

a

—

2L (19)

—

—

2L (16)

—

2L (34)

2L (19) —

9L (18)

6L (35)

tetragonal

Fe(0.76)

—

2L (25)

2L (25)

—

Co(0.78)

—

2L (28)

2L (36)

—

2L (30)

2L (32)

Ni(0.78) Cu(0.69) Cd(0.97) a.

2L (11) —

2L ( 3 3 )

T h e numbers e a c h atom.

2L (38)

—

6L (33)

—

i n parentheses

3L (20) 4L (39)

are the ionic

radius

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

( |+1/2 > t r a n s i t i o n of the esr s p e c t r a of the V bromine and i o d i n e com­ p l e x e s ( f i g u r e 1 5 ) . The s p i n H a m i l t o n i a n p a r a m e t e r s , e x c l u d i n g t h e l i g a n d h y p e r f i n e t e n s o r s a r e shown i n T a b l e 5. The i n c r e a s evident i n both th c o n s t a n t s and i n t h e changes i n t h e g t e n s o r . I n f a c t , we o b s e r v e f o r t h e i o d i n e complex a v e r y u n u s u a l ex­ ample o f an e a r l y t r a n s i t i o n element i n a n e a r l y o c t a ­ h e d r a l f i e l d w i t h both g and g g r e a t e r t h a n t h e f r e e e l e c t r o n g f a c t o r o f 2.0023. T h i s i s q u a l i t a t i v e l y e x p l i c a b l e i n terms o f M c G a r v e y s t h e o r y ( 4 2 ) , ( 4 3 ) o f c o v a l e n c y f o r d systems i n w h i c h t h e l i g a n d m o l e c u l a r o r b i t a l c o e f f i c i e n t s enter i n t o the expression f o r the g t e n s o r , w e i g h t e d by t h e l i g a n d s p i n o r b i t a l c o u p l i n g constant. The l a s t e x p e r i m e n t a l o b s e r v a t i o n t h a t w i l l be d e s c r i b e d i s i l l u s t r a t e d by t h e s i n g l e c r v s t a l e l e c ­ t r o n i c a b s o r p t i o n s p e c t r u m o f C s C r C l (21) ( f i g u r e 16). The c o n c e n t r a t i o n dependence o f t h i s s p e c t r u m i n C s M g C l i s a l s o shown. Two f e a t u r e s o f i n t e r e s t i n t h e s p e c t r u m a r e ( l ) a band a t 22,000 cm" w h i c h i s not ex­ p l a i n e d by l i g a n d f i e l d c a l c u l a t i o n s and (2) an en­ hanced i n t e n s i t y o f s p i n f o r b i d d e n t r a n s i t i o n s by ap­ p r o x i m a t e l y an o r d e r o f magnitude. The p o l a r i z a t i o n o f t h e 22,000 cm" band i s p a r t i c u l a r l y s t r o n g and i t s t e m p e r a t u r e dependence i s not t h a t e x p e c t e d f o r a v i b r o n i c mechanism. The e x p l a n a t i o n f o r t h i s e f f e c t was f i r s t p r o p o s e d by D e x t e r (44) and l a t e r e l a b o r a t e d upon by Day (45) and o t h e r s . In i t s s i m p l e s t form ( f i g u r e 17), a s i n g l e photon r e s u l t s i n t h e f o r m a t i o n o f e i t h e r two e x c i t o n s o r an e x c i t o n and a magnon. I n b o t h c a s e s , t h e t o t a l s p i n symmetry i s c o n s e r v e d . I n C s C r C l , the former r e s u l t s i n a band a t a p p r o x i m a t e l y t w i c e 10 Dq, t h e l a t t e r r e ­ s u l t s i n an enhanced a l l o w e d n e s s f o r t h e s p i n f o r b i d d e n q u i n t e t - t r i p l e t and q u i n t e t - s i n g l e t t r a n s i t i o n . I n summary, t h e i n f i n i t e one d i m e n s i o n a l complexes, RMX , p r o v i d e an u n u s u a l l y b r o a d and i n t e r e s t i n g c l a s s 4

3

->

2

fJ

x

1

3

3

3

1

1

3

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

h

(77°K)

(297°K)

4

l

+ 70 + 2.

4

+ 74. + 1.

B(xl0 cm"

x

+ 70. + 2.

4

A ( x l 0 c m " ) = + 76. + 1.

4

+ 2 + 65. + 2

+ 65.

+ 208O. + 30

2.035 + .002

1.999 + .002

= 1.9760 + .001

2.038 + .002

+ 2.

+ 67.

1.996 + .002

+ 2.

+ 67.

+ 2 1 6 0 + 50.

2.04 + .01

1.9740 + .0006

+ 128Ο + 15.

u

+ 70. + 2.

+ 1320. + 15.

1.999 + .002

3

2.04 + .01

CsMgI

+ 70+2.

+ 1.

+ 4.

3

1.996 + .002

CsMgBr

+ 74. + 1.

= + 76.

+ 961.

= 1.9750 + .0006

1.9730 + .0006

3

Parameters

D ( x l 0 c m " *) = + 8 5 8 . + 7.

g

B(xl0 cm" )

4

A(xl0 cm"

4

D(xi0 cm"

iι

g,,

CsMgCl

Spin Hamiltonian

Table 5

11.

SEKUTOWSKi E T A L .

Linear

Chain

Complexes

161

6.4% Cr^CsMgCI

3

5.0 ί

10.000

15.000

25.000

20.000

Figure 16. Electronic absorption spectrum at 77°K of CsCrCh and CsCrCls doped into CsMgCl (16). Spectra with Ε polarized parallel and perpendicular to the crystallographic axis

29% Cr^CsMgCl

s

5.000

5.000

10,000

10.000

6.000

20.000

25.000

15.000

20,000

25.000

mole

1

(cm ) 1

M°=l S =2 a

ΙΙΙΦ

m-

Cr'

Cr'

ΔΜ;=+Ι

ΔΜ =0

ΔΜ =0

8

8

AS=0

ν - · - exciton • exciton

Figure

17.

Simultaneous

T

ΔΜΪ=*1

ΔΜ!=

pair

AS=0 hi/ w r * - e x c i t o n • magnon

excitation

model

(44, 45)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

cm . 1

162

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

of m a t e r i a l s f o r t h e i n v e s t i g a t i o n o f c o o p e r a t i v e electronic effects. Acknowledgment. The s u p p o r t o f t h e N a t i o n a l S c i e n c e F o u n d a t i o n under G r a n t s NSF-GH-33634 and GP-31016X a r e g r a t e f u l l y acknowledged. The e x p e r i m e n t a l m a g n e t i c s u s c e p t i b i l i t y measurements were made w i t h t h e g r e a t l y appreciated a s s i s t a n c e o f P r o f e s s o r David Hendrickson of t h e U n i v e r s i t y o f I l l i n o i s . Literature Cited 1. 2. 3. 4. 5. 6.

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Salzmann, J. J., H e l v . Chim. A c t a ( 1 9 6 8 ) , 5 1 , 5 2 6 . Vonk, C. G., J. Cryst. M o l . Struct. ( 1 9 7 3 ) , 3, 201. G r u b e r , S. J., J. Chem. Phys. (1968), 49, 2183. G i n s b e r g , A. P., Martin, R. L., and Sherwood, R. C., I n o r g . Chem. ( 1 9 6 8 ) , 7 , 932. C o u t t s , R. S. P., W a i l e s , P. C., and Martin, R. L., J . O r g a n o m e t a l l i c Chem. (1973), 47, 375.

M a r t i n , R. L., in "New Pathways in I n o r g a n i c C h e m i s t r y , " Pages 175-231, E. A. U. E b s w o r t h , A. G. Maddock and A. G. S h a r p e , Cambridge P r e s s , London ( 1 9 6 8 ) . D a h l , L. F. and P e t e r s e n , J. L., private communication. P e t e r s e n , J. L. and D a h l , L. F., J. Amer. Chem. Soc. ( 1 9 7 4 ) , 9 6 , 2248. M a r t i n , R. L. and W i n t e r , G., J. Chem. Soc. (1965),

1965,

4709.

S t u c k y , G. D., A c t a C r y s t . ( 1 9 6 8 ) , B24, 330. Y e l o n , W. B. and Cox, D. Ε., Phys. Rev. B. ( 1 9 7 3 ) , 7 , 2024, and included r e f e r e n c e s . Goodgame, D. M. L. and Weeks, M. J., J. Chem. Soc. ( 1 9 6 4 ) , 1 9 6 4 , 5 1 9 4 . Asmussen, R. W. and Soling, Η., Z. A n o r g . A l l g e m . Chem. (1965), 203, 3. Asmussen, R. W. and B o s t r u p , Ε., A c t a Chem. Scand. (1957), 11, 745.

Li, Ting-i, S t u c k y , G. D. and McPherson, G., A c t a C r y s t . (1973), B29, 1330. McPherson, G. L., K i s t e n m a c h e r , T. and S t u c k y , G. D., J. Chem. Phys. ( 1 9 7 0 ) , 5 2 , 815. McPherson, G. L. and S t u c k y , G. D., J. Chem. Phys. (1972),

57,

3780.

Li, Ting-i, S t u c k y , G. D. and McPherson, G. L., A c t a C r y s t . ( 1 9 7 3 ) , B 2 9 , 1330.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

11.

SEKUTOWSKI

19. 20. 21.

22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45.

E T AL.

Linear

Chain

Complexes

163

Seifert, H. J. a n d Ehrlich, P., Z. A n o r g . A l l g e m . Chem. (1959), 302, 286. Unpublished results. M c P h e r s o n , G. L . , K i s t e n m a c h e r , T. J., Folkers, J. B. a n d S t u c k y , G. D., J. Chem. P h y s . (1972), 57, 3771. Li, Ting-i a n d S t u c k y , G. D., Inorg. Chem. (1973), 12, 4 4 1 . Zalkin, Α . , L e e , K. a n d T e m p l e t o n , D. H., J. Chem. P h y s . ( 1 9 6 2 ) , 37, 697. G o o d y e a r , J. a n d K e n n e d y , D. J., Acta Cryst. (1972), B28, 1640. Kestigian, M., Leipzig, F . D., Croft, W. J. a n d Guidoboni, R. Che (1966) 5 1462 Seifert, H. J. Chem. ( 1 9 6 6 ) , , L o n g o , J. M. a n d Kafales, J . Α., J. Solid State Chem. ( 1 9 6 9 ) , 1, 1 0 3 . Seifert, H. J., Z. A n o r g . A l l g e m . Chem. (1960), 307, 137. Babel, D., Z. Naturforsch. (1965), 20A, 165. T i s h c h e n k o . G. N., Tr. Inst. Krist. A k a d . Nauk SSSR (1955), 11, 9 3 . S t u c k y , G. D., D'Agostino. S. a n d M c P h e r s o n , G. I., J. Amer. Chem. S o c . (1966), 8 8 , 4823. H e x a g o n a l , with a = 12.56, c = 11.56Å. See R e f . 27. Siegel, J . a n d Gebert, Ε . , Acta Cryst. (1964), 17, 790. M o r o s i n , B. a n d G r a e b n e r , E . J. Acta Cryst. (1967), 2 3 , 766. Seifert, H. J. a n d K o k n a t , F. W., Z. A n o r g . Allgem. Chem. (1965), 341, 269. E n g b e r g , A. a n d Soling, H., Acta Chem. S c a n d . (1967), 21, 168. A s m u s s e n , R. W. a n d Soling, H., Z. A n o r g . A l l g e m . Chem. (1956), 2 0 3 , 3 . S c h l u e t e r , A . W., J a c o b s o n , R. A. a n d R u n d l e , R. E . , Inorg. Chem. (1966), 5, 277. Willett, R. D., D w i g g i n s , C., K r u h a r d , R. a n d R u n d l e , R. E . , J. Chem. P h y s . ( 1 9 6 3 ) , 38, 2429. M o r o s i n , B., Acta Cryst. (1972), B28, 2303. M c P h e r s o n , G. L . , K o c h , R. C. a n d S t u c k y , G. D., J. Chem. P h y s . (1974), 6 0 , 1 4 2 4 . M c G a r v e y , B. R., J. Chem. P h y s . (1964), 41, 3743. M c G a r v e y , B. R., J . P h y s . Chem. (1967), 71, 51. D e x t e r , D. L . , P h y s . R e v . ( 1 9 6 2 ) , 126, 1962. D a y , P., Inorganica C h i m i c a Acta R e v i e w s ( 1 9 6 9 ) , 3, 8 1 .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

12 Optical Properties of Linear Chains S. L. H O L T University of Wyoming, Laramie, Wyo.

82070

Compounds o f t h heavy alkali metal ion (Cs or Rb ) o r an organic cation s u c h as (CH3)4N+, M is a first row divalent transition m e t a l ion, and X is Cl¯, Br¯ o r I¯, display unusual magnetic behavior.(1,2) This b e h a v i o r i s predominately o n e - d i m e n s i o n a l and arises in large part because of the o n e - d i m e n s i o n a l m o l e c u l a r structure of these materials. F i g u r e 1 shows t h e chain-like con­ stitution exhibited by all compounds o f this type. These chains consist of face-sharing octahedra. The larger circles r e p r e s e n t the bridging halide ions while the smaller circles r e p r e s e n t the transition ions. As shown in Figure 2, t h e s e c h a i n s a r e physically separa­ ted by t h e cations, in this case, (CD ) N . These cations p r o v i d e the magnetic insulation between c h a i n s with the interchain distance and t h u s t h e strength of interchain interaction being d e p e n d e n t upon t h e size of cation, i.e., the larger the cation the smaller the interchain interaction. 3

4

+

Figure 3 shows t h e p a t h w a y o f t h e e x c h a n g e . As d r a w n , it s u g g e s t s t h a t t h e e x c h a n g e involves primarily the o v e r l a p of the dz orbital o f C a t i o n 1 w i t h t h e ρσ orbital of the bridging anion followed,by the inter­ action of the ρσ electrons w i t h t h e dz o f C a t i o n 2. Indeed, other orbitals may be involved as is schemat­ ically shown in Figure 4. In this c a s e , we h a v e s u p e r e x c h a n g e illustrated for b o t h 9 0 ° b o n d e d c h r o m i u m ( I I I ) i o n s and 90° b o n d e d iron(II) ions. In the upper p a r t o f the figure, we h a v e t h e c h r o m i u m ( I I I ) c a s e and h e n c e , a r e dealing with 3 d-electrons. The d-electrons on C a t i o n 1 a r e f o u n d in t orbitals, as a r e t h o s e on C a t i o n 2. An electron from the ρσ orbital of the ligand is shown as b e i n g virtually exchanged into the e orbitals o f C a t i o n 1. The coupling process of lowest energy requires t h a t the 2

2

2g

g

164

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

12.

HOLT

Linear

165

Chains

(hOl)

ORIENTATION

podia* j

.1200)

I «(IOO I (OOO #

I

NUCLEAR RECIPROCAL RECIPI LATTICE POINTS

Figure

1.

:(OOI)

(002)

I

MAGIN 'OETIC INTENSITY PLANES

ZONE BOUNDARY

A portion of the linear chain of an RMX. compound (adopted from Ref. l)

{

(003)4^

++

Figure

2.

Crystal structure

of (CDJMnCk

(adopted

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

from Réf. I)

166

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Cation 2

Figure 3. The 90° exchange pathway between two interacting metal ions (adopted from Ref. 2)

Cotion I

Cation I Figure bonded

Anion

Anion

Cation 2

Cation 2

4. Superexchange pathway in (a) 90° C^-Cr *; (b) 90° bonded Fe +-Fe * (adopted from Ref. 2). 3

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

12.

HOLT

Linear

Chains

spin that

167

o f t h e v i r t u a l l y e x c h a n g e d e l e c t r o n be t h e same as i n the t2g s e t of C a t i o n 1. The P a u l i E x c l u s i o n P r i n c i p l e t h e n r e q u i r e s t h a t t h e o t h e r e l e c t r o n i n t h e ρσ o r b i t a l o f t h e l i g a n d h a v e a s p i n of o p p o s i t e s i g n ( i n t h i s c a s e down). Because t h e r e a r e no e l e c t r o n s i n t h e eg o r b i t a l s o f C a t i o n 2, o n l y one e x c h a n g e p r o c e s s i s p o s s i b l e , t h a t process t h a t g i v e s r i s e to a - J , i . e . , a n t i p a r a l l e l exchange c o u p l i n g b e t w e e n t h e ρσ e l e c t r o n o f t h e a n i o n and t2g e l e c t r o n s o f C a t i o n 2. The n e t e f f e c t i s t o a l l o w f e r r o m a g n e t i c coupling b e t w e e n C a t i o n 1 and C a t i o n 2 w i t h t h e o v e r a l l be­ h a v i o r shown by t h e compound b e i n g ferromagnetic. I n t h e l o w e r p a r t o f t h e f i g u r e , we h a v e t h e c a s e of 90° exchange betwee tw Fe2+ i o n s I thi t h i n g s are not so c l e a t h e ρσ e l e c t r o n o f eg o r b i t a l s o f C a t i o n 1 i s a n t i f e r r o m a g n e t i c i n n a t u r e . T h i s does not d i c t a t e the o v e r a l l s p i n a r r a n g e m e n t however. As we c a n s e e t h e r e e x i s t s t h e p o s s i b i l i t y for ferromagnetic as w e l l as a n t i f e r r o m a g n e t i c exchange b e t w e e n t h e e l e c t r o n i n t h e ρσ o r b i t a l o f t h e anion and t h e e l e c t r o n s i n t h e d - o r b i t a l s o f C a t i o n 2. The r e s u l t a n t m a g n e t i c b e h a v i o r d e p e n d s o n l y upon w h i c h i s s t r o n g e r , t h e c o u p l i n g b e t w e e n t h e e l e c t r o n i n t h e ρσ o r b i t a l and t h e eg e l e c t r o n s o r t h e c o u p l i n g b e t w e e n t h e ρσ e l e c t r o n and t h e t 2 g e l e c t r o n s . I f t h i s cou­ p l i n g i s s u f f i c i e n t l y s t r o n g to produce p r e f e r e n t i a l a l i g n m e n t and i n v o l v e s s e v e r a l c a t i o n c e n t e r s t h e n c o n d i t i o n s have been f u l f i l l e d f o r the c r e a t i o n of spin-waves. The d r a w i n g i n F i g u r e 5 i s a s c h e m a t i c d e p i c t i o n o f a s p i n - w a v e i n a one d i m e n s i o n a l ferromagnetically coupled system. As one c a n s e e , a s p i n d e v i a t i o n a t one end o f t h e c h a i n c a u s e s s p i n d e v i a t i o n s down t h e l i n e producing the "spin-wave". As a q u a n t u m o f e n e r g y c a l l e d a phonon e x c i t e s a v i b r a t i o n , a quantum of e n e r g y c a l l e d a magnon c a u s e s a s p i n - w a v e . I f one has i s o l a t e d c h a i n s w i t h i n a m a t e r i a l , magnons w i l l i n d u c e spin-waves i n these chains independent of each o t h e r . The p r e s e n c e o f m a g n e t i c c o u p l i n g and t h e induce­ ment o f s p i n - w a v e s c a n be shown e x p e r i m e n t a l l y through i n e l a s t i c - n e u t r o n - s c a t t e r i n g techniques. Figure 6 shows t h e v a r i a t i o n w i t h t e m p e r a t u r e o f t h e e x c i t a t i o n which i s a s s o c i a t e d w i t h the p r e s e n c e of spin-waves i n t h e compound ( C D 3 ) 4 M n C l 3 . As c a n be s e e n t h e s p i n - w a v e i s w e l l d e f i n e d at 1.9°K. T h i s d e f i n i t i o n d e c r e a s e s as one i n c r e a s e s t h e t e m p e r a t u r e t o 4 0 ° K . T h i s marked d e c r e a s e i n i n t e n s i t y i s t o be e x p e c t e d as t h e threedimensional o r d e r i n g t e m p e r a t u r e h a s b e e n shown t o be

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

168

EXTENDED

,., y

y

Figure

5.

y

y

γ

ψ ψ

INTERACTIONS

ψ

γ y

of a spin wave, (a) View in the ac plane, (adopted from Ref. 3).

Schematic

BETWEEN

METAL

y

IONS

y

y

(b) View in the ab plane

TMMC λ · 64 A EXCITATION

* ··· _J

I

I

•

—1

_l

I

I

I

I

L_

V

I

I

l_

Figure 6. Variation with tempera­ ture of the excitation at (0.3, 0,1.10), q,.* = 0.10 reciprocal lattice units compound is (CD ) ~NMnCl (adopted from Réf. 1).

L

Α

_l

_J 0.0

I 1.0

I

I 1 20 30 ENERGY (meV)

h

L.

1 4.0

L_ 5.0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

A

12.

HOLT

Linear

Chains

169

0.84°K. T h u s , as one i n c r e a s e s t h e t e m p e r a t u r e above t h i s , c o n t i n u a l l y g r e a t e r d i s o r d e r and d e c o u p l i n g a l o n g the c h a i n s w i l l o c c u r . One w o u l d e x p e c t , as a g e n e r a l p h e n o m e n o n , t h e p r e s e n c e o f s p i n - w a v e s s h o u l d be l e s s e v i d e n t t h e h i g h e r Τ i s a b o v e TJJ . Our p r i m a r y i n t e r e s t h e r e w i l l be w i t h t h e o p t i c a l m a n i f e s t a t i o n of these spin-waves. F i g u r e 7 shows t h e v a r i o u s t y p e s o f e l e c t r o n i c p r o c e s s e s one i s l i k e l y t o encounter i n a m a g n e t i c a l l y coupled t r a n s i t i o n metal containing material. We s h a l l f o c u s o u r a t t e n t i o n p r i m a r i l y upon t h o s e t r a n s i t i o n s w h i c h a r e s p i n f o r b i d d e n i n n a t u r e , i . e . , those w h i c h undergo a change i n s p i n - m u l t i p l i c i t y , s i n c e i t i s here t h a t the e f f e c t s o f t h e s p i n - w a v e phenomenon on t h e e l e c t r o n i c t r a n s i ­ t i o n s w i l l be s e e n The f i r s t p a r o c c u r s when y o u h a v e a m a g n e t i c a l l y and o p t i c a l l y d i l u t e system i n which the s p i n - m u l t i p l i c i t y may change. T h i s i s a n o r m a l e x c i t a t i o n c a u s e d by a p h o t o n where the s u b l a t t i c e s a c t i n d e p e n d e n t l y o f each o t h e r . In t h e c a s e shown h e r e , we h a v e o n l y t h e e x c i t a t i o n o f an e l e c t r o n f r o m t h e g r o u n d s t a t e on s u b l a t t i c e A i n t o an e x c i t e d s t a t e w i t h t h e a c c o m p a n y i n g s p i n - f l i p . T h i s w i l l u s u a l l y be s e e n i n t h e s p e c t r u m o f a compound as a weak a b s o r p t i o n b a n d . The w e a k n e s s o f s u c h a t r a n s i t i o n a r i s e s b e c a u s e i t i s b o t h p a r i t y and s p i n f o r b i d d e n , AS^O. S u c h i s n o t t h e c a s e i n an o p t i c a l l y c o n c e n t r a t e d s y s t e m where c o o p e r a t i v e e x c i t a t i o n s such as we s e e i n t h e s e c o n d p a r t o f F i g u r e 7 may o c c u r . In t h i s c a s e , by e x c i t i n g an e l e c t r o n f r o m t h e g r o u n d s t a t e i n s u b l a t t i c e A and s i m u l t a n e o u s l y e x c i t i n g an e l e c t r o n w i t h t h e o p p o s i t e s p i n on s u b l a t t i c e B, we c a n c o n s e r v e t h e s p i n - a n g u l a r momentum, i . e . , AS=0. T h i s i s c a l l e d an e x c i t o n · e x c i t o n t r a n s i t i o n . The consequences of the c o n s e r v a t i o n of s p i n i s to p r o v i d e a band or t r a n s i t i o n which i s r e l a t i v e l y i n t e n s e i n comparison to a normal s p i n - f o r b i d d e n t r a n s i t i o n . (In t h e c a s e we h a v e c h o s e n b o t h t h e e x c i t a t i o n on s u b l a t t i c e A and t h e one on s u b l a t t i c e Β a r e by themselves spin-forbidden. T h i s i s n o t an e x c l u s i v e r e q u i r e m e n t f o r an e x c i t o n · e x c i t o n t r a n s i t i o n . ) Note a l s o t h a t i f t h i s e x c i t a t i o n o f A and Β a r e i n d i v i d u a l l y t h e same as t h e s i n g l e e x c i t o n e x c i t a t i o n shown i n t h e f i r s t p a r t of t h i s f i g u r e , then the c o o p e r a t i v e e x c i t o n * e x c i t o n t r a n s i t i o n w i l l o c c u r at a p p r o x i m a t e l y t w i c e the energy of the e x c i t o n - o n l y t r a n s i t i o n . In t h e t h i r d p a r t o f t h e f i g u r e , we h a v e an exciton+magnon t r a n s i t i o n . T h i s i s the f i r s t of these v a r i o u s phenomena w h i c h a r e s t r i c t l y c h a r a c t e r i s t i c o f a m a g n e t i c a l l y concentrated system. The f i r s t two

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

170

EXTENDED

—

* - t é A

INTERACTIONS

—

r

*

ι

- t é A

Β

1

- i é

B

EXCITON

EXCITON • EXCITON

-4-

—μ

1

METAL

r

•

-H-

BETWEEN

I

- t é Α

Β

EXCITON - MAGNON

EXCITON+MAGNON

Figure 7. Exciton and exciton · magnon processes A and B refer to opposite sublattices (adopted from Ref. 2)

FINE S T R U C T U R E O F \ TRANSITION

q

(*S) —

4

T ( G) | f

4

2.2*K

Figure 8. Exciton (El, E2) and exciton + magnon (VI, σΐ, σ2) transitions in MnF (adopted from Réf. 4) 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

12.

HOLT

Linear

Chains

171

p r o c e s s e s , t h e e x c i t o n and e x c i t o n · e x c i t o n t r a n s i t i o n s a r e c h a r a c t e r i s t i c o f m a g n e t i c a l l y d i l u t e s y s t e m s as w e l l as m a g n e t i c a l l y c o n c e n t r a t e d s y s t e m s . The c a s e o f the exciton+magnon e x c i t a t i o n r e q u i r e s a m a g n e t i c a l l y c o u p l e d system, however. The exciton+magnon t r a n s i t i o n is a combination of a normal s p i n - f o r b i d d e n t r a n s i t i o n s u c h as shown on s u b l a t t i c e A a n d a s p i n - d e v i a t i o n o f low e n e r g y , s u c h as shown on s u b l a t t i c e B. Here a g a i n , j u s t as i n t h e e x c i t o n · e x c i t o n t r a n s i t i o n we s e e t h a t t h e s e l e c t i o n r u l e AS=0 i s o b e y e d . I n o t h e r w o r d s , we h a v e an u p - s p i n on A g o i n g t o a d o w n - s p i n s i m u l t a n e ­ o u s l y w i t h a d o w n - s p i n on Β g o i n g t o an u p - s p i n . The d i f f e r e n c e b e t w e e n t h i s and an e x c i t o n · e x c i t o n t r a n s i ­ t i o n i s t h a t t h i s p r o c e s s a r i s e s o n l y i n the case o f magnetically coupled system d th f th allowing excitatio n i t u d e than t h a t f o sition . The l a s t d i a g r a m i n F i g u r e 7 shows a s e c o n d pro­ c e s s by w h i c h t h e p r e s e n c e o f a c o o p e r a t i v e m a g n e t i c i n t e r a c t i o n i n t h e s y s t e m h e l p s t o a l l o w an e l e c t r o n i c excitation. This i s c a l l e d the exciton-magnon. In t h i s c a s e , we h a v e , as b e f o r e , a s p i n - f o r b i d d e n e x c i ­ t a t i o n on s u b l a t t i c e A b u t as o p p o s e d t o t h e e x c i t o n + magnon, we h a v e a l r e a d y c r e a t e d an e x c i t a t i o n on s u b l a t t i c e Β w h i c h t h e n d e c a y s a t t h e same t i m e as t h e e x c i t a t i o n o c c u r s on s u b l a t t i c e A. As we c a n s e e , t h e s p i n i s c o n s e r v e d i n t h i s s y s t e m as w e l l as i n t h e p r e v i o u s two s y s t e m s . One o b s e r v a b l e d i f f e r e n c e b e t w e e n t h e e x c i t o n + magnon a n d e x c i t o n - m a g n o n i s t h e i r p o s i t i o n r e l a t i v e t o the pure e x c i t o n l i n e . I t s h o u l d be c l e a r from F i g u r e 7 t h a t t h e exciton+magnon w i l l occur at h i g h e r e n e r g i e s than the pure e x c i t o n l i n e , w h i l e the exciton-magnon w i l l occur at lower e n e r g i e s than the pure e x c i t o n line. T h e c a s e f o r t h e e x c i t o n + m a g n o n i s shown q u i t e g r a p h i c a l l y i n F i g u r e 8. T h e s e l a t t e r t h r e e e x c i t a t i o n s a l l h a v e one t h i n g i n common and t h a t i s t h e i r u n u s u a l i n t e n s i t y . The v a r i a t i o n o f t h e s e i n t e n s i t i e s , as a f u n c t i o n o f Τ w i l l d i f f e r , however. F i g u r e 9 shows t h e c a l c u l a t e d temp­ e r a t u r e dependence f o r both the exciton+magnon ( c o l d band) and e x c i t o n - m a g n o n ( h o t band) c a s e s . This sug­ g e s t s t h a t a major change i n o s c i l l a t o r s t r e n g t h s h o u l d occur below TJT. The m a g n i t u d e o f t h i s change w i l l d e p e n d u p o n t h e r e l a t i v e c o n t r i b u t i o n o f magnon h o t and c o l d b a n d s and p h o n o n modes. F i g u r e 10 shows t h e c a l c u l a t e d t e m p e r a t u r e depend­ e n c e ( s o l i d l i n e ) f o r a two e x c i t o n t r a n s i t i o n i n RbMnF3. As c a n be s e e n f r o m t h e e x p e r i m e n t a l result

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

i.

INTERACTIONS

BETWEEN

METAL

Spin Wovt Theofy

=> 2 >» w

α

!

5«

^

Hot Bond

100

200

Temperature

300

( K)

Figure 9. Calculated temperature dependence of magnon side bands in MnF> (adopted from Ref. 5)

Two Exciton Transition

Kid*

2[ A ( S)— T, ( G)] e

l e

e

4

4

e

7.0

60«έ

H50 ? H40

100

200

300 Κ

30

Temperature Figure 10. Temperature dependence of two-exciton absorption in RbMnF Ref. 6) {

of the intensity (adopted from

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

12.

Linear

HOLT

173

Chains

(dashed l i n e ) the agreement i s r e l a t i v e l y good. F i g u r e 11 p r o v i d e s us w i t h t h e e x p e r i m e n t a l l y d e t e r m i n e d i n t e n s i t y v a r i a t i o n of exciton-magnon t r a n s i t i o n s in RbMnF3 and MnF2. The c r i t e r i a t h e n f o r i d e n t i f y i n g magnon assisted transitions a r e : 1) t o l o o k t o e n e r g i e s s l i g h t l y h i g h e r and s l i g h t l y l o w e r t h a n t h e p a r e n t spin-forbidden t r a n s i t i o n f o r anomously i n t e n s e m a n i f o l d s and 2) ascertain i f an a n o m o l o u s i n t e n s i t y c h a n g e o c c u r s i n the r e g i o n of the Néel p o i n t . C r i t e r i o n 1) s h o u l d be q u a l i f i e d i n t h a t o n l y t h e magnon a s s i s t e d transitions may be v i s i b l e , t h e p a r e n t l i n e b e i n g t o o weak t o be observed. One o f t h e RMX3 s y s t e m s , i n w h i c h we h a v e h a d considerable interes metal ion n i c k e l . Tabl l o g r a p h i c p a r a m e t e r s f o r a number o f n i c k e l - c o n t a i n i n g compounds. A l s o i n c l u d e d a r e the room t e m p e r a t u r e m a g n e t i c moments, t h e i r W e i s s c o n s t a n t s a n d , w h e r e known, an i n d i c a t i o n o f t h e i r N é e l t e m p e r a t u r e s . As c a n be s e e n , t h e i n t e r c h a i n d i s t a n c e s v a r y f r o m 9.35 A i n t h e c a s e o f ( C H ^ ^ N i B ^ , down t o 6.85 A i n the case of ΤΙΝΙΟΙβ. T h i s t h e n s h o u l d g i v e us a l a r g e r a n g e i n which to look at the e f f e c t of i n t e r - versus intrachain coupling. To t h i s moment, t h e m e a s u r e m e n t s t h a t we h a v e made have been r e s t r i c t e d to ( C H 3 ) 4 N N i C l , C s N i C l 3 , CsNiBr3, RbNiCl3 * RbNiBr3. Indeed, even i n the case of the ( C H 3 ) 4 N i C l 3 we s e e f r o m T a b l e I t h a t i t h a s b e e n f o u n d t o be f e r r o m a g n e t i c , consequently s h o u l d n o t and, i n f a c t , d o e s n o t e x h i b i t any magnon a s s i s t e d transitions in i t s o p t i c a l spectrum. C o n s e q u e n t l y , our d i s c u s s i o n i s r e s t r i c t e d to the a l k a l i m e t a l s a l t s of the nickel halides. I n F i g u r e 12 we s e e t h e s p e c t r u m o f an ^ 2 % solid s o l u t i o n of C s N i C l 3 i n the c o l o r l e s s , non-magnetic diluent CsMgCl3. This i s b a s i c a l l y a normal spectrum f o r the Ni2+ i o n . The a b s o r p t i o n maximum o c c u r i n g a t approximately 7,000 cm" i s t h e ^A2g-*"^T2g(F) t r a n s i ­ tion. T h i s i s f o l l o w e d by t h e s p i n - a l l o w e d ^A2g"** 3 T i g ( F ) t r a n s i t i o n a t 12,000 wave n u m b e r s . Superim­ p o s e d u p o n i t i s t h e s p i n - f o r b i d d e n A2g- * Eg t r a n s i ­ tion. The 3 A 2 g + T 2 g t r a n s i t i o n t h e n l i e s a t ^ 1 8 0 0 0 cm" f o l l o w e d by t h e 3 A 2 - * l A i g t r a n s i t i o n a t 19000 cm" . The s p i n - a l l o w e d A2g^ Ti (P) transition is t h e n o b s e r v e d a t 22000 cm" . This i n turn i s followed by t h e s p i n - f o r b i d d e n A g - ^ T i g ( G) (^24,000 c m " ) , 3 A a - * E ( G ) (25 ,700 cm" ) and 3 A 2 g + T ( G ) (26 ,000 cm-ï) transitions. F i g u r e 13, t h e s p e c t r u m o f C s N i C l 3 a t 5 ° K , shows o

3

a n (

1

3

)

1

1

1

g

1

3

3

g

1

3

1

2

1

2

1

g

1

2 g

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4

3

CsNiBr

R. W. A s m u s s e n

c.

Achiwa, J .

Ν.

b.

3

Phys

43,

-156

-100

-101 C

b

-69

0

b

a

(20°)'°

(20°)

(20°)

(20°)

Appl.

2 .84

3 .34

2 .94

3 .41

(80°)

θ,°κ

TN,°K

1932

11

(1972).

4.5

ferromagnetic

Z. a n o r g . u . a l l g e m . Chem., 2 8 3 , 1 ( 1 9 5 6 ) .

(1969).

eff

3 .16

Willett, J.

7.268(8)

6.955(1)

7.66

7.50

7.18

6.85

6.94

9.35(2)

7.268(8)

J a p a n , 2 7 , 561

a n d H. S o l i n g ,

Phys. Soc.

a n d R. D.

3.104(4)

2.953(1)

3.12

2.97

3.12

3.17(1)

3.577(1)

F . D. G e h r i n g

P6 /mmc

3

Ρ6 /mmc

3

P6 /mmc

3

Ρ6 /mmc

3

P6 /mmc

3

P6 /mmc

Cmcm

3

v

Compounds

,Β .Μ·

o f Some R N i X

Interchain

Parameters

Intrachain

and M a g n e t i c

Group

P6 /m

Space

B. C. G e r s t e i n ,

3

RbNiBr

a.

3

RbNiCl

3

3

CsNiCl

CsNiI

3

3

TlNiCl

3

CH NH NiCl

3

3

3

(CH ) NNiBr

4

3

3

(CH ) NNiCl

Compound

Structural

TABLE I

12.

Linear

HOLT

1 0

,

175

Chains

ι 100

,

ι 200

ι

ι 300

Κ

I

Temperature

Figure 11. Experimental temperature dependence of oscillator strengths for exciton-magnon transition A ( S) -> E,( G), A ( G) in MnF and RbMnF, (adopted from Ref. 7) 6

lff

6

4

4

4

lfl

4

t

Figure

12.

The 5°K

spectrum of CsMgCly.Ni, from Réf. S)

±Z

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(adopted

EXTENDED

_2i

22

IS ENERGY

Figure

13.

14

INTERACTIONS

IQ

BETWEEN METAL

g_

cnrf'XIO"*

The 5 ° Κ spectrum of CsNiCh, from Ref. 8)

±Z

(adopted

ENERGY cn-fXIO"'

Figure 14. The 5°K CsNiBr,, i Z (adopted

spectrum of from Ref. 8)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

12.

HOLT

Linear

Figure

111

Chains

15.

Temperature dependence CsNiBr , ±Z

of the spectrum

of

s

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3

3

CsNiCl

RbNiCl

Compound

u

1

w

1

Polarization

Oscillator

Α

3

A

A

3 2

3

8

g

^A

1

8

l

L

g

l g

(G)

(F)

( 0

( D )

(F)

2g

- T

3

T

3

^ Α

g

l g

2g

- T / E

2 g

(^)

3 A -*· Τ 2g 2g

A

3

3

2

2

2 g

T

3

g

l g

2.5

1.7

2.5

2.2 1.2 4.2

2.7

2.1

2.5

0.3

2.2

1.5

0.8 0.7 1.5

0.6

3.0

0.4

2.9

2.0

0.4

0.2

1.9

1.8

1.8

5.5

1.8

0.4

6.4

4.1

1.1

4.7

1.4

1.1

5

1.8

3

fxl0 (4°K)

i n RNiX

fxl05(77°K)

Transition

fxl0 (300°K)

5

f o r Selected

->\ (G)

3

3

- T

2g*

A

3

A

A

3

2 g

2 g

28-

3

A

A

3

3

A

2

A g^ T2g

3

3

Transition

Strengths

TABLE I I

2

Ο

25

I

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

II

CsNiBr

3

Compound

Table

\\

1

Polarization

Continued

/ E

3

g

- T

A

3

A

A

3

3

%

A

1

(G>

3

A

+ T

* S

2 g ^

2 g

e

A

G

)

(F)

l g

l g

g

(

(F)

l g

l g

v

( D )

l g

2g

- T

3

3 T

2 g ^

2 g

2g^

3

3 A

2

2 g

A „ -*-A, (G) 2g l g

A

3

3

A

3

Transition

·

6

9

7

11.7

·

8.1

'°

3

3

6.0

0.9

5

fxlQ (300°K)

3.6

5.2

2.4

5.4

4.8

0.7

4.8

2.8

8.5

1.4

5.4

1.8

1.9 4.4

1.0

0.9

0.5

2.6

3.3

5 5

fx xl lQ 0 ( (4 4° °K K) ) f

5 5

K) ) fxlQ ( (7 77 7° °K

I*

ο 5

180

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

a marked d i f f e r e n c e i n the r e l a t i v e i n t e n s i t i e s o f s e v e r a l o f t h e b a n d s when c o m p a r e d t o F i g u r e 12. Of p r i n c i p l e i n t e r e s t are t h o s e bands w h i c h are above 18,000 wave n u m b e r s . That band at a p p r o x i m a t e l y 19,000 w h i c h h a s b e e n i d e n t i f i e d as t h e s p i n - f o r b i d d e n ^A2g"** ^ A i g t r a n s i t i o n i s seen to have gained i n t e n s i t y rela­ t i v e t o t h e s p i n - a l l o w e d t r a n s i t i o n s b o t h a b o v e and b e l o w i t . The same may be s a i d f o r t h e h i g h e r e n e r g y s p i n - f o r b i d d e n t r a n s i t i o n s w h i c h l i e above the allowed 3A -* Tig(F) transition. T u r n i n g now to the spectrum of C s N i B r 3 , F i g u r e 14, one c a n a g a i n s e e t h a t t h e same i n t e n s i t y p a t t e r n i s repeated. F i g u r e 15 shows t h e t e m p e r a t u r e d e p e n d e n c e i n the lower energy r e g i o n f o r b o t h the spin-allowed and s p i n - f o r b i d d e n the p r i m a r y f e a t u r r a p i d i n c r e a s e i n the i n t e n s i t y of the f o r m a l l y s p i n f o r b i d d e n bands w i t h d e c r e a s i n g t e m p e r a t u r e . This i s i n c o n t r a s t to the d e c r e a s e i n i n t e n s i t y of the s p i n allowed t r a n s i t i o n s . This rapid increase i n i n t e n s i t y a t T>>T$j i s i n c o n t r a s t t o b o t h work r e p o r t e d by L o h r and M c C l u r e ( ^ ) on some m a n g a n e s e s a l t s and t o t h e w o r k o f F u j i w a r a et_ £ l (_7) c i t e d e a r l i e r , F i g u r e 11. That t h i s b e h a v i o r i s common t o a l l o f t h e antiferromagnetic RMX3 s t u d i e d i s g r a p h i c a l l y shown i n T a b l e I I w h e r e t h e o s c i l l a t o r s t r e n g t h s , both i n the JL C and 11C d i r e c ­ t i o n s , a r e shown as a f u n c t i o n o f t h r e e t e m p e r a t u r e s ; room, 80° and 5 ° K . As may be s e e n , i n a l l c a s e s a r a p i d i n c r e a s e of the i n t e n s i t y of the magnon-assis t e d e x c i t o n l i n e s i n the a l k a l i h a l i d e s i s n o t e d . This s u g g e s t s t h a t the i n t e n s i t y p r o d u c i n g mechanism i s s i m i l a r i n a l l c a s e s and p e r h a p s o f a d i f f e r e n t nature t h a n t h a t s e e n f o r RbMnF3 and MnF2. This l a t t e r point i s one w h i c h d e s e r v e s more s t u d y . Unfortunately, the l i m i t e d amount o f d a t a a v a i l a b l e do n o t a l l o w us t o s o r t out i n t e r - v e r s u s i n t r a c h a i n effects. Close i n s p e c t i o n a l s o shows t h a t t h e p a r e n t e x c i t o n l i n e d o e s n o t a p p e a r t o be d i s c e r n a b l e i n t h e c a s e s shown. The exciton+magnon are c l e a r l y seen, however. 3

2 g

I w o u l d l i k e t o e x p r e s s my t h a n k s t o t h e N a t i o n a l S c i e n c e F o u n d a t i o n f o r s u p p o r t o f t h i s r e s e a r c h and t o J . A c k e r m a n , G. M. C o l e , and Ε . M. H o l t who collabor­ a t e d i n t h i s work. Work p a r t i a l l y supported by the N a t i o n a l Science Foundation grants GP-15432A1 and GP-41506.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

12.

HOLT

Linear

Chains

181

Literature

Cited

1.

H u t c h i n g s , M. T., Shirane, G., Birgeneau, R. and S. L . Holt, P h y s . Rev., (1972), B5, 1999.

2.

A c k e r m a n , J. Inorg. Chim.

3.

Kittel, C., 4th edition,

4.

Sell, Phys.

5.

S h i n a g a w a , K. a n (1971), 30, 1280.

6.

Motizuki, K. a n d (1972), 11, 167.

7.

F u j i w a r a , T., G e b h a r d t , W., P e n t a n i d e s , K. a n d T a n a b e , Υ., J. P h y s . S c o . , J a p a n ( 1 9 7 2 ) , 33, 39.

8.

A c k e r m a n , J., Holt, Ε . M. a n d State Chem., (1974), 9, 279.

9.

L o h r , L . L . a n d M c C l u r e , D. S., J. Chem. ( 1 9 6 8 ) , 49, 3 5 1 6 .

F., Cole, G. M. a n d Acta, (1974),8, 3 2 3 . Introduction W i l e y , New

Holt,

Miyata,

S. L . ,

t o Solid S t a t e Y o r k , (1971) 539.

D. D., G r e e n e , R. L . a n d W h i t e , R e v . , ( 1 9 6 7 ) , 158, 489.

S.,

Sol.

Holt,

J.

Physics,

R. Μ . ,

State

Comm.,

S. L . , J. Sol.

Phys.,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

13 Spectroscopic and Magnetic Properties o f CsMI

3

T y p e Transition Metal Iodides G. L. McPHERSON and L. J. SINDEL

Tulane University, New Orleans,La.70118

As t h e two previous been a great deal of interest in transition m e t a l salts of t h e general formula M(I)M' (II)X (where M(I) is a large univalent cation, Μ' (II) a divalent transi­ tion m e t a l ion, and X a halide ion). These materials often crystallize in hexagonal lattices in which t h e most prominent structural feature is a parallel array of infinite, linear c h a i n s of octahedra sharing faces. The c h a i n s r u n parallel t o t h e crystallographic c-axis with the transition metal i o n s at t h e centers and t h e halide i o n s at t h e c o r n e r s of t h e octahedra (see Figure 1). The magnetic properties o f these types of salts a p p r o a c h t h o s e o f a o n e - d i m e n s i o n a l system o f interacting spins. The hexagonal linear chain struc­ ture is observed with t h e widest variety o f transition metals and halogens when M(I) is a cesium ion. T h i s paper discusses t h e magnetic and spectroscopic proper­ ties o f several cesium metal triiodides which adopt this structure. A l t h o u g h t h e properties o f t h e s e salts are inherently interesting, it is especially informative to compare t h e iodides t o t h e analogous chlorides and b r o m i d e s . 3

The cesium metal triiodides, CsMgI , CsVI , CsCrI , CsMnI and CsNiI have been shown by X-ray studies to adopt the linear chain structure, Table I contains a summary of the crystallographic data for these salts. Although the space groups are not unambiguously determined it is very likely that a l l of the materials except the chromium salt are isostructural with CsNiCl (space group P6 /mmc). Crystallographic studies of CsCrCl3 and CsCrBr3 suggest that the structures of the chromium salts differ somewhat from that of CsNiCl ; however, the basic linear chain feature is still retained. Un­ doubtedly there are minor structural variations among 3

3

3

3

3

1,2

3

3

3

3

182 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4

13.

MCPHERSON

AND

siNDEL

Metal

183

Iodides

the CsMX s a l t s w h i c h r e s u l t f r o m t h e d i f f e r e n c e s i n the s i z e o f t h e h a l i d e i o n . C e r t a i n l y , a v e r y impor­ tant s t r u c t u r a l parameter i s the i n t r a c h a i n metalmetal separation. This separation i s equal t o half of the l a t t i c e dimension i n t h e c d i r e c t i o n and i s e x p e c t e d t o be l a r g e s t i n t h e i o d i d e s a l t s . 3

f f

Table

I.

Crystal

Structural

System:

n

Properties

Salts

2n

hhi,

Space Group:

P6 /mmc, P 6 / m c ,

Lattice

3

Hexagonal

Extinctions:

Mol./unit

of CsMI

3

3

cell:

o r P^2c

Ζ

Constants :

CsVI

a

c

8.21

6.81

a

8.12

6.85

b

8.18

6.95

a

8.00

6.76

a

8.20

7.01

a 3

CsCrI

3

CsMnI

3

CsNiI

3

CsMgI

3

^Reference "Reference

ίέϊ

In a d d i t i o n t o the f a i r l y subtle s t r u c t u r a l v a r i a t i o n s , s i g n i f i c a n t changes i n t h e n a t u r e o f t h e m e t a l - h a l o g e n bond would be e x p e c t e d i n g o i n g f r o m a c h l o r i d e t o a bromide and f i n a l l y t o a n i o d i d e lattice. These bonding d i f f e r e n c e s a r e d r a m a t i c a l l y demonstrat­ ed b y e l e c t r o n s p i n r e s o n a n c e m e a s u r e m e n t s . The e p r spectra of V , Μη , and N i doped i n t o t h e i s o s t r u c t u r a l magnesium s a l t s , C s M g C l , C s M g B r , and C s M g I , have b e e n s t u d i e d . The g- and m e t a l h y p e r ­ fine tensors indicate a considerable v a r i a t i o n i nthe metal-halogen bonding i n the three l a t t i c e s . Table I I g i v e s a summary o f h y p e r f i n e c o n s t a n t s and g - v a l u e s for the three l a t t i c e s . 2 +

2

+

3

9

3

6

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS

Jg4

Table

II.

H y p e r f i n e C o n s t a n t s * and

CsMgCl

V (77°K) g

obs Scale

Mn (77°K) 2 +

fobs

A

Ni (77°K)

4

g-Values "

CsMgBr

3

2 +

BETWEEN M E T A L IONS

CsMgI

3

1.975 1.957 75.

1.994 1.950 70.

2.056 1.942 65.

2.002

2.004 77.

2 .008 75.

2.25

2.25 2.40

2.16

80.

2 +

Sobs Scale

2.58

2.40

*The h y p e r f i n e c o n s t a n t p a r a l l e l and p e r p e n d i c u l a r components and of 10" cm" . (Data t a k e n from (6)) 4

3

are

in

units

1

+

T h e gobs v a l u e s r e p r e s e n t t h e a v e r a g e o f t h e p a r a l l e l and p e r p e n d i c u l a r components o f o b s e r v e d g - t e n s o r . The g a x values f o r the d and d systems are c a l c u ­ l a t e d from simple c r y s t a l f i e l d theory. (Data t a k e n from (6)) 3

C

8

c

One n o t i c e s t h a t t h e r e a r e c o n s i d e r a b l e d i s ­ c r e p a n c i e s between the observed g-values o f V and N i * and t h o s e c a l c u l a t e d f r o m t h e f o l l o w i n g s i m p l e crystal field expression. 2

2

(g = 2 . 0 0 2 3 - § λ ) F u r t h e r m o r e , t h e d i s a g r e e m e n t b e c o m e s more p r o n o u n c e d i n going from c h l o r i d e t o bromide t o i o d i d e . Presum­ a b l y t h e d i s a g r e e m e n t between t h e o b s e r v e d and c a l c u ­ l a t e d values a r i s e s from a l i g a n d c o n t r i b u t i o n t o the g-value. The l i g a n d c o n t r i b u t i o n i n c r e a s e s a s t h e s p i n o r b i t c o n s t a n t o f t h e l i g a n d i n c r e a s e s and a l s o as the d e r e a l i z a t i o n o f the u n p a i r e d e l e c t r o n s from the metal to the ligands i n c r e a s e s . " I n view o f the observed g-values, i t appears that the metalh a l o g e n b o n d i n g b e c o m e s more c o v a l e n t p r o c e e d i n g through the s e r i e s from c h l o r i d e t o i o d i d e . The Mn and V hyperfine constants support t h i s conclusion, s i n c e t h e c o n s t a n t s show a s t e a d y d e c r e a s e i n g o i n g from c h l o r i d e t o i o d i d e . A decrease i n the metal 7

1 0

5 5

5 1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

13.

MCPHERSON

AND

siNDEL

Metal

185

Iodides

h y p e r f i n e constant suggests an i n c r e a s e i n the metal to ligand delocalization. (Reference ( 6 ) g i v e s a more thorough d i s c u s s i o n of the epr parameters.) Although the trend i n the nature of the metal-halogen bonding i n the CsMX s e r i e s i s p e r h a p s i n t u i t i v e l y obvious, the epr s t u d i e s p r o v i d e a very s a t i s f y i n g experimental verification. P l o t s of the r e c i p r o c a l of the molar s u s c e p t i b i l ­ i t y versus the a b s o l u t e temperature f o r C s N i I , CsMnI , and C s C r I a r e shown i n F i g u r e s 2, 3 , a n d 4, respec­ tively. Both CsNiI and C s M n I o b e y t h e C u r i e - W e i s s l a w a b o v e 190°K, b u t show s i g n i f i c a n t d e v i a t i o n a t 77°K. The l o w t e m p e r a t u r e d e v i a t i o n s a n d t h e l a r g e negative Weiss constants i n d i c a t e that these s a l t s are antiferromagnetic. t h e C u r i e - W e i s s la T h i s m a t e r i a l , h o w e v e r , has a l a r g e n e g a t i v e W e i s s constant which indicates that i t i s a l s o antiferromag­ netic. The m a g n e t i c p r o p e r t i e s o f C s V I d i f f e r f r o m those of the three p r e v i o u s l y mentioned s a l t s . The v a n a d i u m s a l t has a s m a l l p a r a m a g n e t i c s u s c e p t i b i l i t y ( 2 . 3 χ 10" esu/mole) which i s e s s e n t i a l l y independent of temperature. This o b s e r v a t i o n suggests that the antiferromagnetic i n t e r a c t i o n s i n this material are s i g n i f i c a n t l y s t r o n g e r than those of the other s a l t s . These i n t e r a c t i o n s a r e e f f e c t i v e e v e n a t room tempera­ ture. A l t h o u g h t h e s e s u s c e p t i b i l i t y s t u d i e s do n o t completely c h a r a c t e r i z e the magnetic behavior of the i o d i d e s , t h e r e i s l i t t l e doubt about the antiferromag­ n e t i c nature of these m a t e r i a l s . The m a g n e t i c s u s c e p t i b i l i t i e s o f a number o f t h e a n a l o g o u s b r o m i d e s and c h l o r i d e s have a l s o b e e n studied. D a t a has b e e n r e p o r t e d f o r C s V C l s , CsCrCl , CsMnBr , CsNiBr , and CsNiCl . ~ A f u n d a m e n t a l q u e s t i o n t o be c o n s i d e r e d when d i s c u s ­ s i n g the m a g n e t i c p r o p e r t i e s o f t h e CsMX s a l t s i s whether the magnetic exchange i n t e r a c t i o n s are d i r e c t (through space) or i n d i r e c t (through l i g a n d ) . In p r i n c i p l e b o t h mechanisms a r e p o s s i b l e , s i n c e t h e metal-metal separations w i t h i n a chain are f a i r l y s h o r t (~3Â) and e a c h m e t a l i o n s h a r e s t h r e e h a l i d e ligands w i t h the neighboring metal ions i n the chain. The c o m p a r i s o n o f t h e s u s c e p t i b i l i t y d a t a f o r t h e C s M X s a l t s s h o w n i n T a b l e I I I g i v e s some q u a l i t a t i v e insight into this question. 3

3

3

3

3

3

3

3

1 1

1 2

3

1 3

3

1 4

1 4

3

3

3

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1 6

EXTENDED

INTERACTIONS

BETWEEN

METAL

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

MCPHERSON

AND

siNDEL

Metal

187

Iodides

100

50

-100°

0°

100° 200° 300° T(°K)

Figure 3. Reciprocal molar susceptibility of CsMnlg vs. absolute temperature. Curie-Weiss constants for the linear re­ gion: C = 4.97; θ = -165°.

Figure 4. Reciprocal molar suscepti­ bility of CsCrI vs. absolute temperature. Curie-Weiss constants: C = 3.08; θ = -163°. s

T(°K)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

188

EXTENDED INTERACTIONS BETWEEN METAL IONS

Table I I I .

M a g n e t i c P r o p e r t i e s o f CsMX Cs7X χ,(297°Κ) 3

M-M D i s t a n c e

(Α)

Cl

3.01

1370

I

3.40

2220

Salts

3

Χ(77 Κ) ό

a

a

1440 2360

CsCrX (A.) X(297°K) 4500° 3

Cl Br I

M-M D i s t a n c e 3.11 3.25 3.42

(Α)

3.26 3.47

12800

X(297°K) 11000 10750

2.98 3.12 3.38

X(77°K) 18500 17200

e

CsNils (A) X(297°K)

M-M D i s t a n c e Cl Br I

D

67ΟΟ

M-M D i s t a n c e Br ι

X(77°K) 5300

3800 3650 2940

e

X(77°K) 9200^ 8l50 4350

α

d

d

X v a l u e s a r e i n u n i t s o f 10"" esu/mole ^Reference i l l ) R e f e r e n c e ΓΤ2) Reference Π 3 ) R e f e rence ( T i ) 6

D

c

d

D i r e c t exchange i s a f u n c t i o n o f t h e d i s t a n c e between i n t e r a c t i n g i o n s and would be expected t o d i m i n i s h as t h e m e t a l - m e t a l s e p a r a t i o n i n c r e a s e s . On the o t h e r hand, i n d i r e c t exchange depends more on t h e covalency of the metal-ligand-metal l i n k a g e . Since t h e m e t a l - m e t a l s e p a r a t i o n s i n t h e CsMX s e r i e s a r e d i r e c t l y dependent on t h e s i z e o f t h e h a l i d e i o n , t h e s t r e n g t h o f d i r e c t e f f e c t s would be e x p e c t e d t o f o l l o w the o r d e r : Cl>Br>I. I n c o n t r a s t , t h e i n d i r e c t e f f e c t s would be e x p e c t e d t o e x h i b i t t h e o p p o s i t e o r d e r . The data f o r the cesium n i c k e l t r i h a l i d e s i n d i c a t e t h a t the s t r e n g t h o f the a n t i f e r r o m a g n e t i c i n t e r a c t i o n s i s g r e a t e s t f o r C s N i I and s m a l l e s t f o r C s N i C l . This o b s e r v a t i o n s u g g e s t s t h a t i n d i r e c t exchange i s p r e ­ dominant i n t h e s e s a l t s . T h i s c o n c l u s i o n i s q u i t e reasonable i n l i g h t o f simple c r y s t a l f i e l d theory. 3

3

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

13.

MCPHERSON

AND

Metal

SINDEL

189

Iodides

8

The u n p a i r e d e l e c t r o n s o f a d system i n an o c t a h e d r a l complex o c c u p y t h e e~ s e t o f o r b i t a l s w h i c h a r e d i r e c t ed t o w a r d t h e l i g a n d s . For a d s y s t e m s u c h as Mn d i r e c t as w e l l as i n d i r e c t i n t e r a c t i o n s m i g h t be exp e c t e d s i n c e t h e u n p a i r e d e l e c t r o n s o c c u p y t h e t 2 g and e orbitals. The d a t a i n d i c a t e t h a t t h e c o u p l i n g i n CsMnBr i s perhaps a l i t t l e stronger than i n CsMnI , b u t t h e s u s c e p t i b i l i t i e s o f t h e two s a l t s a r e v e r y similar. I t appears that there are considerably stronger interactions i n CsCrCl than i n C s C r I . It i s p o s s i b l e t h a t d i r e c t exchange i s dominant i n a d system s i n c e the m a j o r i t y of the unpaired e l e c t r o n s o c c u p y o r b i t a l s ( t g ) w h i c h a r e d i r e c t e d away f r o m t h e ligands. We h e s i t a t e t o s p e c u l a t e o n t h e C s V X salts since the s u s c e p t i b i l i t i e s e n t i a l l y independen s u s c e p t i b i l i t i e s may r e s u l t f r o m a t e m p e r a t u r e i n d e p e n d e n t p a r a m a g n e t i s m w h i c h has n o t h i n g t o do w i t h t h e normal paramagnetism a s s o c i a t e d w i t h the unpaired e l e c t r o n s o f t h e V"*" i o n . Clearly, rather strong antiferromagnetic i n t e r a c t i o n s are p r e s e n t i n these vanadium s a l t s . One v e r y i m p o r t a n t p o i n t h a s b e e n n e g l e c t e d i n the q u a l i t a t i v e d i s c u s s i o n of the magnetic p r o p e r t i e s o f t h e CsMX s a l t s . I t has b e e n f i r m l y e s t a b l i s h e d t h a t t h e i n t e r a c t i o n b e t w e e n two p a r a m a g n e t i c i o n s i s c r i t i c a l l y d e p e n d e n t on t h e m e t a l - l i g a n d - m e t a l angle. W h i l e t h e s t r u c t u r e s o f t h e s a l t s t h a t have b e e n d i s cussed are a l l s i m i l a r , t h i s c r i t i c a l angle undoubtedl y v a r i e s t o some e x t e n t f r o m l a t t i c e t o l a t t i c e . Unfortunately, s u f f i c i e n t precise c r y s t a l l o g r a p h i c data are not p r e s e n t l y a v a i l a b l e t o d i s c u s s t h i s important point. The e l e c t r o n i c s p e c t r a o f C s V I , C s C r I , C s M n I , and C s N i I a r e shown i n F i g u r e s 5 and 6. In general, t h e s p e c t r a show t h e l i g a n d f i e l d t r a n s i t i o n s t h a t w o u l d be e x p e c t e d f r o m o c t a h e d r a l c o m p l e x e s o f t h e s e t r a n s i t i o n metal ions. The s p e c t r u m o f C s C r I shows an i n t e n s e a b s o r p t i o n edge a t a p p r o x i m a t e l y 10,000 cm" . The m a t e r i a l a b s o r b s s t r o n g l y t h r o u g h o u t t h e visible region. T h i s i n t e n s e a b s o r p t i o n may be due t o charge-transfer transitions. Charge-transfer absorpt i o n would be e x p e c t e d t o a p p e a r a t l o w e r e n e r g i e s i n these i o d i d e s than i n s i m i l a r c h l o r i d e or bromide complexes. The s h o u l d e r o n t h e a b s o r p t i o n e d g e o f t h e CsCrI s p e c t r u m has b e e n t e n t a t i v e l y a s s i g n e d t o t h e s p i n a l l o w e d , E-+ T , l i g a n d f i e l d transition. S i m i l a r l y , the C s N i I s p e c t r u m has a n i n t e n s e a b s o r p t i o n edge w h i c h a p p e a r s a t a p p r o x i m a t e l y 13000 cm" and p r e s u m a b l y r e s u l t s f r o m c h a r g e - t r a n s f e r t r a n s 5

2

g

3

3

3

3

4

2

3

2

3

3

3

3

3

3

1

3

5

5

2

3

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

190

EXTENDED

INTERACTIONS

BETWEEN

METAL

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

13.

MCPHERSON AND

Metal

siNDEL

191

Iodides

itions. The s p e c t r a o f C s V T a n d C s M n I do n o t s e e m t o be p a r t i c u l a r l y u n u s u a l . T a b l e I V g i v e s t h e band a s s i g n m e n t s f o r t h e i o d i d e s a l t s b a s e d on a n o c t a h e ­ dral ligand f i e l d . 3

Table

IV.

Spectroscopic CsVI

Assignment 4A

4 4

T

Assignments

3

CsCrI Energy 3

1

Energy

(cm"" )

7700 12000

2

2

-

3

Τχ(Ρ)

Assignment

1

(cm"" )

9000 (sh)

5E -* [ 5 T ] 2

13000(sh) 4

-

15300 1870

Ti(P)

CsMnI Assignment E n e r g y (cm" ) Αχ Ti(G) I78OO 3

1

6

4

-* « T (G) 2

4e,4

—

4

T

2

A 3 L

(g)

(d)

~» 4E (D)

Assignment 3

A

3 2

CsNils Energy

6500 8000

T [IE] 2

20900 22100 255ΟΟ

3

1

(cm" )

10900

T (F) X

26400

28600

4 T i (P)

s h = shoulder Brackets designate

assignments which are

uncertain.

3

4

Spectroscopic studies of C s C r C l , C s C r B r , CsMnBr CsNiCl and C s N i B r ' have b e e n r e p o r t e d . A com­ p a r i s o n o f t h e Dq v a l u e s o f t h e CsMX s a l t s i s p r e ­ s e n t e d i n T a b l e V. 3

1 7

3

3

1 3

3

1 8

3

3

Table

V.

Dq

CsVX

V a l u e s f o r the CsCrX

3

1000

CI Br

—

I

770

114 5

1150 900

b

3

CsMX CsMnX

3

3

Salts CsNiX

3

695*

a

680 605

c

655 650

d

Reference Reference ^Reference ^Reference

(13 (Tf

The t r e n d s i n t h e Dq v a l u e s a p p e a r t o f o l l o w t h a t w h i c h would be p r e d i c t e d b y t h e s p e c t r o c h e m i c a l series. I t s h o u l d be m e n t i o n e d t h a t t h e Dq v a l u e f o r C s M n I was d e r i v e d f o l l o w i n g t h e p r o c e d u r e p r e s e n t e d 3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

,

192

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

in

reference (13). The e l e c t r i c a l r e s i s t i v i t i e s o f s i n g l e c r y s t a l s o f C s N i I have b e e n s t u d i e d a s a f u n c t i o n o f tempera­ ture. The m a t e r i a l a p p e a r s t o be a s e m i c o n d u c t o r w i t h a room t e m p e r a t u r e r e s i s t i v i t y o f 1 0 t o 10 ohm cm a n d a n e n e r g y o f a c t i v a t i o n o f a p p r o x i m a t e l y 0 . 7 ev. For a m a t e r i a l t h a t i s an i n t r i n s i c semiconductor the band gap ( t h e e n e r g y s e p a r a t i n g t h e v a l e n c e and c o n d u c t i o n bands) should be e q u a l t o twice t h e energy of a c t i v a t i o n o f c o n d u c t i o n . The i n t e n s e a b s o r p t i o n edge a p p e a r s i n t h e s p e c t r u m o f C s N i I a t a p p r o x i m a t e ­ l y 1.5 e v w h i c h i s a b o u t t w i c e t h e o b s e r v e d e n e r g y o f activation. T h i s s u g g e s t s t h a t a t room t e m p e r a t u r e the m a t e r i a l i s a n of the l i n e a r c h a i material i s essentially isotropic. I n c o n c l u s i o n , we h o p e t h a t we h a v e shown t h a t the c e s i u m m e t a l t r i i o d i d e s have r a t h e r i n t e r e s t i n g s o l i d s t a t e p r o p e r t i e s a n d t h a t t h e s e compounds w i l l be u s e f u l f o r f u r t h e r s t u d i e s i n t o t h e n a t u r e o f t h e linear chain M(I)M'(II)X compounds. 3

1 9

7

8

3

3

Literature Cited 1. Li, Ting-i, S t u c k y , G. D., a n d M c P h e r s o n , G. L . , Acta Crystallogr., (1973), B 2 9 , 1330. 2. M c P h e r s o n , G. L . a n d Data.

Quarls,

H.

F.,

Unpublished

3. B.,

M c P h e r s o n , G. L . , K i s t e n m a c h e r , T . K. Folkers, J. a n d S t u c k y , G. D., J. Chem. P h y s . , ( 1 9 7 2 ) , 57,

4.

Li,

3771.

Ting-i

and

Stucky,

G. D.,

Acta

Crystallogr.,

(1973), B29, 1529. 5. M c P h e r s o n , G. L . , K i s t e n m a c h e r , Τ . Κ . , a n d S t u c k y , G. D., J. Chem. P h y s . , (1970), 5 2 , 815. 6. M c P h e r s o n , G. L . , K o c h , R. C., a n d S t u c k y , G. J. Chem. P h y s . , (1974), 6 0 , 1424.

7.

M c G a r v e y , B . R., J. Chem. P h y s . ,

(1964),

41,

D.,

3743.

8. Garrett, B . B., D e A r m o n d , K., a n d G u t o w s k y , H. S., J. Chem. P h y s . , ( 1 9 6 6 ) , 4 4 , 3393.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

13.

MCPHERSON

AND

SINDEL

Metal

Iodides

193

9 . M e s e t i c h , A. A. and Buch, T., J. Chem. P h y s . , (1964), 4 1 , 2524. 10.

M e s e t i c h , A. A. and Watson, R. Ε., P h y s . Rev.,

(1966),

11.

143, 3 5 5 .

12.

H. J., Fink, Η., and Just. Ε., Naturwiss.,

Seifert,

(1968), 55,

297.

L a r k w o r t h y , L. F. and Trigg. J. Κ., Chem. Commun.,

(1970),

1221.

13. McPherson, G. L., Aldrich, H. S., and Chang, J. R., J. Chem. P h y s . ( 1 9 7 4 ) 6 0 534 14. Asmussen, R. W. Chem., (1956), 2 8 3 , 1. 15.

A c h i w a , N., J. Phys. S o c . ( J a p a n ) , (1969), 2 7 ,

561.

16. S m i t h J., Gerstein, B. C., Liu, S. Η., and S t u c k y , G., J. Chem. Phys., (1970), 53, 4 1 8 . 17.

McPherson, G. L. and S t u c k y , G. D., J. Chem.

Phys.,

(1972),

57,

3780.

18. Ackerman, J., Holt, Ε . M., and Holt, S. L., J. Solid S t a t e Chem., in press. 19. McPherson, G. L., Wall, J. E., Jr., and Hermann, A. M., I n o r g . Chem., in press.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

14 Magnetic and Thermal Properties of the Linear Chain Series

J. N.

3 3

McELEARNEY,

R. L .

MX ·2H O

[(CH ) NH]

G.

E.

3

SHANKLE,

2

D. B.

LOSEE,

S.

MERCHANT,

and

CARLIN

U n i v e r s i t y of Illinois, C h i c a g o , Ill. 60680

Introduction Several previous p a p e r s in this s y m p o s i u m h a v e discussed properties of members o f t h e linear chain series RMX (where M=transition metal, X = h a l i d e a n d R= (CH ) N, Cs or Rb). T h i s p a p e r will be c o n c e r n e d w i t h t h e new linear chain series [(CH ) NH]MX ·2H O, where M=(Mn, Co, Ni, Fe o r Cu) a n d X = ( B r o r Cl). Large single crystals suitable for optical and oriented mag­ netic field studies may be easily obtained for most members o f t h e series. Single crystal near-zero-field magnetic susceptibility d a t a ( m e a s u r e d using a m u t u a l inductance t e c h n i q u e ) and heat capacity data (measured using standard heat pulse techniques) are presented h e r e w h i c h show several of the interesting features of this series: anisotropic magnetic b e h a v i o r greater than n o r m a l with t h e p r e s e n c e o f low-dimensional character­ istics a s well a s spin-canting in the ordered state. 3

3

4

3

3

3

2

Crystal Structures To a large e x t e n t the magnetic b e h a v i o r o f these compounds is quite clearly related to their structure. N o t all t h e members o f t h e series are isomorphic, al­ though they probably are a l l isostructural, as inferred from their magnetic properties. Structures have been obtained o n l y for t h e n o n - i s o m o r p h i c (M=Co, Cu; X=C1) compounds (1,2.), although X-ray studies indicate that t h e (M=Co, Mn; X=C1) compounds b o t h crystalize in the s p a c e g r o u p Pnma a n d a r e p r o b a b l y i s o m o r p h i c . The m o s t i m p o r t a n t structural characteristics of t h e s e compounds may be s e e n by considering the projec­ tions o f t h e (M=Co; X=C1) p r o t o t y p e s h o w n in P i g s . 1 a n d 2. The structure consists of chains of edge-shar­ ing trans-[CoCl (0Η ) ] octahedra. The cobalt atoms 4

2

2

194

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

14.

MCELEARNEY

Ο

Cl

New

E T AL.

a

H 0

·

2

Linear

Chain

195

C

Figure 1. Projection of the unit cell of [(CH ) NH]CoCl * 2H O onto the ac plane. Height above this plane of several of the atoms is indicated. (All cobalt atoms are at the same height.) s 3

3

s

Ο

,

ο

-Ο

Figure 2. Projection of a portion of the crystal structure of [(CH ) NH] CoCl · 2H 0 onto the be plane. Portion used was a 3.33-A thick layer taken parallel to the be plane and centered about the cobalt atoms. Dashed lines give unit cell bounda­ ries. S 3

s

2

Ο ci

· Co

β

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

HO 2

r

196

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

a r e 3-637 A a p a r t ( C o - C l d i s t a n c e s a r e 2.456 and 2.503 A) and the i n t e r n a l c h l o r i n e b r i d g i n g a n g l e s a r e 93.14° and 95-52°. A n i o n i c c h l o r i n e s t i e t o g e t h e r c h a i n s which l i e i n t h e be p l a n e v i a hydrogen b o n d i n g . I t i s i m p o r t a n t t o note the r e l a t i v e t i l t i n g o f t h e [ C o C l ι» ( 0 H ) 2 ] m o l e c u l a r u n i t s w i t h r e s p e c t t o each o t h e r as seen i n the p r o j e c t i o n s . I t i s t h i s t i l t i n g , t a k e n w i t h t h e tendency o f the s p i n s t o a l i g n i n some manner c o n s i s t e n t w i t h the O-Co-0 v e c t o r s , w h i c h l e a d s to the u n u s u a l magnetic p r o p e r t i e s o f t h i s s e r i e s . 2

C(CH )3NH]CuCl3'2ΚιΟ

Properties of

3

The most u n u s u a l p r o p e r t y o f t h i s compound ( i n view o f the p r o p e r t i e f t h o t h e compounds) i t h a t i t behaves as a norma o n l y s l i g h t s i g n s o magneti exchang prio o r d e r i n g a t 0.15°K (3) and t h a t thus i n t r a c h a i n magnet­ i c exchange i n t h i s m a t e r i a l must be q u i t e s m a l l . Thus i t s measured heat c a p a c i t y (which v e r y n e a r l y f o l l o w s a T law) has been used i n c o n j u n c t i o n w i t h a c o r r e s p o n d ­ i n g s t a t e s p r o c e d u r e t o determine l a t t i c e c o n t r i b u t i o n s to the heat c a p a c i t i e s o f the o t h e r members o f t h e series. 2

P r o p e r t i e s o f C ( C H ) *NH]CoX 3'2H 0 3

3

2

(X=C1, B r )

Both o f t h e s e compounds behave v e r y s i m i l a r l y . The (M=Co; X=C1) compound m a g n e t i c a l l y o r d e r s a t 4.14°K, w h i l e the B r a n a l o g does so a t 3.86°K. The d a t a and t h e o r e t i c a l f i t s f o r b o t h a r e n e a r l y i d e n t i c a l so o n l y the r e s u l t s f o r the (X=C1) compound w i l l be g i v e n h e r e . The measured heat c a p a c i t y i s shown i n P i g . 3 and the e x t e n s i v e amount o f s h o r t - r a n g e o r d e r above t h e o r d e r ­ i n g t e m p e r a t u r e i s c l e a r l y e v i d e n t . More t h a n 9056 o f the e x p e c t e d e n t r o p y change o c c u r s above t h e t r a n s i ­ tion. This short-range order i s i n d i c a t i v e of the l o w e r e d d i m e n s i o n a l i t y o f the s p i n system and i s c o n ­ s i s t e n t w i t h the s h e e t - l i k e nature of the s t r u c t u r e . Thus s i n c e C o ( I I ) u s u a l l y has I s i n g - l i k e c h a r a c t e r i s ­ t i c s , Onsager s s o l u t i o n t o t h e a n i s o t r o p i c two-dimen­ s i o n a l I s i n g model (k) has been used t o f i t t h e d a t a . B o t h the magnetic heat c a p a c i t y d e r i v e d from t h e d a t a and the f i t t e d curve a r e shown i n P i g . 4. The v a l u e s f o r the exchange parameters d e t e r m i n e d from t h e f i t are J/k=7.7°K and J /k=0.09°K. (The f i t o f t h e B r a n a l o g r e s u l t s i n J/k=7.0°K and J /k=0.09°K.) C l e a r l y the s p i n system i s not f a r from b e i n g o n e - d i m e n s i o n a l . Thus t h e measured magnetic s u s c e p t i b i l i t i e s , shown i n F i g s . 5 and 6, s h o u l d be n e a r l y one-dimension1

f

f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

14.

New Linear

MCELEARNEY E T AL.

197

Chain

(C H

3) 3 Ν Hi'C 0 C 1 3 · 2 H 0 2

9 T E M P E R A T U R E

Figure

3.

12

13

3

14

15

( K)

3

3

2.5

Π

Zero-field heat capacity data for [(CH,) NH"\CoCh · 2H 0 and 15°K. Solid line represents the lattice contribution.

[ (C Η >

«

10 e

2

Ν Η] C 0 C Ι · 2

between

3°

H 0

3

2

Ι­

ο

ε ^

2.0

J 0

,

2

3

4

5

6

L. 7

8

T E M P E R A T U R E

Figure

ι

I

9

10

_J

L_ Π

12

13

I

1

14

15

(·Κ)

4. Magnetic heat capacity (solid line) of l(CH ) NH]CoCl · 2H 0. Dashed line is the fit to Onsagers two-dimensional anisotropic Ising model solution. 3 3

s

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

198

EXTENDED

INTERACTIONS

BETWEEN

f[CH ) NH]CoCI - 2 3

5

3

x

METAL

IONS

18

20

H 0 2

· · ·

h

0. I 6 h

4

6

θ

10

12

TEMPERATURE

Figure

5.

14

16

(°K )

Zero-field magnetic susceptibility of [(CH )sNH] 2H 0 measured parallel to the b axis S

2

[(CH,) NH]CoCI,' 2 s

H 0 2

Xc

è * f

ΔΔΔΔ X

e

•••• Χ

β

*

; .'i

*l

m

8

I I 10

ί

t 12

T E M P E R A T U R E (°K )

Figure 6. Zero-field magnetic susceptibilities of [(CH ). NH"\CoCl 2H 0 measured along a and c axes. Results of measurements the a axis on two different crystals are shown. s

2

l

3

· along

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

CoCl

s

·

14.

MCELEARNEY

New

ET AL.

Linear

199

Chain

al i n character. The n e a r l y d i s c o n t i n u o u s b e h a v i o r n e a r t h e t r a n s i t i o n t e m p e r a t u r e s h o u l d be n o t e d . It i s e v i d e n t t h a t t h e r e i s a n e x t r e m e amount o f a n i s o t r o p y , X b e i n g g e n e r a l l y v e r y much s m a l l e r t h a n X and X , a n d t h a t t h e r e a r e i n d i c a t i o n s o f a f e r r o m a g n e t i c moment p a r a l l e l t o t h e a a x i s i m m e d i a t e l y b e l o w t h e transition. A d d i t i o n a l l y , at the lowest temperatures X and are constant, while X decreases with dec r e a s i n g t e m p e r a t u r e , as m i g h t be e x p e c t e d f o r a n a n t i ferromagnetic m a t e r i a l f o rwhich c i s t h e easy a x i s . These f a c t s l e a d t o a s p i n arrangement model i n w h i c h s p i n s l i e n e a r l y a l o n g t h e c a x i s w i t h some c a n t ing towards the a a x i s . A l lspins are perpendicular to the b a x i s . A l l t h e s p i n s i n e a c h b e s h e e t o f Co i o n s are a l i g n e d p a r a l l e and between c h a i n s b o r i n g p a i r o f be s h e e t s a r e a l i g n e d s o a s t o g i v e a n e t moment a l o n g t h e a a x i s w i t h n o n e t moment a l o n g the c a x i s . Thus t h e s p i n arrangement i s t h a t o f a c a n t e d a n t i f e r r o m a g n e t o r a weak f e r r o m a g n e t . The i n t r a e h a i n exchange v a l u e determined from t h e heat c a p a c i t y d a t a has been used w i t h t h e e q u a t i o n s d e r i v e d (5.) f o r a o n e - d i m e n s i o n a l I s i n g m o d e l t o f i t t h e Xb a n d Xc d a t a . To t a k e a c c o u n t o f t h e a a x i s c a n t i n g , t h e r e s u l t s d e r i v e d b y M o r i y a (6_) f o r t h e s u s c e p t i b i l i t y o f a c a n t e d a n t i f e r r o m a g n e t have been a p p l i e d t o the X data. T h e r e s u l t s o f t h e f i t s a r e shown i n Fig. 7 a n d a r e s e e n t o be e x t r e m e l y g o o d . b

a

a

c

c

a

Properties

o f [ (CH 3) 3NH]MnX 3 *2H 2O

(X=C1, B r )

B o t h o f t h e s e compounds b e h a v e s i m i l a r l y , w i t h t h e (X=C1) compound e x h i b i t i n g a t r a n s i t i o n a t 0 . 9 8 ° K , w h i l e t h e ( X = B r ) compound o r d e r s a t 1.58°K. Because o f t h e s i m i l a r i t y o f t h e d a t a f o r t h e s e compounds o n l y t h e l a t t e r w i l l be d i s c u s s e d h e r e . The m e a s u r e d h e a t c a p a c i t y i s shown i n F i g . 8. The s h a r p s p i k e c o r r e s p o n d s t o t h e m a g n e t i c o r d e r i n g w h i l e t h e b r o a d hump i s i n d i c a t i v e o f the short-range order expected f o ra low-dimensional system. More t h a n 80% o f t h e m a g n e t i c e n t r o p y c h a n g e e x p e c t e d f o r a n S=5/2 s y s t e m i s g a i n e d above 1.58°K. Likewise, the single c r y s t a l susceptib i l i t i e s m e a s u r e d f o r t h i s compound, shown i n F i g . 9* show t h e e f f e c t s o f e x t e n s i v e s h o r t - r a n g e o r d e r . The b r o a d maximum i n X s i g n i f i c a n t l y a b o v e t h e o r d e r i n g temperature i s e s p e c i a l l y i n d i c a t i v e o f t h e behavior w h i c h s h o u l d be e x p e c t e d f o r a l i n e a r c h a i n . It is a l s o i m p o r t a n t t o n o t e t h e amount o f a n i s o t r o p y b e l o w 10°K and t h a t X and X behave as p a r a l l e l a n d p e r p e n dicular antiferromagnetic s u s c e p t i b i l i t i e s , respecc

c

a

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

200

Figure

EXTENDED

7.

INTERACTIONS

Results of the fits (solid lines) to the [(CH, ),NH]CoCl. susceptibility data t

t

BETWEEN

· 2H 0 2

METAL

principal

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

axis

MCELEARNEY

New Linear

E T AL.

Chain

201

4.8 ~

[(CH ) NH]MnBr -2H 0 3

3

3

2

4.0 -

32 24

0-8 I

_i

0.0

I

ι

ι

I

ι

I

»

ι

•

I

ι

I 8

ι

I

ι

I 9

ι

! 10

e

TEMPERATURE ( K)

Figure

8.

Zero-field

heat capacity

data for [(CH ) NH]

MnBr

3 3

3

·

2H 0 2

1.12 |CH ) NH]MnBr - 2 H 0 3

3

3

2

0.96

0.80

°

0.64

Φ

0.48

x 0.32 Xo-».«.—~

~rr ·.·.*· .* ; · « * «

0.16

0.00

I

I

i

1

I

I

6

I

I

8

I

I

i

10

TEMPERATURE

Figure

9.

Zero-field

I

12

14

16

_l

I I 18

I 20

( K) e

magnetic susceptibilities of [(CH ) NH"]MnBr sured along the a, b, and c axes 3 3

3

· 2H 0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

mea­

202

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

tively. As M n ( I I ) h a s a n S g r o u n d s t a t e , i t m i g h t b e e x p e c t e d t h a t the H e i s e n b e r g l i n e a r c h a i n model p r e v i o u s l y a p p l i e d t o o t h e r M n ( I I ) c h a i n s (7., 6kG. T h i s type o f p l o t i s o f t e n used to determine the paramagnetlc s u s c e p t i b i l i t y o f a sample i n the presence o f ferromagnetic i m p u r i t i e s which can be saturated by the a v a i l a b l e magnetic f i e l d s (19). The i n t e r c e p t i n Figure h as determined by a l e a s t squares f i t o f the data i s 1 0 χ . * 1980 ± 60 cgs/mole at 300°K. À study o f the temperature dependence o f the f i e l d dependent s u s c e p t i b i l i t y would a s c e r t a i n the p o s s i b l e maintenance o f the Curie Law f o r X^. This r e q u i r e s l a r g e f i e l d s t o magnetic a l l y saturate the sample, but the use o f a Cryostat r e s t r i c t s our a v a i l a b l e f i e l d s t use o f a few data p o i n t TT°K. A p p l i c a t i o n o f the above technique t o two d i f f e r e n t s o l i d samples a t TT°K produced values o f l O * ^ o f 69IO and 5990 cgs/ mole. The values o f χ^ r e v e a l an i n t e r e s t i n g value of the mag­ n e t i c moment. At 300°K u = 2.15 BM/Fe and a t 7T°K = 1.9 t o 2.1 HM/Fe. Therefore, w i t h i n experimental e r r o r , the polymers e x h i b i t a paramagnetic moment of 2.1 BM/Fe which i s temperature indepen­ dent between 77 and 300°K. T h i s agrees with the measured moment (between h.2 and 50°K) o f TPPFe ( ImH) C 1 where u » 2.36 BM/Fe (13). These values a r e a l l c o n s i s t e n t with low s p i n F e ( l l l ) ex­ h i b i t i n g some s p i n o r b i t coupling. The l i n e a r i t y shown i n Figure h implies that the f i e l d de­ pendent p o r t i o n o f the s u s c e p t i b i l i t y o f the sample i s saturated when H > 6kG. This i s shown i n Figure 5 as molar magnetization vs f i e l d . In t h i s case the magnetization ( l =* X^H) i s d e t e r ­ mined from the observed χ^ l e s s the f i e l d independent χ^; taken as I920 χ 1 0 ~ cgs/mole i n accord w i t h the r e s u l t o f Figure k. The room temperature s a t u r a t i o n magnetization, f o r TPPFe Im · HgO i s found as 11.5 cgs units/mole. This corresponds to an extremely small moment a t s a t u r a t i o n (300°K) o f 2.06 χ 1CT BM/Fe, i f a l l the i r o n present i s assumed to be i n v o l v e d . As small as t h i s value i s , TPPFeIm · HgO shows magnetic o r d e r i n g a t room temperature. The study o f the temperature dependence o f the s u s c e p t i b i l ­ i t y o f TPPFeIm · HgO was c a r r i e d a t a f i x e d f i e l d o f 6*kk KGauss and the r e s u l t s are presented i n Table I. The d e t * i n d i c a t e s a n o n - l i n e a r r e l a t i o n s h i p between l/χί and Τ w i t h l/χ^ decreasing f a s t e r than the Curie-Weiss Law p r e d i c t s as temperature de­ creases. T h i s behavior (downward curvature o f a l/χ vs Τ p l o t ) i s reminiscent o f a f e r r i m a g n e t i c m a t e r i a l above i t s Neel tem­ perature ( 2 0 ) . However, that i n t e r p r e t a t i o n can be r e j e c t e d s i n c e the Neel temperature according t o t h i s data would have t o β

8

f

f

u

2

e f f

e

3

c

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

COHEN

AND

OSTFELD

Iron(IH)

Hemin

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

INTERACTIONS

BETWEEN

METAL

Figure 5. Molar magnetization at 300°Κ vs. Η for TPPFelm * H 0. The independent portion of the susceptibility, χι = 1920 X 10' cgs/mole, subtracted from the observed χ °, M = H(x — χι)2

6

c

Μ

M

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

field was

IONS

16.

C O H E N

A N D

Iron(III)

OSTFELD

Hemin

227

be b e l o w 100° K. I b i s i t i n c o n s i s t e n t w i t h the f i e l d dependent behavior of the saae s a m p l e , b e c a u s e s a t u r a t i o n can be o b s e r v e d at room t e m p e r a t u r e . TABLE I . M a g n e t i c P r o p e r t i e s o f TPPFelm · % 0 T°K

10%

~ff 89 10k 127 15* 185 217 251

1%

8200

122

7719 Tlk6 6372 5766

129 ΐ4θ

1*300 1*200 i*08i* 3978

319 3^ β. =

0

%

157 m 189 204

5295 1*908

286 300

1

i o %

c l c / 7792 67^1 5769 472% 3986

4o8 8 9 T

1377 1648 1870 2052

3243 2765

238

2143

2200 2203

2000 1881 L744

245

b

t l i e

% cale Paramagnetic s u s c e p t i b i l i t y c a l c u l a t e d f o r a s y s t e m w i t h u = 2·2 BM. b c % " % cale ' d e p e n d e n t p o r t i o n o f the s u s s

t

l

i

e

f

ceptibility. Inasmuch a s the p a r a m a g n e t i c moment o f T P P F e l m · HgO was shown t o remain between 2 and 2.2 BM from 77 t o 344°K; t h e c o r ­ c r e s p o n d i n g f i e l d independent s u s c e p t i b i l i t y c a n be c a l c u l a t e d (χ^ ) e a c h t e m p e r a t u r e . S u b t r a c t i o n o f | from leaves~>N the f i e l d dependent p o r t i o n o f at each temperature a

t

c

a

c

(Table I j . T h e v a r i a t i o n o f χ^ a t c o n s t a n t f i e l d w i t h Τ i s shown i n F i g u r e 6. It i s apparent that the extent o f magnetiza­ t i o n decreases c o n s i d e r a b l y w i t h d e c r e a s i n g t e m p e r a t u r e . An ex­ t r a p o l a t i o n o f t h e c u r v e i m p l i e s t h a t χ^ s h o u l d become n e g l i g i b l e b e l o w 70°Κ a n d t h e n t h e t o t a l o b s e r v e d s u s c e p t i b i l i t y s h o u l d be due t o χ^ a l o n e . The o b s e r v a t i o n o f t h e l o s s o f m a g n e t i z a t i o n a t low temperature i s i n d i c a t i v e o f a f e r r o m a g n e t i c system w h i c h u n d e r g o e s a m a g n e t i c p h a s e change a t low t e m p e r a t u r e o r a f e r r i magnetic system which possesses a h i g h Neel temperature and a low t e m p e r a t u r e c o m p e n s a ! i o n p o i n t (20). The d e p e n d e n c e is fit

of ^

shown i n F i g u r e 7 ·

It

on Τ a t low temperature is evidently

t h e C u r i e - W e i s s Law i n a n y r e g i o n o f T .

ledge o f the value

of χ β

that the t o t a l

is o n l y about 2 BM.

observed

T h e l a c k o f know­

the constant

It

i s also rather

susceptibility

10$ o f t h e χ^ c a l c u l a t e d

Clearly,

field

but t h e anomalous b e h a v i o r o f

s a m p l e a t a b o u t 6θ Κ i s e v i d e n t .

to note

and h i g h

complex and does n o t

b e l o w 77°Κ p r o h i b i t s s e p a r a t i o n o f

i n t o χ^ a n d χ^ a t e a c h t e m p e r a t u r e the

quite

important

a t low t e m p e r a t u r e

on the basis

of

=

moment o b s e r v e d a b o v e 77°Κ i s n o t

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

228

EXTENDED

INTERACTIONS

BETWEEN

METAL

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

COHEN

Iwn(IH)

A N D OSTFELD

ΗβΤΠΪΠ

-

7.0

" G G

G

— G

ο

o

o

Q

G G ) -

G

G

G

Θ G

3.( 0

Figure 7.

1 40

Observed

1 80

1 1 120 160 TEMPERATURE °K

χ ° for TPPFelm Μ

1 200

· H,Ο vs. temperature

1 240

at 15 KGauss

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

230

E X T E N D E D

constant

INTERACTIONS

B E T W E E N

M E T A L

IONS

b e l o w 77° K.

The a p p r o a c h used here to determine χ. i n the presence o f a f i e l d dependent moment i s u s u a l l y r e q u i r e d f o r t h e study o f sam­ ples c o n t a i n i n g f e r r o m a g n e t i c i m p u r i t i e s . In a d d i t i o n , the very s m a l l moment o b s e r v e d by d e t e r m i n i n g t h e s a t u r a t i o n m a g n e t i z a ­ t i o n o f TPPFeIm · HgO s u g g e s t s t h a t t h e o r d e r e d s y s t e m i s i n f a c t o n l y an i m p u r i t y i n the sample. I t c o u l d be a r g u e d t h a t the [TPPFeIm] s y s t e m i s b e h a v i n g l i k e [ T P P F e ( l m H ) ] * and p o s ­ ses a temperature independent o f a b o u t 2.3 BM a n d t h e i m ­ p u r i t y a l o n e i s r e s p o n s i b l e f o r the f i e l d dependent e f f e c t s . The a n a l y s e s c e r t a i n l y do n o t r u l e out s u c h a p o s s i b i l i t y . On t h e o t h e r h a n d , t h e s t a r t i n g m a t e r i a l T P P F e B F d o e s n o t show any 2

4

and s i m p l y g r i n d i n g t h e s o l i d p r o d u c t w h i c h i s n o t e x p e c t e d t o e f f e c t t h e amount o f a n i m p u r i t y d o e s c h a n g e t h e v a l u e o f y^ It i s also rather s i g n i f i c a n w i t h imidazole produce f i e l d d e p e n d e n t moment and t h e t r e a t m e n t o f TPPFe B F w i t h OH" produces ( T P P F e ) 0 w h i c h a l s o d o e s n o t e x h i b i t any f i e l d d e p e n ­ dent magnetic p r o p e r t i e s . We do n o t t h e r e f o r e e x p e c t t h a t t r e a t ­ ment o f T P P F e B F by ImH a n d O H * t o g e t h e r w i l l p r o d u c e a f e r r o ­ magnetic i m p u r i t y i n the p r o d u c t . A n o t h e r s t r o n g argument a g a l n s t an i m p u r i t y b e i n g r e s p o n s i b l e f o r the f i e l d dependence f o l l o w s a c o n s i d e r a t i o n o f t h e l o w t e m p e r a t u r e d e p e n d e n c e o f y^. Below 60°R t h e v a l u e o f y^ d r o p s w e l l b e l o w t h a t c o n s i s t e n t w i t h a u = 2 system. T h e r e i s n o way t h a t a m a g n e t i c i m p u r i t y c a n s u b t r a c t from t h e t o t a l s u s c e p t i b i l i t y o f a s a m p l e ; t h a t I s , t h e minimum χ f o r a n i m p u r e s a m p l e must be due t o t h e p a r a m a g n e t i t ­ self. E i t h e r t h e m a g n e t i c b e h a v i o r a n d t h e a n o m a l y a t 60°K i s c h a r a c t e r i s t i c o f t h e b u l k s a m p l e o r we must r e q u i r e t h a t t h e i m ­ p u r i t y a n d t h e p o l y m e r b o t h c o i n c i d e n t a l l y u n d e r g o some m a g n e t i c p h a s e c h a n g e a t a b o u t 60°K. I t i s a l s o s i g n i f i c a n t t o n o t e t h a t n o EPR s p e c t r u m c o u l d be o b s e r v e d a t room t e m p e r a t u r e o r 7 7 ° Κ f o r T P P F e Im · ΗβΟ. How­ ever even 1 p a r t per thousand o f e i t h e r i r o n metal o r magnetite c o u l d be o b s e r v e d i n t h e EPR s p e c t r u m when t h o s e m a t e r i a l s w e r e s p e c i f i c a l l y added t o a s o l i d p o r p h i n e . F i n a l l y , e x a m p l e was p r e p a r e d w h i c h showed a l a r g e f i e l d d e p e n d e n c e o f y^ ( i . e . 133$ change i n when Η v a r i e d f r o m 2 t o 6 K G a u s s ) . The s a m p l e was suspended i n C % C 1 a n d g a s e o u s HC1 was b u b b l e d t h r o u g h t h e s u s ­ pension for 1 mln. The s o l i d i m m e d i a t e l y d i s s o l v e d . The s o l u ­ t i o n was n o t d i s t u r b e d f o r 5 m i n . and t h e n t h e s o l v e n t was r e ­ moved i n a stream o f N . T h e s o l i d was d r i e d i n v a c u o a t room t e m p e r a t u r e and was n o t p u r i f i e d o r a d d i t i o n a l l y t r e a t e d i n any manner. T h e f i n a l s o l i d a s was o b t a i n e d was e x a m i n e d m a g n e t i ­ c a l l y and t h e was o b s e r v e d t o change by o n l y 6.3$ between h i g h and l o w f i e l d . F u r t h e r m o r e , t h e y?. o b s e r v e d c o r r e s p o n d e d t o a u £ £ b e t w e e n 5.6 and 5.8 B M / F e . The v i s i b l e s p e c t r u m i n ­ d i c a t e d t h a t t h e t r e a t m e n t w i t h HC1 h a d c o n v e r t e d t h e p o l y m e r t o T P P F e C l [known u = 5.9 BM ( 2 1 ) ] . 4

2

4

c

2

2

e

e

f

f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

16.

C O H E N

A N D

osTFELD

Iron(III)

231

Hemin

We c o n c l u d e that the f i e l d dependent m a g n e t i c behavior o f the sample i s c h a r a c t e r i s t i c o f t h e p o l y m e r b e c a u s e m i l d t r e a t ­ ment w h i c h b r e a k s u p the polymer a l s o e l i m i n a t e s t h e f i e l d d e p e n ­ dence o f χ^. The very high s u s c e p t i b i l i t i e s and f i e l d dependence observed i n d i c a t e s that [ T P P F e l m ] i s capable o f magnetic order­ ing. The f a c t t h a t the f i e l d dependent magnetization can be c

n

s a t u r a t e d ( a t room t e m p e r a t u r e ) e l i m i n a t e s a n t i f e r r o m a g n e t i c c o u p l i n g and i m p l i e s t h a t f e r r o - o r f e r r i - m a g n e t i c c o u p l i n g i e

o c c u r r i n g , The temperature dependence o f χ^ does n o t a l l o w t h e s t r a i g h t f o r w a r d a s s i g n m e n t o f e i t h e r f e r r o - o r a n t i f e r r o m a g n e t ism t o t h i s eyetern* A f e r r i m a g n e t i c model r e q u i r e s the presence o f two d i f f e r e n t t y p e s o f m a g n e t i c s i t e s f o r t h e i r o n w i t h a n a n t i ­ f e r r o m a g n e t i c i n t e r a c t i o n b e t w e e n them (20). I t i s e x t r e m e l y d i f f i c u l t t o e n v i s i o n how t h i s c o u l d o c c u r f o r t h i s s y s t e m . In particular, interchai l a r g e s e p a r a t i o n betwee phine r i n g . However, t h e u n u s u a l p r o p e r t i e s o b s e r v e d may be due t o the l i n e a r s t r u c t u r e o f the polymer. The p r o b l e m o f u n i d i m e n s i o n a l m a g n e t i c a l l y o r d e r e d systems has been o f c o n s i d e r a b l e t h e o r e t i c a l and e x p e r i m e n t a l i n t e r e s t for several years. T h i s i s i n p a r t d u e t o t h e p r o o f by M e r m i n and Wagner (22) t h a t a n i n f i n i t e o n e d i m e n s i o n a l s y s t e m a t f i ­ n i t e temperature cannot e x h i b i t l o n g range o r d e r and thus c a n ­ not a c t a s a p e r f e c t f e r r o - o r ant i ferromagne t . I t i s however s t i l l possible f o r s h o r t range i n t e r a c t i o n s t o o c c u r which can lead t o both types o f magnetic o r d e r i n g . S e v e r a l groups have s u b s e q u e n t l y d i s c u s s e d t h e t h e o r y o f t h e s e s y s t e m s (23%24). Most o f t h e e x p e r i m e n t a l work has concerned a n t i f e r r o m a g n e t i c systems (25) b u t C s N i F h a s b e e n f o u n d t o a c t a s one d i m e n s i o n a l f e r r o magnet b y s u s c e p t i b i l i t y a n d n e u t r o n d i f f r a c t i o n s t u d i e s (26). The r e s u l t s o b s e r v e d f o r [ T P P F e I m ] a p p e a r t o o c o m p l i c a t e d t o attempt any q u a n t i t a t i v e f i t t o t h e t h e o r i e s a t t h i s t i m e . How­ e v e r t h e v e r y l o w s a t u r a t i o n moment d e r i v e d f r o m t h e f i e l d d e p e n ­ dent magnetization o f [TPPFeIm] i s c o n s i s t e n t w i t h the o c c u r ­ r e n c e o f o n l y weak, s h o r t range o r d e r i n g o f the f e r r o m a g n e t i c type. I f o n l y a s m a l l p o r t i o n o f the b u l k sample i s o r d e r e d t h e n the remainder o f the i r o n s i t e s could continue t o e x h i b i t normal p a r a m a g n e t i s m a n d c o n t r i b u t e t o a f i e l d i n d e p e n d e n t moment o f reasonable magnitude as observed. T h i s model c o u l d a l s o e x p l a i n t h e e x t r e m e v a r i a b i l i t y o f χ^ b e t w e e n s a m p l e s b e c a u s e t h e c o n d i ­ t i o n s o f p r e c i p i t a t i o n and presence o f s o l v e n t c o u l d g r e a t l y change t h e e x t e n t o f t h e s m a l l o r d e r i n g t h a t o c c u r s . In o r d e r t o b e t t e r a s c e r t a i n the a p p l i c a b i l i t y o f a l i n e a r f e r r o m a g n e t i c m o d e l f o r [ T P P F e l m ^ we w i l l r e q u i r e c o n s i d e r a b l y m o r e i n f o r m a t i o n d e r i v e d f r o m low t e m p e r a t u r e s u s c e p t i b i l i t y a n d Mossbauer s t u d i e s . Knowledge r e g a r d i n g t h e u b e l o w 60°K a n d t h e n a t u r e o f t h e a n o m a l y a b o u t 60°K i s c u r r e n t l y b e i n g s o u g h t . T h e f a c t s t h a t t h e p o l y m e r c a n be p r e p a r e d a n d t h e i m i d a z o ­ l a t e bridges allow metal-metal Interactions to occur, support our i n t e r e s t i n t h i s system. Thus o u r c u r r e n t s t u d y o f t h e r o l e o f 3

n

n

e

f

f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

232

E X T E N D E D

I N T E R A C T IONS

B F T W E E N

M E T A L

IONS

imidazole i n oxidation reduction reactions i n v o l v i n g i r o n porphy­ r i n s may p r o v i d e a model f o r p a r t o f t h e b i o l o g i c a l e l e c t r o n transport c h a i n * We thank A r t h u r T a u b e r a t t h e U . S . Army E l e c t r o n i c s Command f o r t h e measurement o f s u s c e p t i b i l i t y b e l o w 77° Κ a n d t h e P u b l i c H e a l t h S e r v i c e f o r s u p p o r t o f t h i s w o r k t h r o u g h G r a n t AM-11355·

Literature Cited 1. Cohen, I. A., Jung, C. and Governo, T., J. Amer. Chem. Soc. (1972)94,3003. 2. Abbreviations: TPP=the tetraphenylporphine dianion, C H N4=; Im=the imidazolate anion, C3H3N2¯;X =the field independent magnetic susceptibility; X = the field dependent magnetic susceptibility; X calc. = the field in­ dependent magnetic susceptibility calculated from an as­ sumed moment;X susceptibility. 3.(a)Nicholas, Μ., Mustacich, R. and Jayne, D., J. Amer. Chem. Soc. (1972) 94, 4518. (b) Hoffman, A. B., Collins, D. Μ., Day, V. W., Fleischer, E. B., Srivastava, T. S. and Hoard, J. L., ibid (1972) 94, 3620. (c) Cohen, I. A., ibid (1969) 91, 1980. Also see references found within (a) and (b). 4. Bolton, W. and Perutz, M. F., Nature (1970) 228, 551. 5. Dickerson, R. E., Tarkano, T., Eisenberg, D., Kallai, O. Β., Samson, L., Cooper, A. and Margoliash, E., J. Biol. Chem. (1971) 246,1511. 6. Ogoshi, H., Watanabe, E., Yoshida, Z., Kincaid, J. and Nakamoto, K., J. Amer. Chem. Soc. (1973)95,2845. 7. LaMar, G. N. and Walker, F. Α., ibid (1973)95,1782. 8. Duclos, J. Μ., Bioinorganic Chem. (1973) 2, 263. 9. Coyle, C. L., Rofson, P. A. and Abbott, Ε., Inorg. Chem. (1973) 12, 2007. 10. LaMar, G. N. and Walker, F. A., J. Amer. Chem. Soc. (1972) 94, 8607. 11. Kolshi, G. B. and Plane, R. A. ibid (1972)94,3740. 12. Collins, D. Μ., Countryman, R. and Hoard, J. L., ibid (1972) 94, 2066 13· Epstein, L. M., Straub, D. K. and Maricondi, C., Inorg. Chem. (1967) 6, 1720. 14.(a)Baraniak, E., Freeman, H. C. and Nockolds, C. E., J. Chem. Soc. (1970) A, 2558. (b) Eilbeck, W. J . , Holmes, F. and Underhill, Α. Ε., ibid (1967) 757. (c) Yamane, T. and Davidson, Ν., J. Amer. Chem. Soc. (1960) 82, 2118. 15. Freeman, H. C., Advan. Protein Chem. (1967) 22, 257. 16. Jarivis, J. A. J. and Wells, A. F., Acta Crystallogr. (1960) 13, 1028. 17. Brown, G. P. and Aftergut, S., J. Polym. Sci. (1964) 2, Part A-2, 1839. 18. Inuve, M., Kishita, M. and Kubo, Μ., Inorg. Chem. (1965) 4, 626. 44

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19.

20.

21.

COHEN

AND

OSTFELD

Iron(III)

288

Hemin

W i l l i a m s , D . E . G., "The M a g n e t i c P r o p e r t i e s o f M a t t e r " , p. 9 9 , E l s e v i e r P u b l i s h i n g Co., I n c . , New Y o r k , Ν. Y. 1966. M o r r i s h , A. Η . , "The P h y s i c a l Principles o f M a g n e t i s m " , pp. 486-498, J o h n W i l e y a n d S o n s , Inc., New York, Ν. Y. 1965. Maricondi,

C.,

Swift,

W.

and

Straub,

D . Κ . , J. A m e r .

Chem.

Soc. (1969) 91, 5205. M e r m i n , N . D . a n d W a g n e r , Η . , P h y s . R e v . Lett. (1966) 17, 1133. 23. K o n d o , J. a n d Y a m a j i , Κ . , P r o g . T h e o r . P h y s . (1972) 47, 807. 24. B o n n e r , J. C . a n d Fisher, M . F., P h y s . R e v . (1964) 135, A640. 25. F o r a few e x a m p l e s s e e : ( a ) M c E l e a r n e y , J. Ν., M e r c h a n t , S . and Carlin, R. L., D. Y . a n d Halfield, 3055. ( c ) H u t c h i n g s , Μ. Τ., Shirane, G., B i r g e n e a u , R. J. and Holt, S . L., P h y s . R e v . B . (1972) 5, 1999. ( d ) S w e e n e y , W. V . a n d C o f f m a n , R . E., B i o c h i m . B i o p h y s . A c t a (1972) 286, 26. ( e ) C a s e y , Α. Τ., Morris, B . S., Sinn, E. a n d T h a c k e r a y , J. R., Aust. J. Chem. (1972) 25, 1 1 9 5 · (f) Griffiths, R. Β., P h y s . R e v . (1964) 135, Α659. 2 6 . ( a ) S t e i n e r , M . a n d D o r n e r , Β . , Solid State Comm. (1973) 12, 537. (b) Steiner, Μ., ibid (1972) 11, 73. ( c ) Steiner, M. and D a c k s , H., ibid (1971) 9, 1603. (d) Steiner, Μ., Kruger, W. a n d Bakel, D., ibid ( 1 9 7 1 ) 9, 227. (e) Steiner, Μ., Z. Angew. Physik (1971) 32, 116.

22.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17 Local and Collective States i n Single and M i x e d Valency Chain Compounds P. DAY

University of Oxford, South Parks Rd., Oxford ΟΧ1 3QR, England

Abstract Compounds c o n t a i n i n g chains o f metal atoms are found with four d i s t i n c t c l a s s e s of s t r u c t u r e , each of which, as a r e s u l t of the widely v a r y i n g strength of the metal-metal i n t e r a c t i o n , has a s s o c i a t e d w i t h it its own c h a r a c t e r i s t i c p a t t e r n of p h y s i c a l p r o p e r t i e s . Thus one may have e i t h e r s i n g l e or mixed v a l e n c y phases w i t h near neighbor contact between metal ions formed e i t h e r d i r e c t l y or through anion b r i d g e s . The o p t i c a l , X-ray photo e l e c t r o n , magnetic and t r a n s p o r t p r o p e r t i e s of examples of each c l a s s are surveyed, with p a r t i c u l a r emphasis on work c a r r i e d out at Oxford, to h i g h l i g h t the r e l a t i v e importance of s i n g l e center and c o l l e c t i v e e x c i t e d s t a t e s in each category, in r e l a t i o n to t h e i r s t r u c t u r e s and the magnitude of the metal-metal i n t e r a c t i o n . I t is pointed out that a c o l l e c t i v e d e s c r i p t i o n may be a p p r o p r i a t e f o r magnetic and e l e c t r o n i c e x c i t a t i o n s even without e l e c t r o n delocalization.

1.

Introduction

Only i n the l a s t few years have i n o r g a n i c chemists i n t e r e s t e d i n the e l e c t r o n i c s t r u c t u r e s of metal complexes p a i d much a t t e n t i o n to the consequences of the i n t e r a c t i o n s which can take place between molecules and ions placed next to each other i n the s o l i d s t a t e . At t h e i r weakest such I n t e r a c t i o n s may manifest themselves only i n r e l a t i v e l y s u b t l e s h i f t s and s p l i t t i n g s of e l e c t r o n i c absorption bands, and i n magnetic o r d e r i n g at low temperatures. On the other hand, strong i n t e r m o l e c u l a r i n t e r a c t i o n s lead to spectacular c o l o r changes or the appearance of new a b s o r p t i o n bands not found i n the spectra of the i s o l a t e d 234

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17.

DAY

Chain

Compounds

235

molecules or i o n s . F i n a l l y , i n extreme cases the e n t i r e range of p h y s i c a l p r o p e r t i e s of the c r y s t a l may be profoundly d i f f e r e n t from those expected of ordinary i o n i c or molecular s o l i d s . Of the l a t t e r s i t u a t i o n , the most famous instance i s c e r t a i n l y the one-dimensional m e t a l l i c conducting behavior of the p a r t i a l l y o x i d i z e d platinum compounds such as K2Pt(CN)4Bro,3 3H20. The e x c e p t i o n a l p r o p e r t i e s of t h i s m a t e r i a l have served to focus a t t e n t i o n on metal atom chain compounds i n general. For a number of years we ourselves have been i n t e r e s t e d i n both magnetic and charge t r a n s f e r i n t e r a c t i o n e f f e c t s i n i n o r g a n i c c r y s t a l s . The purpose of the present paper i s to b r i n g together some of these observations, p r i m a r i l y concentrating on one-dimensional examples, to exemplify how the o p t i c a l , magnetic and e l e c t r o n transport p r o p e r t i e s are influenced by the strengt atom chain compounds ar and are formed by a l a r g e number of d i f f e r e n t elements. I t may be u s e f u l t h e r e f o r e to attempt some general c l a s s i f i c a t i o n of the p a t t e r n of p h y s i c a l behavior c h a r a c t e r i s t i c of each s t r u c t u r e type, and of the strength of the metal-metal i n t e r a c t i o n . One reason why t h i s c l a s s of m a t e r i a l s i s so i n t e r e s t i n g i s that they span the e n t i r e range from l o c a l i z e d to c o l l e c t i v e behavior and thus should be a good t e s t i n g ground f o r t h e o r e t i c a l models of the s o l i d s t a t e . e

2.

L o c a l i z e d and

C o l l e c t i v e States

Before d i s c u s s i n g the v a r i o u s c l a s s e s of metal chain compound, a general point which needs to be touched on concerns the meaning of the words ' l o c a l i z e d and ' c o l l e c t i v e as used i n t h i s context. U n l i k e s o l i d s t a t e p h y s i c i s t s , i n o r g a n i c chemists are not u s u a l l y accustomed to c o n s t r u c t i n g wavefunctions which a r e i n v a r i a n t to the operations of t r a n s l a t i o n w i t h i n a p e r i o d i c l a t t i c e . Nevertheless, i f a c r y s t a l such as ^ P t (CN)^ absorbs a photon of such an energy that one of the c o n s t i t u e n t complex anions undergoes a l i g a n d f i e l d t r a n s i t i o n , i t i s an inescapable f a c t that we do not know which of the 1023 anions i n the c r y s t a l has been e x c i t e d , and that as a r e s u l t , the e x c i t e d s t a t e wavefunction must allow an equal p r o b a b i l i t y of each i o n being e x c i t e d : i n other words the wavefunction i s a Bloch f u n c t i o n . In one sense the s t a t e which i t describes i s ' c o l l e c t i v e , since the e x c i t a t i o n belongs to the whole l a t t i c e . How f a r and how f a s t i t can move i n p r a c t i c e , however, depends on the magnitude of i n t e r - i o n i c coupling or t r a n s f e r i n t e g r a l s compared with e i t h e r i n t r a - i o n i c e l e c t r o n c o r r e l a t i o n or r e p u l s i o n e f f e c t s on the one hand, or e l e c t r o n i c - v i b r a t i o n a l i n t e r a c t i o n s on the other. Crudely, i f the e x c i t a t i o n i s capable of being t r a n s f e r r e d from molecule to molecule f a s t e r than each molecule can accommodate i t s geometry to the e x c i t a t i o n , then the e x c i t a t i o n , or 'exciton,' 'belongs' to the whole l a t t i c e . 1

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

236

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Otherwise, i t s range of t r a v e l i s l i m i t e d to a few adjacent molecules (or f i n a l l y , only to one), and i t i s l o c a l i z e d . D e r e a l i z a t i o n of e x c i t a t i o n , however, i s f a r from implying d e r e a l i z a t i o n of e l e c t r o n s , or the occurrence of e l e c t r o n i c c o n d u c t i v i t y . E x c i t o n m i g r a t i o n over hundreds of angstroms i s f a m i l i a r i n aromatic molecular c r y s t a l s , f o r example, yet i n t h e i r ground s t a t e s they are e x c e l l e n t i n s u l a t o r s . For c o n d u c t i v i t y i t i s obvious that we need to form c r y s t a l s t a t e s i n which the e l e c t r o n occupancy at d i f f e r e n t s i t e s has been a l t e r e d , e.g., c r e a t i n g Pt(CN)|~ and Pt(GN)^ i n K ^ P t i C N ) ^ In o r d i n a r y i o n i c and molecular c r y s t a l s such processes r e q u i r e a l a r g e energy i n p u t , and i t i s t h e r e f o r e i n a p p r o p r i a t e to t a l k of c o l l e c t i v e e l e c t r o n s t a t e s i n these m a t e r i a l s . I n t e r i o n i c e l e c t r o n t r a n s f e r s t a t e s are only formed r e a d i l y i n s o l i d s c o n t a i n i n g an i n t e g r a e l e c t r o n concentratio e l e c t r o n from the p o s i t i v e hole l e f t on the i o n from which i t o r i g i n a t e d . In c o n t r a s t , i f d i f f e r e n t s i t e s i n the c r y s t a l are already occupied by d i f f e r e n t numbers of e l e c t r o n s when the l a t t i c e i s formed, much l e s s energy i s needed to form s t a t e s i n which these s i t e s are merely interchanged. I t may then be p o s s i b l e to form conducting s t a t e s at lower e l e c t r o n c o n c e n t r a t i o n s , or with smaller overlap between i o n s , than one would need to generate energy bands of f i n i t e width i n s t o i c h i o m e t r i c s i n g l e valence s o l i d s . T h i s i s the s i g n i f i c a n c e of mixed valency, as we s h a l l see i n the survey of metal i o n chain compounds which follows." 3.

Types of T r a n s i t i o n Metal Chain Compound

T r a n s i t i o n metal compounds i n which each c a t i o n has only two nearest neighbors are formed both i n s i n g l e and mixed valency s i t u a t i o n s . In t u r n , each of these may be formed e i t h e r with or without anion b r i d g e s between the neighboring c a t i o n s . Some examples of each type of compound are given i n Table 1. The p r i n c i p a l c l a s s of anion bridges s i n g l e valence chains i s that of the hexagonal p e r o v s k i t e s ABX3. When X i s a h a l i d e i o n and A a u n i v a l e n t i o n the l a t t i c e can be thought of as made up of a c l o s e packed a r r a y of A and X, i f they are of comparable s i z e , e.g., Cs* and C l ~ . Now i n a c l o s e packed assembly of ions the number of o c t a h e d r a l holes equals the number of c l o s e packed atoms, but i n the system AXo o n l y οne_quarter of the o c t a h e d r a l holes are surrounded e x c l u s i v e l y by X ions and are hence a v a i l a b l e f o r occupation by the B^+ c a t i o n s . The ABX3 etoichiometry would then be generated by f i l l i n g a l l the holes of t h i s type. I f the l a t t i c e of (A+3X) i s cubic c l o s e packed we have the cubic p e r o v s k i t e s t r u c t u r e formed by many oxides and f l u o r i d e s . However another p o s s i b i l i t y i s hexagonal c l o s e packing of (A+3X). In that case the o c t a h e d r a l BX5 groups are stacked i n columns s h a r i n g p a i r s of opposite faces so that the

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

Direct

Anion Bridged

n

2

2

4

2

Ir(C0) acac

APt(CN).

3

Pt(NH ) PtCl

2

4

2

(M - N i , Pd, P t )

K PtCl^, PtenCl

M(DMG)

8 8 8 Square planar 3d , 4d , 5d

3

CsMCl .2H 0(M » Mm, Fe, Co)

M « V, Cr, Mh, Fe, Co, N i )

3

AMC1 (A « a l k a l i metal or NR*,

Hexagonal p e r o v s k i t e s , ABX^:3d

Valence

3

A

2

n

P t ( C

4

4

n

A _ Pt(CN) 2-n 2°4>2

2

A Pt(CN) X

n

l i n e a r / s q u a r e planar h a l i d e s

CsAuCl

:d

8

Wolframs r e d s a l t

3

Square planar 5d**

d

10

Valence

square p l a n a r / o c t a h e d r a l h a l i d e s

Mixed

Compounds

PtenCl

6

d :d

8

Examples o f T r a n s i t i o n Metal Chain

Single

Table 1.

238

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Β to Β separation i s much smaller p a r a l l e l to the c - a x i s o f the hexagonal u n i t c e l l than perpendicular to i t . I f the BX* octahedra were u n d i s t o r t e d , and i f the i o n i c r a d i u s of A+ were equal to that of X~ the r a t i o o f the nearest neighbor B-B d i s t a n c e along the stacks to the d i s t a n c e between stacks would be 2.449. In many examples, however, the r a t i o i s l a r g e r s i n c e one can use c a t i o n s such as Ν ζ Ο Η β ) ^ which a r e bigger than X". On the other hand, with u n d i s t o r t e d B X 5 the nearest neighbor B-B d i s t a n c e i s 2//3 times the B-X bond l e n g t h . I t i s c l e a r t h e r e f o r e that the anions w i l l p l a y an important r o l e i n any exchange processes between the Β i o n s . Whatever the pathway o f the B-B i n t e r a c t i o n though. A convincing demonstration o f i t s one-dimensionality i s provided by the r a t i o of i n t r a - c h a i n and i n t e r - c h a i n exchange i n t e g r a l s i n a compound such as N ( Œ ) 4 M n C l 3 , which ha The other c l a s s o anion b r i d g i n g p l a y s a r o l e a r e the hydrated ternary t r a n s i t i o n metal h a l i d e s . Here the c o o r d i n a t i o n of each b i v a l e n t metal i o n i s again o c t a h e d r a l , c o n s i s t i n g of four h a l i d e ions and two cis-water molecules (3). The octahedra a r e j oined i n t o chains through approximately 180° bridges i n v o l v i n g the trans h a l i d e ions. H a l i d e bridges between c a t i o n s o f d i f f e r i n g o x i d a t i o n s t a t e are mainly confined t o s i t u a t i o n s i n which one of the c a t i o n s has a low s p i n d^ c o n f i g u r a t i o n , and hence b a s i c a l l y square planar c o o r d i n a t i o n . The vacant s i t e s perpendicular to the plane c o n t a i n i n g the l i g a n d s may then be occupied by h a l i d e ions which a r e themselves coordinated t o another c a t i o n . I n the most common examples of t h i s type, the second c a t i o n has a low s p i n c o n f i g u r a t i o n and thus octahedral c o o r d i n a t i o n . The h a l i d e bridges may a l s o be the t e r m i n a l groups of l i n e a r l y coordinated d*0 c a t i o n s such as A u . I n f a c t , a famous example of the l a t t e r i s the b l a c k compound known as Wells' s a l t , which has the e m p i r i c a l formula CSAUCI3. The s t r u c t u r e , determined many years ago by E l l i o t and P a u l i n g (4) contains chains o f a l t e r n a t i n g l i n e a r A u C l and square planar AuClT._ However, each A u C l a l s o has four c h l o r i d e ions from four AuCl^ coordinated a t r i g h t angles t o i t s p r i n c i p a l a x i s , so i n t o t o , the l a t t i c e could be viewed as a d i s t o r t e d v e r s i o n of the cubic c l o s e packed perovskite. To achieve a c l o s e enough approach between metal ions f o r d i r e c t i n t e r a c t i o n between them to outweigh i n t e r a c t i o n s through b r i d g i n g l i g a n d groups, the most f a v o r a b l e s i t u a t i o n i s c l e a r l y to have complexes which are c o o r d i n a t i v e l y unsaturated, so that another metal i o n , or complex, can a c t , as i t were, as a l i g a n d . The most f a m i l i a r examples of c o o r d i n a t i v e unsaturation a r e square planar complexes, so i t i s among low s p i n d$ compounds that some o f the most spectacular i n t e r m e t a l l i c i n t e r a c t i o n e f f e c t s are found, both i n s i n g l e and mixed valency compounds. A l s o , because i t i s the trans s i t e s i n such a geometry, which 3

1

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

17.

DAY

Chain

Compounds

239

are a v a i l a b l e f o r c o o r d i n a t i o n , polymerization i s bound to g i v e l i n e a r systems. S i n g l e valence examples are found with 3d**, 4 d and 5d8 c o n f i g u r a t i o n s , both where t h e _ u n i t s are n e u t r a l molecules l i k e P t e n C l , anions l i k e Pd(CN)?~ or a l t e r n a t i n g anions and c a t i o n s , as i n Magnus' Green S a l t . In a l l of these, important i n t e r m e t a l l i c i n t e r a c t i o n e f f e c t s are observed, as we s h a l l i n d i c a t e l a t e r . At the present time examples of d i r e c t metal-metal i n t e r a c t i o n s i n mixed valence chains are only known f o r systems based on p a r t i a l o x i d a t i o n of square planar complexes i n the t h i r d t r a n s i t i o n s e r i e s . We and others have made repeated e f f o r t s to prepare N i and Pd analogues of the p a r t i a l l y o x i d i z e d Pt c h a i n compounds, but w i t h no success. Even attempts to dope K P t ( C N ) B r . 3 0 · 3 H 0 with Pd(CN) %~ or Ni(CN)f" by c o - c r y s t a l l i z i n g i t i n the presence of l a r g e excesses of these two ions do not lead to an d e t e c t a b l e i n c o r p o r a t i o n of 3 d or 4 d complex. Whether extension of the 5d o r b i t a overlap between the c a t i o n s t o s t a b i l i z e the band s t r u c t u r e , or whether i t i s connected with the smaller nd-(n+l)p s e p a r a t i o n i n the t h i r d t r a n s i t i o n s e r i e s we cannot say. Following t h i s b r i e f and g e n e r a l i z e d survey of the main s t r u c t u r e types of the metal chain compounds, some account can now be given of the p h y s i c a l p r o p e r t i e s a s s o c i a t e d with each type. In order to avoid t h i s having too much of the character of a review, reference w i l l mainly be made to work on chain compounds which has been c a r r i e d out i n Oxford i n the l a s t few years. 8

2

2

4

0

2

8

8

4.

S i n g l e Valence Metal Chain Compounds

(a) Anion Bridged Compounds. An o u t l i n e of the p r o p e r t i e s of s i n g l e valence metal chain compounds i s given i n Table 2, which h i g h l i g h t s the d i f f e r e n c e s found between compounds c o n t a i n i n g anion bridges r a t h e r than d i r e c t l y i n t e r a c t i n g metal i o n s . The key observation i s that none of the anion bridged compounds have any low energy e x c i t e d s t a t e s of metal-to-metal charge t r a n s f e r type. Since, as we have noted, i t i s mixing of t h i s k i n d of s t a t e i n t o the ground s t a t e which leads to c o l l e c t i v e e l e c t r o n i c behavior, i t i s no s u r p r i s e that a l l known examples are i n s u l a t i n g , with l o c a l i z e d ground s t a t e s . Ligand-to-metal charge t r a n s f e r s t a t e s e x i s t i n the u l t r a v i o l e t , however, and i n s o f a r as these i n v o l v e the b r i d g i n g l i g a n d s , t h e i r mixing i n t o the ground s t a t e provides a mechanism f o r superexchange, l e a d i n g to magnetic o r d e r i n g at low temperatures. At l o t has been w r i t t e n about one-dimensional magnetic o r d e r i n g , but a s i n g l e r e p r e s e n t a t i v e example w i l l i l l u s t r a t e some of the c h a r a c t e r i s t i c s of t h i s k i n d of system. U n l i k e CsCuCl^, which d i s t o r t s from the hexagonal p e r o v s k i t e s t r u c t u r e so as to provide each Cu atom with four short and two long bonds, the other f i r s t t r a n s i t i o n s e r i e s i o n which

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974. charge t r a n s f e r states

(unusual temperature depen-

Transport

Magnetic

Insulator

Weak superexchange

magnetic a t low temperatures

(anisotropic?)

(probably e x t r i n s i c )

Semiconductor

Diamagnetic

t r a n s i t i o n s only

transfer) Ferromagnetic or a n t i f e r r o -

when En chain

Usual h a l i d e ·*· metal

(b) C o l l e c t i v e

(d-*p or charge

dence of i n t e n s i t y )

s t r o n g l y perturbed by UV

exciton-magnon i n t e r a c t i o n

Large Davydov s h i f t s

field)

Vibronic transitions

Direct Interaction

Weakly perturbed by

Anion Bridged

P r o p e r t i e s of S i n g l e Valence T r a n s i t i o n Metal Chains

None a t low energy

(ligand

(a) L o c a l l y e x c i t e d

Optical

Table 2.

17.

DAY

Chain

241

Compounds

1 1

c u s t o m a r i l y shows strong J a h n - T e l l e r d i s t o r t i o n , C r , forms a compound CsCrCl3 which, a t l e a s t above 172 K, has an u n d i s t o r t e d hexagonal p e r o v s k i t e s t r u c t u r e (5). That one-dimensional s p i n c o r r e l a t i o n s are important i n t h i s compound f o l l o w s a t once from the temperature v a r i a t i o n of the magnetic s u s c e p t i b i l i t y which has a broad maximum near 170 K, then decreases with the onset of antiferromagnetism. In f a c t though, three dimensional magnetic o r d e r i n g does not take place u n t i l the Neel temperature of 16 Κ i s passed, so between 170 and 16 Κ only antiferromagnetic i n t e r a c t i o n s w i t h i n the chains are of any importance. An even more d i r e c t measure of the one-dimensionality of the s p i n c o r r e l a t i o n s i s the d i s p e r s i o n of the spin-wave e x c i t a t i o n s p a r a l l e l and perpendicular to the chain ( 6 ) . J u s t as we have to w r i t e the e l e c t r o n i c e x c i t e d s t a t e wavefunctions of a c r y s t a l as Bloch f u n c t i o n s , s t allo th e x c i t a t i o l p r o b a b i l i t y of r e s i d i n the same way, must a magneti (tha , of a s p i n from the d i r e c t i o n i t would have i n the t o t a l l y ordered l a t t i c e ) be d e l o c a l i z e d . Spin waves of d i f f e r i n g wavelengths (or wave v e c t o r s ) have d i f f e r e n t energies, which can be determined by i n e l a s t i c neutron s c a t t e r i n g , thus b u i l d i n g up a p i c t u r e of the d i s p e r s i o n curve experimentally. The experimental spin-wave d i s p e r s i o n of C s C r C ^ near 4 Κ p a r a l l e l and perpendicular to the chains i s shown i n F i g u r e 1. Although the r e s u l t s do not cover the e n t i r e B r i l l o u i n zone, because of the small s i z e of the c r y s t a l a v a i l a b l e to us, they do show very c l e a r l y that the d i s p e r s i o n perpendicular to the £-axis i s e s s e n t i a l l y zero. The exchange i n t e g r a l between neighboring Cr ions i n the b a s a l plane i s t h e r e f o r e n e g l i g i b l y small w h i l s t w i t h i n the chains curve f i t t i n g to the experimental p o i n t s on the d i s p e r s i o n curve gives J ^ ^ 26.7 cm""*. Since customarily there are no charge t r a n s f e r s t a t e s i n the v i s i b l e or near u l t r a v i o l e t i n t h i s c l a s s of compound, the lowest energy e x c i t e d s t a t e s are l i g a n d f i e l d i n type, and can t h e r e f o r e be described as F r e n k e l , or t i g h t - b i n d i n g , e x c i t o n s . Many l i g a n d - f i e l d e x c i t e d s t a t e s have s p i n p r o j e c t i o n s d i f f e r e n t from the ground s t a t e and t r a n s i t i o n s to them should consequently be e l e c t r i c d i p o l e forbidden. I t has been known f o r a number of years, however, that such t r a n s i t i o n s can be rendered allowed i n antiferromagnetic compounds by u s i n g the exchange I n t e r a c t i o n to couple an e x c i t o n formed w i t h decrease of s p i n (e.g., sextet to quartet) with a s p i n d e v i a t i o n among the r e s t of the ions i n the l a t t i c e , which remain i n t h e i r ground s t a t e s . The most famous examples of such 'exciton-magnon' combination bands are found i n manganese s a l t s ( 7 ) , so i t i s i n t e r e s t i n g to see how the e f f e c t manifests i t s e l f i n M n s a l t s of the hexagonal p e r o v s k i t e type, which c o n t a i n chains of a n t i f e r r o m a g n e t i c a l l y coupled ions l i k e that found above i n C s C r C ^ . In o v e r a l l appearance the l i g a n d f i e l d spectra of Mh chain compounds are much l i k e those of other s i x - c o o r d i n a t e Mh 11

11

11

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

242

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

complexes, confirming a g a i n that the metal-metal i n t e r a c t i o n s i n anion bridged s i n g l e valence chains are weak. Two f e a t u r e s stand out, however. F i r s t , the o s c i l l a t o r strengths of the_ t r a n s i t i o n s are greater than i n non-bridged ('outer-sphere') s a l t s where magnetic i n t e r a c t i o n s are n e g l i g i b l e . Second, the temperature dependences of the o s c i l l a t o r strengths f o l l o w a r a t h e r c u r i o u s p a t t e r n , q u i t e u n l i k e that of the u s u a l 'coth' p l o t f o r a simple v i b r o n i c a l l y induced t r a n s i t i o n . Some experimental examples are shown i n F i g u r e 2(a) (8). In a l l cases the o s c i l l a t o r strength increases r a p i d l y a t f i r s t when the temperature i s r a i s e d above 4.2 K, then passes through a broad maximum a t a temperature which v a r i e s from one compound to another, but appears t o be the same f o r a l l the bands i n each compound. Then a f t e r dropping s l i g h t l y i t f i n a l l y i n c r e a s e s slowly and monotonically towards room temperature This v a r i a t i o n has some g e n e r a s u s c e p t i b i l i t y v s . temperatur f a c t , the r e s u l t s i n F i g u r e 2(a) stimulated Tanabe and Ebara t o c a l c u l a t e the i n t e n s i t y , frequency and l i n e w i d t h v a r i a t i o n of the magnon sidebands i n l i n e a r antiferromagnets, w i t h the r e s u l t shown i n F i g u r e 2(b) (10). I t can be seen that the o v e r a l l form of the v a r i a t i o n i s n i c e l y reproduced. Tanabe's theory r e q u i r e s that the temperature a t which the broad maximum occurs i n the i n t e n s i t y i s approximately |J/k|S(S+1). For the three compounds we i n v e s t i g a t e d , the v a l u e s of J/k derived from the spectra are l i s t e d i n Table 3, which a l s o shows that f o r the two compounds f o r which we have s u s c e p t i b i l i t y data (9,11), the l e v e l of agreement between the l a t t e r and the o p t i c a l l y derived v a l u e of J/k i s v e r y s a t i s f a c t o r y . (b) D i r e c t l y I n t e r a c t i n g Metal Ions. When metal ions are brought c l o s e enough together i n a c h a i n to i n t e r a c t d i r e c t l y , without the intermediary of a b r i d g i n g l i g a n d , the e f f e c t of the neighboring ions on the e l e c t r o n i c s t a t e s of each metal i o n i s n a t u r a l l y i n c r e a s e d . U n f o r t u n a t e l y no examples of such chains are known i n which the c o n s t i t u e n t ions have unpaired spins because, as we noted a l r e a d y , they are a l l based on square planar complexes. The s i n g l e exception known to us i s P t Q i R ^ ^ C u C l ^ which, however, c o n t a i n s chains of a l t e r n a t i n g Pt(NH3)^ and CuCl^" and so has a diamagnetic i o n between each p a i r of i o n s . Consequently i t has a Neel temperature of o n l y 0.5°K, from which near neighbor exchange i n t e g r a l of about 0.25 cm~l can be estimated (12). Turning to o p t i c a l p r o p e r t i e s , one f i n d s two d i f f e r e n t types of s i t u a t i o n s i n the s i n g l e valence chains w i t h d i r e c t l y i n t e r a c t i n g metal i o n s , depending on whether the lowest energy e x c i t e d s t a t e s of the c o n s t i t u e n t complexes are forbidden or allowed. The former a r e most l i k e l y t o be l i g a n d f i e l d s t a t e s , which are p a r i t y forbidden from the ground s t a t e i n centrosymmetric complexes l i k e PtCl£~ and may i n a d d i t i o n be +

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17.

DAY

Chain

Compounds

243

Figure 1. Dispersion of spin waves propagating (a) par­ allel and (b) perpendicular to the Cr chains in the linear antiferromagnet CsCrCl>

ο

ο

ο

ο

ο

ο

ο

Ο '

Λ

ο

ο

ο

ο ο ο

Ο

?0

60

ΙΟΟ

τικ

ο ΙΘΟ

2όΟ Figure 2(a). Temperature depend­ ence of oscillator strengths of ligand field transitions in linear antiferromagnets NtCHsh MnCl and CsMnX, · 2H 0 (X = Cl, Br) (8) 3

2

Figure 2(b). Calculated temperature variation of oscillator strengths of exciton-magnon transitions in linear antiferromagnets and ferromagnets (10)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

244

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

spin-forbidden. The l a t t e r may be e i t h e r 'atomic' (e.g., 5d ·> 6p i n P t complexes) or charge t r a n s f e r . Cases where the lowest e x c i t e d s t a t e s are of l i g a n d f i e l d type are the s a l t s I ^ P t O ^ , PtenX (X-Cl,Br) and Pt ( N H ^ P t C l ^ , Magnus' Green S a l t (MGS), although some of the e a r l i e r workers thought that the unusual c o l o r of the l a s t example pointed to 'metal-metal bonding,' and the existence of new e x c i t e d s t a t e s which could not be traced back i n parentage to any intramolecular t r a n s i t i o n s of the c o n s t i t u e n t anion or c a t i o n (13). At f i r s t s i g h t i t does seem p e c u l i a r that MGS should be green, s i n c e P t ( N H ) | i s c o l o r l e s s , both i n s o l u t i o n and i n P t i N ^ ^ C l ? , while P t C l | ~ i s pink i n s o l u t i o n and i n I ^ P t C l ^ . Nevertheless, by l o o k i n g a t the way the p o l a r i z e d c r y s t a l spectra vary along a s e r i e s of MGS analogues Pt (RNH^^PtClA as the R group i s made more bulky, we showed i n MGS are a l l t r a c e a b l i n the spectrum of P t C l | i n I ^ P t C l ^ , a l b e i t w i t h red s h i f t s up to 4000 cm~l. Examples of these are shown i n Figure 3. They are a l s o considerably i n t e n s i f i e d compared to l ^ P t C l ^ , most d r a m a t i c a l l y when the i n c i d e n t e l e c t r i c vector i s p a r a l l e l to the metal stack (15). The reason f o r the i n t e n s i f i c a t i o n i s assumed to l i e i n the f a c t that the t r a n s i t i o n s borrow t h e i r i n t e n s i t y v i b r o n i c a l l y from allowed t r a n s i t i o n s out i n the u l t r a v i o l e t which are themselves s t r o n g l y r e d - s h i f t e d by the intermolecular i n t e r a c t i o n . Intense absorption when the i n c i d e n t e l e c t r i c v e c t o r i s p a r a l l e l to the metal chains has o f t e n been taken as evidence for metal-metal bonding, but as we f i r s t pointed out (16), t h i s conclusion may be based on a f a l s e i n t e r p r e t a t i o n of the a v a i l a b l e absorption mechanisms. I t i s undoubtedly t r u e , of n i c k e l dimethylglyoximate f o r example, that the c r y s t a l , which contains metal chains, has an Intense absorption band i n the v i s i b l e , p o l a r i z e d p a r a l l e l to the chains, which does not appear i n the s o l u t i o n spectrum. I t i s equally t r u e , however, that the f i r s t intense band of n i c k e l N-methyl-salicycaldimine, which does not form chains i n the c r y s t a l , i s a l s o p o l a r i z e d perpendicular to the molecular planes. In both cases the t r a n s i t i o n i s most probably a d ττ* charge t r a n s f e r , but the f e a t u r e d i s t i n g u i s h i n g them i s f h a t when the molecules are stacked plane to plane the t r a n s i t i o n d i p o l e s are a l l p a r a l l e l , so i n t e r a c t i o n s between them are maximized and very l a r g e Davydov s h i f t s occur. No intermolecular e l e c t r o n exchange need be invoked i n e i t h e r case. In f a c t , we b e l i e v e that i n general, when the lowest energy e x c i t e d s t a t e s of the molecular u n i t s are p o l a r i z e d perpendicular to t h e i r planes, the lowest e x c i t e d s t a t e s of the stacks of molecules i n the c r y s t a l are always n e u t r a l Frenkel excitons r a t h e r than i o n i c e x c i t o n s . This p r e j u d i c e i s based on the magnitude of the Davydov s p l i t t i n g to which the n e u t r a l excitons w i l l be subject (a 'back of an envelope' c a l c u l a t i o n suggests around 14,000 cnT^ f o r t r a n s i t i o n 1 1

2

+

3

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17. D A Y

Chain

Table 3.

Temperature Dependence of I n t e n s i t y of Magnon Sidebands i n L i n e a r Antiferromagnets

Compounds

Τ

max

245

J/k

(exp.) r

20-40 Κ

CsMnBr .2H 0 2

30-60

4.5

(NMe )MnCl

3

50-80

6.7

2

3

4

(susceptibility) 3.0 Κ

3.3 Κ

CsMnCl .2H 0 3

J/k

6.3

Figure 3. Polarized absorption spectra of (a) Pt(CH iNH ),,PtCl Pt(C H NH ) PtCl and (c) K PtCl Dotted lines are Ε c, full Ε c (15). 2

2

5

2 >l

f>

2

r

h

(b) lines

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

246

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

d i p o l e length of U i n a stack of molecules separated by 3.25Â). On the other hand, i f the parent t r a n s i t i o n i s p o l a r i z e d w i t h i n the molecular planes, Davydov s p l i t t i n g of i t s n e u t r a l excitons i n the c r y s t a l i s smaller ( a c t u a l l y one h a l f of the other i n the p o i n t d i p o l e approximation) and i o n i c e x c i t o n s may become observable. An example here i s the work of M a r t i n and h i s colleagues on P t e n X (17,18). One way of v e r i f y i n g that 'new electronic transitions appearing i n chain compounds with p o l a r i z a t i o n s p a r a l l e l to the stacks are a c t u a l l y Davydov components of t r a n s i t i o n s which take p l a c e i n the u l t r a v i o l e t i n the i s o l a t e d u n i t s i s to determine how t h e i r energy v a r i e s as one changes the i n t e r m o l e c u l a r spacing i n the stacks. In the p o i n t d i p o l e approximation the separation between the Davydov components i s p r o p o r t i o n a l to |M|2/R3 where M i s the t r a n s i t i o n d i p o l the centers of the molecule set of molecules to exemplify t h i s expression i s that of the t e t r a c y a n o - p l a t i n i t e s . Many s a l t s of t h i s anion contain stacks of Pt(CN)|~ w i t h t h e i r planes p a r a l l e l , though w i t h changing c a t i o n the Pt-Pt spacing can be v a r i e d from 3.13Â (Mg) to 3.69Â (Rb) (20). Although the lowest e x c i t e d s t a t e of Pt(CN)|~ i n s o l u t i o n l i e s a t 35,600 cm"*, i n a l l such s a l t s there i s an extremely intense absorption band i n the v i s i b l e or near u l t r a v i o l e t , p o l a r i z e d e n t i r e l y along the d i r e c t i o n of the stacks. With i n c r e a s i n g Pt-Pt s e p a r a t i o n the band moves to higher energy, and i n F i g u r e 4 we p l o t i t s energy against 1/R**. Both f o r the t e t r a c y a n o - p l a t i n i t e s and the isomorphous t e t r a c y a n o p a l l a d i t e e , which are included on the same p l o t , there i s a very s a t i s f a c t o r y c o r r e l a t i o n between these two q u a n t i t i e s . Furthermore, e x t r a p o l a t i n g the two s t r a i g h t l i n e s of Figure 4 to R » », i . e . , to the i s o l a t e d molecule, we f i n d energies of 44,800 cm" (Pt(CN)|~) and 52,900 cm"" (Pt(CN)|~) which, bearing i n mind that the aqueous s o l u t i o n s p e c t r a of the ions do not s t r i c t l y represent those of the i s o l a t e d molecules, i s w e l l w i t h i n the energy range of the z - p o l a r i z e d allowed t r a n s i t i o n s assigned by Mason and Gray (21). 2

1

f

1

5.

1

Mixed Valence Metal Chain Compounds

About f o r t y elements form compounds i n which, to s a t i s f y the observed s t o i c h i o m e t r y , one has to a s s i g n d i f f e r e n t o x i d a t i o n numbers to d i f f e r e n t metal ions of the same element i n the same lattice. These s o - c a l l e d 'mixed valence' compounds o f t e n have p r o p e r t i e s f a r from a simple s u p e r p o s i t i o n of those we would p r e d i c t f o r each of the o x i d a t i o n s t a t e s taken s e p a r a t e l y . In p a r t i c u l a r , i f the s i t e s occupied by the metal ions of d i f f e r i n g valency have s i m i l a r c o o r d i n a t i o n numbers and l i g a n d environments, the energy needed to t r a n s f e r an e l e c t r o n from one to the other, i . e . , to interchange the v a l e n c i e s , may be v e r y low. Indeed, i f the metal ion s i t e s i n the l a t t i c e of our

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(ESCA, Mossbauer

Class

I n s u l a t o r a t 4K

(probably

Mixed Valence

M e t a l l i c a t 300K

Semiconductor

Transport

(Metal atoms equivalent)

(Valences

trapped)

IIIB ( I n c i p i e n t l y II??)

II

intrinsic)

Dlamagnetic

Plasma edge i n v i s i b l e

f o r En chains

Opaque a t a l l frequencies

E J L chains

Weak allowed t r a n s i t i o n s when

Dlamagnetic

etc)

metal charge

t r a n s f e r f o r Ε it chains

Metal

when Ex chain

No l i g a n d f i e l d s t a t e s seen

Direct Interaction

Chains

Magnetic

or m e t a l l i c )

(charge t r a n s f e r

(b) C o l l e c t i v e

(ligand f i e l d )

Bridged

Only weakly perturbed

Anion

P r o p e r t i e s of Mixed Valence T r a n s i t i o n Metal

(a) L o c a l l y e x c i t e d

Optical

Table 4.

2* ο ο

248

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

mixed valence compound a l l t u r n out to be c r y s t a l l o g r a p h i c a l l y i d e n t i c a l , then one may have a n o n - i n t e g r a l number of e l e c t r o n s at each s i t e , even i n the ground s t a t e . Searching out the known mixed valence compounds from a l l corners of the P e r i o d i c T a b l e , and c o r r e l a t i n g t h e i r p r o p e r t i e s with the observed s t r u c t u r e s , we f i n d that they can be d i v i d e d i n t o three broad c l a s s e s (22). I f the l i g a n d f i e l d s around the ions of d i f f e r i n g valence are v e r y d i f f e r e n t , i o n i z e d e x c i t o n s , or i n chemists language metal-to-metal charge t r a n s f e r s t a t e s , l i e a long way above the ground s t a t e whose p r o p e r t i e s , along with those of many lower e x c i t e d s t a t e s , are very c l o s e to being j u s t a s u p e r p o s i t i o n of the p r o p e r t i e s of the c o n s t i t u e n t i o n s . T h i s category, c a l l e d c l a s s I, i s of l i t t l e i n t e r e s t here. At the opposite extreme, a t r u l y n o n - i n t e g r a l number of e l e c t r o n s at each metal i o n s i t e , which would be r e q u i r e i d e n t i c a l , can only b continuous. T h i s category we c a l l c l a s s I I I . Between c l a s s e s I and I I I l i e s a continuous spectrum of cases ( c l a s s II) i n which two types of metal i o n s i t e s can be d i s t i n g u i s h e d by c r y s t a l l o g r a p h y , but i n which i o n i z e d e x c i t o n s l i e c l o s e enough to the ground s t a t e that they c o n t r i b u t e to the o p t i c a l spectrum i n the v i s i b l e or even the near i n f r a r e d . Some of the i o n i z e d excitons may even have the c o r r e c t symmetry to mix with the ground s t a t e , p a r t l y smudging out the formal o x i d a t i o n numbers defined by counting e l e c t r o n s a t the two metal i o n s i t e s . We w i l l now examine the p r o p e r t i e s of the known mixed valence metal c h a i n compounds i n the l i g h t of t h i s simple c l a s s i f i c a t i o n . J u s t as i n the s i n g l e valence chains, we d i s t i n g u i s h the two cases of anion b r i d g i n g and d i r e c t l y i n t e r a c t i n g metal i o n s . 1

(a) Anion Bridged Compounds. A l l the anion bridged Pt and Au compounds have two s e t s of c r y s t a l l o g r a p h i c a l l y d i s t i n c t metal i o n s i t e s , and may t h e r e f o r e be c h a r a c t e r i z e d as c o n t a i n i n g trapped valences i n t h e i r ground s t a t e s , (II,IV) i n the former and ( I , I I I ) i n the l a t t e r . X-ray photoelectron (XPS) spectroscopy should be a powerful t o o l f o r d e c i d i n g whether d i f f e r e n t p a r t i a l changes must be assigned to the ions or each type of s i t e , because we might hope to see core i o n i z a t i o n s from each type. I f we do not (assuming that the reason i s not simply the poor r e s o l u t i o n of the technique), then the p a r t i a l charges are equal o r , s a i d another way, the e l e c t r o n s are exchanging between the s i t e s more r a p i d l y than the time taken f o r the core e l e c t r o n to be e j e c t e d . F i g u r e 5 shows some XPS spectra of Pt chain compounds i n the 4f r e g i o n (23), i n c l u d i n g the mixed valence PtenCl3 (en: ethylenediamine). Crystallographically, the l a t t e r can be thought of as made up of a l t e r n a t i n g square planar P t e n C l and o c t a h e d r a l P t e n C l 4 molecules. Comparison between the spectra of PtenCl3 and i t s separated c o n s t i t u e n t molecules i s made more d i f f i c u l t by the ease with which PtenCl4 and PtenCl3 reduce i n the X-ray beam. However, by c a r e f u l l y I I

I v

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17.

DAY

Chain

Compounds

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

249

250

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

f o l l o w i n g the spectra of these two compounds as a f u n c t i o n of i r r a d i a t i o n time one f i n d s that the PtenCl3 spectrum i s indeed c l o s e to a s u p e r p o s i t i o n of those of PtenC±2 and PtenCl/ (Figure 6). However, the same cannot be s a i d of the o p t i c a l p r o p e r t i e s , f o r i n a d d i t i o n to weak absorption bands, p o l a r i z e d perpendicular to the chains, which do i d e n t i f y c l o s e l y with l o c a l l y e x c i t e d l i g a n d from t r a n s i t i o n s of the c o n s t i t u e n t s , PtenCl3 and other s i m i l a r s a l t s a l l have intense absorption across most of the v i s i b l e , p o l a r i z e d e n t i r e l y p a r a l l e l to the chains (24)» This we a s s i g n as due to metal to metal charge t r a n s f e r , i . e . , to i o n i z e d e x c i t o n s t a t e s , and the compounds belong to c l a s s I I defined above. Because the valences i n the ground s t a t e are trapped, though, the c r y s t a l s are semiconducting r a t h e r than metallic. (b) D i r e c t l y I n t e r a c t i n g Metal Ions. In the s i n g l e valence chains when the metal ions were brought c l o s e enough together to i n t e r a c t d i r e c t l y , i n s t e a d of through a b r i d g i n g group, l a r g e Davydov s h i f t s of the n e u t r a l e x c i t o n s t a t e s showed that the metal-metal i n t e r a c t i o n was very much increased. A comparable e f f e c t on the i o n i z e d e x c i t o n s t a t e s of a mixed valence chain might b r i n g them c l o s e enough to the ground s t a t e f o r the system to f i n d i t s e l f on the other s i d e of a Mott t r a n s i t i o n , and to have a conducting ground s t a t e i n which the e l e c t r o n s t a t e s were t r u l y c o l l e c t i v e . T h i s i s the background to the recent i n t e r e s t i n compounds l i k e K2Pt(CN) Br ,3o3H 0(KCP) (25). At f i r s t s i g h t i t has a l l the c h a r a c t e r i s t i c s expected of a c l a s s I I I mixed valency compound: d e s p i t e the n o n - i n t e g r a l o x i d a t i o n number, a l l the Pt atoms appear to occupy c r y s t a l l o g r a p h i c a l l y equivalent s i t e s ; w i t h the i n c i d e n t e l e c t r i c v e c t o r p a r a l l e l to the chains i t i s opaque throughout the v i s i b l e and i n f r a r e d down to very low f r e q u e n c i e s , and has a plasma edge i n the r e f l e c t i v i t y i n the v i s i b l e ; a t room temperature i t i s a m e t a l l i c conductor. Unfortunately, i f more i n t e r e s t i n g l y , however, there are some f e a t u r e s which complicate such a simple view. At low temperature, the compound i s not m e t a l l i c , but semiconducting. There i s a l s o evidence of a tendency f o r the Pt atoms to become i n e q u i v a l e n t as a r e s u l t of long wavelength a c o u s t i c phonon instabilities. I f such an i n s t a b i l i t y , which showed up by i n e l a s t i c neutron s c a t t e r i n g as a Kohn anomaly i n the a c o u s t i c phonon d i s p e r s i o n became locked i n as a s t a t i c d i s t o r t i o n , i t would provide enough reason f o r the energy gap a t low temperature. A c r u c i a l experiment would t h e r e f o r e be to examine the c r y s t a l s t r u c t u r e a t l i q u i d helium temperature. We r e c e n t l y c o l l e c t e d X-ray d i f f r a c t i o n data from a s i n g l e c r y s t a l of KCP a t room temperature, 77° and 4.2°K (26). The arrangement of the d i f f r a c t o m e t e r and c r y o s t a t r e q u i r e d one to concentrate on one c r y s t a l a x i s at a time, and we chose the (0,0,A). Within that l i m i t a t i o n we found no s i g n of Bragg peaks 4

0

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17.

DAY

Chain

Compounds

Figure

251

6. XPS spectrum of PtenCl in the 4f region after surface cleaning 3

Pt

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

252

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

corresponding to ' l o c k i n g i n ' a P e i e r l s d i s t o r t i o n at the repeat d i s t a n c e (6·67 times the Pt-Pt spacing) suggested by the neutron s c a t t e r i n g r e s u l t s . What we d i d f i n d , however, i s a set of seven peaks indexing as (0,0,ε), where ε •= η/ζ. The peaks w i t h η odd are weaker than those with η even, and a l e a s t squares f i t to the Bragg angles of those with η - 1,2,3,4,6,8,10 y i e l d s a v a l u e f o r ζ of 8.97. A key o b s e r v a t i o n i s that the peaks are present a t room temperature as w e l l as a t 4.2°K, and that they do not vary i n any discontinuous way as the temperature i s lowered. They must t h e r e f o r e r e s u l t from some s u p e r l a t t i c e o r d e r i n g which has not p r e v i o u s l y been detected. Our present view i s that the s u p e r l a t t i c e i s probably connected with o r d e r i n g among the bromide i o n s , supposed i n the o r i g i n a l c r y s t a l s t r u c t u r e determination to be randomly d i s t r i b u t e d on three f i f t h s of the a v a i l a b l of Pt(CN>4 groups. Fro such as the hexagonal tungsten bronzes, we b e l i e v e a random d i s t r i b u t i o n of anions i s a. p r i o r i u n l i k e l y , a view r e i n f o r c e d by the v e r y narrow composition range which KCP e x h i b i t s . Since i t has become such an important model compound f o r one-dimensional m e t a l l i c behavior the s u b t l e r d e t a i l s of i t s s t r u c t u r e o b v i o u s l y deserve the c l o s e s t a t t e n t i o n . L i t e r a t u r e Cited 1.

W e l l s , A. F., "Structural Inorganic Chemistry," 3rd ed., p. 375, Oxford U n i v e r s i t y Press, Oxford, 1962.

2.

Hutchings, M. T., Shirane, G., Birgeneau, R. J . and H o l t , S. L., Phys. Rev. (1972), B5, 1999.

3.

Jensen, S. J . , Andersen, P. and Rasmussen, S. Ε., Acta Chem. Scand. (1962), 16, 1890.

4.

Elliot, 1846.

5.

McPherson, A. L., Kistenmacher, T. J . , F o l k e r s , J . B. and Stucky, G. D., J . Chem. Phys. (1972), 57, 3771.

6.

Hutchings, M. T., Day, P., Gregson, Α. Κ., Leech, D. H. R a i n f o r d , B. D., P h y s i c a l Society Meeting, Manchester, England, January 1974.

7.

S e l l , D. D., Greene, R. L. and White, R. Μ., Phys. Rev. (1967), 158, 489.

8.

Day, P. and D u b i c k i , L., J.C.S. Faraday I I (1973), 69,

N. and P a u l i n g , L., J . Amer. Chem. Soc. (1938), 60,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

and

363.

17. D A Y

9.

Chain

Compounds

253

D i n g l e , R., L i n e s , M. E. and H o l t , S. L., Phys. Rev. (1969), 187, 643.

10. Ebara, K. and Tanabe, Υ., J. Phys. Soc. Japan (1974), 36, 93. 11. Smith, T. and F r i e d b e r g , S. Α., Phys. Rev. (1968), 176, 660. 12. Soos, Z. G., Huang, T. Z., V a l e n t i n e , J. S. and Hughes, R. C., Phys. Rev. (1973), B8, 993. 13. Yamada, S. J. Amer. Chem. Soc. (1951), 73, 1579. 14. Day, P., Orchard, A. F., Thomson, A. J. and W i l l i a m s , R. J. P., J. Chem. Phys. 15. idem.,

ibid.

(1965), 43, 3763.

16. Day, P., Inorg. Chim. Acta Rev. (1969), 3, 81. 17. M a r t i n , D. S., Hunter, L. D., Kroening, R. F. and Coley, R. F., J . Amer. Chem. Soc. (1971), 93, 5433. 18. Kroening, R. F., Hunter, L. D., Rush, R. Μ., Clardy, J. C. and M a r t i n , D. S., J. Phys. Chem. (1973), 77, 3077. 19. C r a i g , D. P. and Walmsley, S. Η., "Exitons in Molecular C r y s t a l s , " W. A. Benjamin Inc., New York, 1968. 20. Moreau-Colin, M. L., S t r u c t u r e and Bonding

(1972), 10, 167.

21. Mason, W. R. and Gray, H. B., J. Amer. Chem. Soc. (1968), 90, 5721. 22. Robin, M. B. and Day, P., Adv. Inorg. Chem. and Radiochem. (1967), 10, 247. 23. McGilp, J. Chemistry, Part I I T h e s i s , Oxford, 1973. 24. Yamada, S. and Tsuchida, R., 29, 894.

Bull.

Chem. Soc. Japan

(1956),

25. See papers by K. Krogman and H. R. Z e l l e r i n t h i s symposium. 26.

Griffiths,

D., Day, P. and Wedgwood, F. Α., unpublished.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

18 Evidence for Extended Interactions between Metal Atoms from Electronic Spectra o f Crystals with Square Complexes DON S. MARTIN, JR. A m e s L a b o r a t o r y of the U . S . A . E . C .

Abstract

Frequently, the square-planar molecular or ionic complexes of platinum(II) or palladium(II) in crystals are aligned directly over one another in one dimensional stacks. The electronic ab­ sorption spectra of the crystals for polarized light in favorable circumstances provide information about the crystal interactions and possible intermolecular electron transfers in the solid state. For crystals with large metal-metal separations, >4.1Å, the d-d spectra correlate closely in energies and intensities with solu­ tion spectra. The spectra of crystals with Pt Br 2- ions indi­ cate metal-metal electron transfer between metals at 3.55Å, al­ though the transfer may be influenced by the bromide bridges. For Magnus green salt the spectral changes of PtCl 2- are con­ sistent with large energy shifts of Frenkel excitons by crystal effects. However, for some molecular complexes with separations of 3.4-3.5Å, electron transfers to ionic exciton states occur. 2

6

1

4

Introduction The electronic absorption spectra have been measured recently for single crystals of a number of ionic or molecular square planar complexes of platinum(I I). Frequently, such complexes stack face to face in linear arrays in crystals to provide strong anisotropics in the absorption of polarized light and in the electrical conductivities. Our concern here will be for the ev­ idence of transfer of electrons between the platinum atoms. If such electron transfer transitions can be identified, their wave lengths provide directly the energy required for the transfer. Our work to the present has been restricted to the complexes with halide and amine ligands. The absorption bands for the platinum complexes at room tem­ perature are rather broad; half-widths of approximately 2,000 cm"* are common at 300°K. A transition bandais normally characterized by the wave length, λ, or wave number, v, of the maximum, by the 254 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

18.

Crystals

MARTIN, JR.

with

Square

Complexes

255

intensity, € , m o l a r a b s o r p t i v i t y a t t h e maximum o r as t h e o s c i l l a t o r s t r e n g t h : f = 4.32 χ 10" J*e dv and by t h e t e m p e r a t u r e dependence o f t h e i n t e n s i t y . For s i n g l e c r y s t a l s t h e r e i s a l i m i t upon t h e i n t e n s i t y imposed by t h e p r a c t i c a l lower l i m i t s o f c r y s ­ t a l t h i c k n e s s , w h i c h has been e x t e n d e d down t o 1-20μ. T h u s , t h e s t u d i e s a r e l i m i t e d t o bands w i t h e l e s s t h a n 1,000-2,000 cnP-M" . These a r e the n o r m a l l y f o r b i d d e n t r a n s i t i o n s . Hence, t h e a l l o w e d t r a n s i t i o n s a r e not c h a r a c t e r i z e d by c r y s t a l a b s o r p t i o n s p e c t r o s ­ copy except i n t h e i r absence. The t e m p e r a t u r e dependence o f i n t e n s i t y i s an e s p e c i a l l y powerful d i a g n o s t i c property. For t h e d«-d t r a n s i t i o n s , f o r exam­ p l e , w h i c h a r e symmetry f o r b i d d e n , an a s y m m e t r i c v i b r a t i o n may s e r v e as a p e r t u r b a t i o n t o g i v e a v i b r o n i c wave f u n c t i o n i n t o w h i c h i s mixed an a s y m m e t r i c e l e c t r o n i c wave f u n c t i o n . Thus t h e t r a n s i t i o n s may o b t a i able intensity. Sinc w h i c h s e r v e s as t h e p e r t u r b a t i o n , temperatures, t h e r e i s n o r m a l l y a d e c r e a s e i n i n t e n s i t y . £or a b a n d , e x c i t e d vibronic ly by a v i b r a t i o n o f wave number, V j , t h e i n t e n s i t y dependence (\) i s g i v e n by t h e e q u a t i o n : m a x

9

1

f ( T ) = f(0°K) c o t h ( h V j / 2 k T ) .

(I)

However, f o r a band w i t h a n o n - z e r o t r a n s i t i o n d i p o l e , s i n c e t h e i n t e g r a t e d i n t e n s i t y i s not t e m p e r a t u r e d e p e n d e n t , t h e peak height, e , w i l l i n c r e a s e as t h e band n a r r o w s a t l o w e r t e m p e r a ­ ture. F o r t h e h a l i d e and a m i n e - h a l i d e c o m p l e x e s o f p l a t ? n u m ( 1 1 ) t h e c r y s t a l s p e c t r a have i n d i c a t e d t h e o n e - e l e c t r o n o r b i t a l o r d e r i n g i n F i g u r e 1, w h i c h a p p l i e s t o t h e t e t r a g o n a l symmetry 4hThe l o w e s t u n o c c u p i e d o r b i t a l i s t h e σ" o r b i t a l b a s e d on the d _ y 2 w i t h b symmetry. W i t h t h e o t h e r d - o r i b t a l s f i l l e d , t h e g r o u n d s t a t e w i l l be a A i g ; and d-d t r a n s i t i o n s t o t h e s i n g l e t and t r i p l e t s t a t e s o f A g , Eg and B i g r e s p e c t i v e l y s h o u l d o c c u r , the t r i p l e t s t a t e s l y i n g below the corresponding s i n g l e t s . W i t h t h e h i g h s p i n - o r b i t c o u p l i n g o f a heavy P t - a t o m t h e s p i n d e s i g n a t i o n s a r e not e x a c t , and t h e s t a t e s must be d e s c r i b e d by t h e d o u b l e symmetry g r o u p , i n t h i s c a s e D4. Interactions along a s t a c k of square p l a n a r ions w i l l normally i n v o l v e the d s o r b i t a l , which possesses sigma c h a r a c t e r about the s t a c k i n g a x i s . F o r c o m p l e x e s w i t h low e n e r g y π a c c e p t o r o r b i t a l s s u c h as c y a n i d e and o x a l a t e , t h e d o r b i t a l may be much h i g h e r , and i n d e e d has been c o n s i d e r e d by S c h a t z and c o w o r k e r s (2) t o be t h e h i g h e s t f i l l e d orbital for Pt(CN) ". Even i n t h e h a l i d e c o m p l e x e s t h e assignment of the t r a n s i t i o n t o s t a t e (σ * - d 2 ) i s somewhat mx

n

x 2

i g

1

2

z

z 2

3

4

z

open t o q u e s t i o n s i n c e t h e a s s i g n e d band i s s u r p r i s i n g l y weak. B e l o w t h e d o r b i t a l s i n F i g u r e 1 a r e shown t h e o r b i t a l s d e r i v e d f r o m π and σ o r b i t a l s on t h e l i g a n d s . O n l y t h e odd o r b i t a l s a r e d e s i g n a t e d f o r t h e s e can p r o v i d e i n t e n s e P t ( a " ) g * ~ L t r a n s i t i o n s . A p o s s i b l e a l t e r n a t i v e a s s i g n m e n t o f i n t e n s e t r a n s i t i o n s may u

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

256

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

be 6p «-5d, f o r t h e 6 p o r b i t a l i s t h e s e c o n d l o w e s t u n f i l l e d orb i t a l . F u l l y a l l o w e d m o l e c u l a r t r a n s i t i o n s i n D^h must be A s ( z p o l a r i z a t i o n ) or Ε (χ,ν^ p o l a r i z a t i o n ) . A l l o t h e r t r a n s i t i o n s a r e s e e n a s a c o n s e q u e n c e o f v i b r o n i c and s p i n - o r b i t p e r t u r b a t i o n s , w h i c h m i x t h e s e s t a t e s i n t o t h e wave f u n c t i o n s . In a c o n s i d e r a ­ t i o n of a l l t h e v i b r a t i o n s o f an i o n such as P t B r " i t i s found t h a t t h e ΙΚ2^ s t a t e i s v i b r o n i c l y forbidden i n z-polarization but a l l o w e d i n x , ^ . The E and * B i g t r a n s i t i o n s a r e a l l o w e d i n t h e t h r e e p o l a r i z a t i o n s x , ^ and z. Since the s i n g l e t spin func­ t i o n s have t h e symmetry, Α χ , and t r i p l e t s a r e A^ and E i în t h e d o u b l e g r o u p , Oi, t h e t r i p l e t s t a t e s e l e c t i o n r u l e s a r e d i f f e r e n t from those o f the s i n g l e t s t a t e s . For c r y s t a l l i n e s t a t e s i n which extended i n t e r a c t i o n s a r e small, there i s n e g l i g i b l c o m p l e x e s , and e l e c t r o n l o c a l l i z e d , however, and o t h e members o t h e c r y s t a influence t r a n s i t i o n energies. In s u c h c a s e s t h e t h e o r y o f F r e n k e l t y p e e x c i t o n s f o r m o l e c u l a r c r y s t a l s c a n a p p l y . The c r y s t a l ground s t a t e wave f u n c t i o n w i l l have t h e f o r m z

z

1

u

τ

υ

2

4

τ

1

Φ°

0

* -d. The A u s t a t e w o u l d c o r r e s p o n d t o 6p «-5d . The v e r y weak peak a t c a . 3 0 , 5 0 0 cm" seen i n both p o l a r i ­ z a t i o n s d e s e r v e s some comment. In c - p o l a r i z a t i o n , a t l e a s t , i t seems t h a t a v i b r o n i c l y e x c i t e d t r a n s i t i o n w o u l d be a p p a r e n t i n t h e 300°K s p e c t r u m . Even t h o u g h t h e a b s o r p t i o n i s r i s i n g r a p i d l y i n t h i s r e g i o n , i t can not be d i s c e r n e d a t a l l . I t a p p e a r s t h e r e ­ f o r e t o have a s m a l l but n o n - z e r o t r a n s i t i o n d i p o l e , and i t i s a s s i g n e d as E . The t r a n s i t i o n t o t h i s s t a t e i s d i p o l e - a l lowed 1

1

1

1

4

2

4

m x

2

4

1

2

4

1

1

1

2

2

4

2

6

1

1

m x

1

1

4

1

1

v

U

u

1

2 U

2

2 u

1

1

1

2

2

4

1

1

u

1

2

2

4

4

2

4

1

2

z

z2

1

3

U

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

260

EXTENDED

ι ι ι ι I

I

I

I

I

j

1

I

I

I

I

I

!

I

INTERACTIONS

BETWEEN METAL

IONS

!

Kj>PtBr

4

100

15

Figure

3.

20

Polarized

25

crystal absorption

30

35

spectra for

K PtBr 2

k

50i J

45

A

2

u

40 :,

E - B, (E,' ); U

3

U

U

235 χ

3

B,u

ε 30

-/

.—'Eg

υ - — 'A2a . - B , Q (Ei )

la

3

fl

25 20 h

K PtBr 2

4

K PtCI 2

4

Figure 4. Excited states for K PtBr,, and KjPtCl,. Length of the line for each state is 15 proportional to log €„,„, (15°K) for the crystal transitions and log «,„„, (soin) for the intense transitions. 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

18.

Cry stab with

MARTIN, JR.

by

Square

Complexes

virtue of the spin-orbit coupling

Magnus

1

Green S a l t .

261

i n both p o l a r i z a t i o n s .

PtÇNHaUPtCU.

I t h a s l o n g been r e c o g n i z e d t h a t t h e r e must be s t r o n g c r y s ­ t a l e f f e c t s on t h e s p e c t r a i n Magnus' g r e e n s a l t (MGS), Pt(NH3) PtCl . The component c a t i o n , P t ( N H 3 ) , i s c o l o r l e s s i n aqueous s o l u t i o n o r h a l i d e s a l t s w h e r e a s t h e a n i o n , P t C l " , i s r e d . A s t h e name i m p l i e s t h e MGS i s a d a r k g r e e n , v e r y i n s o l u b l e salt. I t p o s s e s s e s a t e t r a g o n a l c r y s t a l s t r u c t u r e (9) w i t h two of each ion p e r u n i t c e l l . There a r e s t a c k s o f a l t e r n a t i n g ions w h i c h a r e i l l u s t r a t e d i n F i g u r e 5 w i t h a s p a c i n g o f 3.24A between t h e a n i o n s and c a t i o n s i n t h e c h a i n s . The p o l a r i z e d c r y s t a l s p e c ­ t r a a r e p r e s e n t e d i n F i g u r e 6. The c r y s t a l s a r e h i g h l y d i c h r o i c . A c r y s t a l w h i c h a p p e a r s d a r k g r e e n i n c_-polar i z a t i o n i s p a l e y e l low i n ^ - p o l a r i z a t i o n much h i g h e r i n t h e c - p o l a r i z a t i o r e s u l t s f r o m a window a t a b o u t 20,000 c m " . The waves i n t h e r e c o r d i n g a t l o w a b s o r b a n c e a r e due t o i n t e r f e r e n c e by m u l t i p l y r e f l e c t e d l i g h t (10). A l t h o u g h t h e i n s t r u m e n t a t i o n d i d n o t p e r ­ m i t measurements b e l o w 17,000 c m " , i t i s c l e a r t h a t t h e l o w e n e r g y band i n £-polarization i s v i b r o n i c i n c h a r a c t e r . A t h i g h e r e n e r g i e s t h e a b s o r p t i o n i n c r e a s e s beyond a n ε o f 700 cm" Μ " , t h e l i m i t o f t h e measurements. In a_-polar i z a t i o n t h e s p e c ­ t r u m i s d o m i n a t e d by a s i n g l e peak a t a b o u t 25,000 c m " , a l t h o u g h t h e r e i s a s h o u l d e r a t 23,000 cm" and a weak s p i n - f o r b i d d e n band b e l o w 17,000 c m " . The wave numbers o f t h e t h r e e p e a k s i n K s P t C l s o l u t i o n a r e shown by t h e 3 a r r o w s i n F i g u r e 6. I t i s concluded t h a t t h e s p i n - f o r b i d d e n p e a k o f P t C l " has been r e d - s h i f t e d b y a b o u t 4,000 cm" i n MGS. The AQQ t r a n s i t i o n and t h e trans­ i t i o n s have c o a l e s c e d t o t h e s i n g l e peak a t a b o u t 25,000 cm" . T h i s c o r r e s p o n d s t o a r e d s h i f t o f n o t more t h a n 1,000 cm" f o r t h e ^ g s t a t e a n d a b o u t 4000-5000 cm" f o r t h e ^-Eg s t a t e . The r e l a t i v e s h i f t o f t h e s e t r a n s i t i o n s i s a t t r i b u t a b l e t o t h e D' t e r m o f e q u a t i o n 5. The a n g u l a r d e p e n d e n c e o f t h e d o r b i t a l s i n ­ v o l v e d i s shown i n F i g u r e 7 . T h u s , f o r t h e t r a n s i t i o n t o t h e A s g s t a t e , a"(d 2_y2)*~d y, both o r b i t a l s a r e concentrated i n the plane o f t h e i o n and t h e i r e l e c t r o n s s u f f e r s i m i l a r i n t e r a c t i o n s w i t h t h e e l e c t r o n c l o u d s o f t h e adj'acent i o n s . For the d o r b i t a l , however, t h e r e i s a c o n s i d e r a b l e e l e c t r o n d e n s i t y o u t o f t h e p l a n e o f t h e i o n . The e n e r g y o f t h e e l e c t r o n i n t h i s o r b i t a l i s t h e r e f o r e r a i s e d by t h e e l e c t r o n - e l e c t r o n r e p u l s i o n s , and a red s h i f t f o r t h e t r a n s i t i o n o c c u r s . The h i g h a b s o r p t i o n i n t h e c - p o l a r i z a t i o n i s c l a r i f i e d f r o m d i f f u s e r e f l e c t a n c e s p e c t r a f o r K P t C l and MGS w h i c h a r e shown i n F i g u r e 8. I t c a n be s e e n t h a t t h e maximum, w h i c h c o r r e s p o n d t o a v e r y i n t e n s e t r a n s i t i o n , has been s h i f t e d f r o m c a . 44,000 cm" i n K P t C l t o 35,000 cm" i n MGS. T h i s band i s a p p a r e n t l y i n t h e c - p o l a r i z a t i o n . Such a l a r g e r e d s h i f t must be l a r g e l y c a u s e d b y t h e I t e r m o f e q u a t i o n 5. I t w o u l d r e q u i r e s t r o n g 2 +

4

4

4

2

4

1

1

1

1

1

1

1

4

2

4

1

1

1

1

1

X

X

y

2

1

4

1

2

4

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

z

262

EXTENDED

Figure

Figure 6. Polarized crystal absorption trum for Magnus' green salt

INTERACTIONS

5.

Alternate Magnus'

BETWEEN

METAL

stacking of green salt

spec-

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

ions

in

18.

MARTIN, JR.

Figure

7.

Angular

Crystals

with

Square

Complexes

portions of ά-orbitals involved tral transitions of MGS

Figure

8.

263

in spec­

Diffuse reflectance spectra and K.FtCh, at 300°Κ

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

for

MGS

264

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

2

2 +

t r a n s i t i o n s o f c o m p a r a b l e e n e r g y i n b o t h P t C l ~ and P t ( N H 3 ) . T h i s i s the a l l o w e d t r a n s i t i o n from which the v i b r o n i c e x c i t a t i o n s i n c - p o l a r i z a t i o n b o r r o w i n t e n s i t y , and t h e p r o x i m i t y o f t h i s band e n h a n c e s t h e i r i n t e n s i t y by a b o u t an o r d e r o f m a g n i t u d e . The s p e c t r a o f MGS t h e r e f o r e i n d i c a t e t h a t t h e s t r i k i n g c o l o r changes r e s u l t f r o m c r y s t a l i n t e r a c t i o n s , p r i m a r i l y w i t h a d j a c e n t n e i g h b o r s , w h i c h m o d i f y t h e m o l e c u l a r t r a n s i t i o n s but w i t h o u t d e l o c a l i z a t i o n of e l e c t r o n s . 4

Pt Br 2

2 6

"

4

Spectra

A d i m e r complex p r o v i d e s an o p p o r t u n i t y f o r t h e d e m o n s t r a ­ t i o n o f e l e c t r o n d e l o c a l i z a t i o n on a l i m i t e d s c a l e by a b s o r p t i o n spectroscopy. The d i m e r i c a n i o n c r y s t a l l i z e s i n t h e t e t r a e t h y l ammonium s a l t w h i c h i w h i c h t h e s t r u c t u r e wa t i o n o f d i m e r i c i o n on t h e c l e a v a g e f a c e s show Figure 9. T h e r e i s one a n i o n p e r u n i t c e l l . The p l a t i n u m atoms i n a d i m e r a r e s e p a r a t e d 3.55Â, and t h e a n i o n s a r e s e p a r a t e d 7 . 6 i f r o m t h e i r n e a r e s t n e i g h b o r s by t h e b u l k y c a t i o n s . The l i g h t beam t h e r e f o r e e n t e r s d i m e r a n i o n n e a r l y end on. The m o l e c u l a r a x e s o f heavy a n i o n a p p a r e n t l y e s t a b l i s h the axes of the i n d i c a t r i x , so t h e r e i s v e r y l i t t l e d i r e c t i o n a l change o f t h e s e a x e s w i t h wave l e n g t h . One e x t i n c t i o n f o r c r y s t a l s was a l i g n e d , as a c c u r a t e l y as c o u l d be d e t e r m i n e d , w i t h t h e p r o j e c t i o n o f t h e y m o l e c u l a r a x i s upon t h e c r y s t a l f a c e o v e r t h e e n t i r e wave lengtTh r e g i o n w h i c h was s t u d i e d . The a b s o r p t i o n i n t h i s e x t i n c t i o n d i r e c t i o n gave a v e r y c l e a n intermediate a x i s or ^ - p o l a r i z a t i o n . The o t h e r e x t i n c t i o n p r o v i d e d p r i m a r i l y the s h o r t a x i s or x - p o l a r i z a t i o n . Preliminary s p e c t r a a r e shown i n F i g u r e 10. The a b s o r p t i o n i n ^ - p o l a r i z a t i o n i s much h i g h e r t h a n i n t h e x - p o l a r i z a t i o n . T h u s , a t room tempera­ t u r e t h e r e i s a peak i n γ p o l a r i z a t i o n a t 2 3 , 5 0 0 cm" very c l o s e t o t h e 24,000 cm" f i r s t spin-allowed transition in K P t B r . However, t h e e f o r t h e band a t t h i s V i n K P t B r was o n l v c a . 100 c n P - M . In t h e P t B r " s a l t i t was 800 cm' /H(?t Br ~J7 But when t h e c r y s t a l was c o o l e d , t h e peak h e i g h t i n c r e a s e d t o 1600 c n T V M , w h i c h i n d i c a t e d t h a t t h e t r a n s i t i o n was d i p o l e a l l o w e d r a t h e r than v i b r o n i c . In t h e x - p o l a r i z a t i o n t h e r e was not much i n d i c a t e d s t r u c t u r e i n t h e s p e c t r u m b u t t h e a b s o r p t i o n i n c r e a s e d as ν i n c r e a s e d . A peak a t 3 6 , 5 0 0 cm" occurs j u s t where t h e MH. c h a r g e t r a n s f e r band o c c u r s i n P t B r " s o l u t i o n . T h i s may be t h e n an x - p o l a r i z a t i o n o r o u t - o f - p l a n e M*-L w h i c h a p ­ p a r e n t l y i s v e r y weak i n P t B r " as w e l l . A t 15°K, band maxima a t 29,000 cm" and 32,000 cm" a r e s e e n as w e l l , and t h e r e i s a s h o u l d e r a t 26,000 cm" . In c o n s i d e r a t i o n o f t h i s s p e c t r u m e a c h p l a t i n u m atom can be c o n s i d e r e d t o b r i n g i n t o t h e d i m e r i t s complement o f f i v e 5 d orbitals. M o l e c u l a r o r b i t a l s f o r t h e d i m e r a r e c o n s t r u c t e d by t a k i n g t h e sum o r d i f f e r e n c e o f c o r r e s p o n d i n g d - o r b i t a l s on d i f ­ f e r e n t p l a t i n u m atoms. W i t h t h e c h o i c e o f a x e s t h e σ'* o r b i t a l s o

1

1

2

m a x

2

- 1

2

2

1

2

6

1

2

4

2

4

1

4

4

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

e

u

Cry stab with

MARTIN, JR.

Square

Complexes

[N(C H ) ]JPt Br ] 2

5

4

2

6

b

Figure

9.

Projection

of the dimeric ions, Pt Br ~ of[N(C H )J [Pt Br l 2

2

I

2000 "

1

1

1

I

J

1

1

5

2

1

2

6

upon the 001 cleavage

2

(

1

ι

ι

l—ι

j

ι

ι

ι

2

5

4

2

2

ι

Ϊ

1

6

300°Κ/^

- x polarization ^-

c

r* +

l 7

c 4p 2

z

,

N i

(4)

w h e r e Wj r e f e r s t o t h e a p p r o p r i a t e l i n e a r c o m b i n a t i o n o f a n t i - b o n d i n g l i g a n d ττ-orbitals and c ^ and c are mixing coefficients. I n t h e G r a y and B a l l h a u s e n p i c ­ t u r e , t h e n , one w o u l d h a v e i n s o l u t i o n a s i t u a t i o n w h e r e c\ » c g and be d e a l i n g , a s n o t e d a b o v e , w i t h an essentially metal-to-ligand transition. A n e x and K r i s t (3) h a v e a r g u e d t h a t one c a n t h e n r a t i o n a l i z e g r o w t h i n i n t e n s i t y , r e d s h i f t , and s h a r p e n i n g o f the c r y s t a l t r a n s i t i o n r e l a t i v e to i t s s o l u t i o n counter­ p a r t i n t e r m s o f t h e s o l i d - s t a t e p e r t u r b a t i o n , whose n a t u r e was n o t s p e c i f i e d , l o w e r i n g t h e 4 p orbital in e n e r g y t o t h e p o i n t w h e r e c-i « C 2 « One w o u l d t h e n be dealing with e s s e n t i a l l y a d 2 > p t r a n s i t i o n con­ f i n e d t o the m e t a l . E x p l a n a t i o n s f o r the development o f the g r e e n b a n d o f NiDMG t h a t i n v o l v e s p e c i f i c h y p o t h e s e s c o n ­ c e r n i n g the n a t u r e o f the s o l i d - s t a t e p e r t u r b a t i o n s have been proposed (5,6). These can most f r u i t f u l l y be d i s c u s s e d a f t e r t h e r e l a t e d s y s t e m s t o be dealt w i t h here have been reviewed. 2

z

z

z

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

19.

The

ANEx

Complexes

1

Magnus

with

Extended

Cliains

285

Salts

The q u a l i t a t i v e o b s e r v a t i o n o f t h e c o l o r c h a n g e s i n v o l v e d i n t h e f o r m a t i o n o f Magnus green s a l t , [ P t ( N H 5 ) 4 ] [ P t C l ^ ] , make i t a p p e a r t h a t t h e p h e n o m e n a observed there p a r a l l e l rather c l o s e l y those d e s c r i b e d p r e v i o u s l y f o r NiDMG. One t h u s f i n d s t h a t i f a n aqueous K o P t C l 4 s o l u t i o n , which i s r e d , i s combined w i t h a c o l o r l e s s aqueous Pt ( N H ^ ^ C l o s o l u t i o n , a g r e e n p r e c i p i t a t e o f M a g n u s g r e e n s a l t (MGS) i s o b t a i n e d . A c r y s t a l c o l o r d i f f e r e n t than t h a t o f e i t h e r o f t h e components m i g h t t h u s l e a d one t o assume t h a t once a g a i n o n e h a s some "new a b s o r p t i o n band a p p e a r i n g i n the v i s i b l e p a r t o f t h e spectrum. Day and coworkers (7) a n d , more r e c e n t l y sured thev i s i b l e absorptio c r y s t a l s a n d shown t h e m t o b e p e r t u r b e d P t C l 2 | 2 spec t r a , and n o t t o p o s s e s s any bands t h a t c o u l d n o t be t r a c e d t o those found i n t h e a n i o n i c s p e c i e s i n t h e KgPtClij. c r y s t a l and i t s aqueous s o l u t i o n . The g r e e n c o l o r o f t h e MGS c r y s t a l r e s u l t s f r o m a r e d s h i f t a n d i n t e n s i f i c a t i o n o f t h e P t C l i j " b a n d s i n MGS l e a d i n g t o a "window" i n t h e g r e e n r e g i o n o f t h e s p e c t r u m , r a t h e r than from t h e development o f an a d d i t i o n a l band i n t h e v i s i b l e p a r t o f t h e spectrum. (KgPtC^ also has a c r y s t a l s t r u c t u r e i n which t h e complex s p e c i e s are stacked, b u t here the metal-metal s e p a r a t i o n i s 4.13 a n d o n e may e x p e c t t h a t s t r o n g i n t e r c o m p l e x i n t e r a c t i o n s w i l l n o t be m a n i f e s t e d - an assumption t h a t i sborn o u t by t h e reasonably c l o s e correspondence o f t h e s o l u t i o n and c r y s t a l a b s o r p t i o n f o r t h i s system.) A c o n n e c t i o n between t h e o p t i c a l phenomena o b s e r v e d i n MGS-type s a l t s and t h o s e d i s c u s s e d p r e v i o u s l y f o r n i c k e l g l y o x i m a t e s was t o b e c o m e a p p a r e n t , h o w e v e r . Day jet a l . c a r r i e d o u t a s e r i e s o f p a r t i c u l a r l y s i g n i f i e a n T ~ e x p e r ime n t s, a n a l o g o u s t o t h o s e d e s c r i b e d i n the p r e c e d i n g s e c t i o n on t h e g l y o x i m a t e s , i n which t h e s p a c i n g b e t w e e n c o m p l e x e s i n t h e MGS c r y s t a l was i n creased by modifying t h e nature o f the ligands i n a manner e x p e c t e d t o a f f e c t t h e " s i n g l e - m o l e c u l e " s p e c t r a i n a minimal fashion (7 £). MGS-type s a l t s were t h u s p r e p a r e d i n w h i c h t h e NH-j l i g a n d s o f P t ( N H - j ) w e r e r e p l a c e d b y C H 3 - N H 2 (Me-MGS) a n d CH-z-Ct^-NiL (Et-MGS), r e s p e c t i v e l y . T h e MGS a n d Me-MGS c r y s t a l s h a v e t h e same m e t a l - m e t a l s p a c i n g ( 3 . 2 5 Â 1 , w h i l e i n Et-MGS t h i s p a r a m e t e r h a s a v a l u e o f 3 . 4 A ( 5 ) . T h e v i s i b l e s p e c t r a o f MGS a n d Me-MGS a r e v e r y s i m i l a r , a n d a s o n e m i g h t e x p e c t , t h e Et-MGS s p e c t r a a r e i n t e r m e d i a t e b e t w e e n t h o s e o f MGS a n d K ^ P t C l ^ ( 7 , 9 ) . Of 1

1

11

2

5

2 +

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

286

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

e q u a l i m p o r t a n c e , D a y et; a l . a l s o c a r r i e d o u t d i f f u s e r e f l e c t i o n s t u d i e s o n t h e s e same s y s t e m s , a n d f o u n d a s t r o n g c o r r e l a t i o n between m e t a l - m e t a l d i s t a n c e and the l o c a t i o n o f a s t r o n g u l t r a v i o l e t t r a n s i t i o n :

Compound

Metal-Metal Distance

P o s i t i o n o f uv T r a n s i t i o n (Diffuse Reflectance)

MGS

3.25 S

3^.5 kK

Me-MGS

3.25

34.5

Et-MGS

3.4

40

K PtCl^

4.1

2

* Actual

absorption

t o blue

of this

(11).

I t was t h u s s u g g e s t e d t h a t t h e e f f e c t s o b s e r v e d i n t h e v i s i b l e spectrum o f t h e Magnus s a l t s c o u l d be u n d e r stood i n terms o f t h e v i s i b l e bands borrowing intens i t y from t h e s t r o n g m e t a l - m e t a l s e p a r a t i o n dependent u l t r a v i o l e t band. I n such a s i t u a t i o n , the c l o s e r the uv b a n d was t o v i s i b l e t r a n s i t i o n s , t h e g r e a t e r i t s i n f l u e n c e on them c o u l d be e x p e c t e d t o be. Further, t h e f a c t t h a t t h e i n t e n s i t y p e r t u r b a t i o n s were s t r o n g er f o r t h e o u t - o f - p l a n e bands than t h e i n - p l a n e , l e d t o t h e c o n c l u s i o n t h a t t h e p e r t u r b i n g band must have out-of-plane polarization. T h e s e o b s e r v a t i o n s and t h e c o n c l u s i o n s drawn f r o m them l e d d i r e c t l y t o t h e s t u d y o f t h e a l l o w e d bands i n t h e same s e r i e s o f t h r e e M a g n u s s a l t s i n our laboratories (10). The r e s u l t s were s t r i k i n g l y i n a c c o r d w i t h t h e s u g g e s t i o n s made b y D a y et> a l . , a s may b e s e e n i n F i g u r e 6, w h i c h p r e s e n t s t h e c r u c i a l p o r t i o n s of the out-of-plane absorption spectra obtained by Kramers-Kronig a n a l y s i s o f the corresponding reflection spectra. The s t r o n g u l t r a v i o l e t a b s o r p t i o n band i s f o u n d t o o c c u r a t 3 4 . 5 , 3 4 . 4 , a n d 3 9 - 9 kK r e s p e c t i v e l y f o r MGS, Me-MGS a n d Et-MGS, i n e x c e l l e n t a g r e e ment w i t h t h e r e s u l t s o f d i f f u s e r e f l e c t i o n . I t was a l s o p o s s i b l e t o develop a q u a n t i t a t i v e c o r r e l a t i o n between t h e p o s i t i o n and i n t e n s i t y o f t h e s t r o n g u l t r a v i o l e t band i n these systems and t h e c o r r e s p o n d i n g outof -plane v i s i b l e i n t e n s i t y . S t r o n g e v i d e n c e was t h u s p r o v i d e d f o r t h e p r o p o s a l t h a t i n t e r a c t i o n between t h e e x c i t e d s t a t e s r e s p o n s i b l e f o r t h e v i s i b l e bands and that o f the u l t r a v i o l e t t r a n s i t i o n plays a substantial role i n the perturbation of thev i s i b l e spectra. 1

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

19.

Complexes

ANEx

400

with

Extended

WAVELENGTH πψ 500

Chains

600

16

12

25

2 FREQUENC Journal of the American Chemical Society

Figure 5. Portions of out-of-plane absorption spectra for four nickel dimethylglyoximates ob­ tained by direct absorption studies (3)

WAVELENGTH 300

30 Journal of Chemical Physics

Figure 6. Kramers-Kronig M G S (A), Me-MGS (B)

y

250

35 40 F R E Q U E N C Y kK

absorption curves for and Et-MGS (C) (10)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

288

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

The b a n d s shown i n F i g u r e 6 w e r e t h o u g h t b y b o t h Day a n d c o w o r k e r s a n d o u r g r o u p t o be c o r r e l a t e d w i t h t h e s o l u t i o n b a n d o f K ^ P t C l i j . t h a t p e a k s a t 46.3 kK w i t h a maximum m o l a r e x t i n c t i o n c o e f f i c i e n t o f 1 0 2 3 0 crrf-Hd"" (11). T h i s t r a n s i t i o n would c l e a r l y h a v e u n d e r g o n e a n i n t e n s i f i c a t i o n i n t h e MGS crystal, but i n e s t i m a t i n g the magnitude o f t h i s e f f e c t one must f i r s t o b s e r v e t h a t t h e e - v a l u e s r e p o r t e d i n F i g ­ u r e 6 must be d i v i d e d by 3 f o r c o m p a r i s o n t o s o l u t i o n v a l u e s and, s e c o n d l y , n o t e t h a t a t r u e comparison of i n t e n s i t i e s r e q u i r e s i n t e g r a t i o n s o v e r t h e b a n d s . Re­ c e n t s t u d i e s (12) i n d i c a t e t h a t t h i s band i n t e n s i f i e s b y a f a c t o r o f a p p r o x i m a t e l y 2.5 i n g o i n g f r o m ^ P t C l ^ t o MGS. Thus, a l t h o u g h the b e h a v i o r o f the u l t r a v i o ­ l e t band i n the Magnus the v i s i b l e band o i n one c r u c i a l f a c t o r : i t d o e s n o t show t h e d r a m a t i c i n t e n s i f i c a t i o n o b s e r v e d f o r t h e g r e e n b a n d o f NiDMG i n going from e s s e n t i a l l y i n f i n i t e metal-metal sepa­ r a t i o n t o one o f 3.25 This intensity behavior i s i n a c c o r d w i t h t h e p i c t u r e d e v e l o p e d f o r NiDMG, i f one r e c a l l s t h a t i n Magnus s a l t s one i s d e a l i n g w i t h l i g a n d s t h a t do n o t p o s s e s s u n s a t u r a t i o n . In Equation 4, t h e r e f o r e , c-^ w o u l d become z e r o f o r MGS and r e l a t e d c o m p o u n d s a n d t h e t r a n s i t i o n w o u l d be e s s e n t i a l l y d —> ρ at a l l stages of s o l i d - s t a t e perturbation. H e n c e one w o u l d n o t e x p e c t i n t h i s c a s e t o s e e a d r a ­ m a t i c d r o p i n i n t e n s i t y a s one moves t o t h e " s i n g l e molecule spectrum. T h i s i n t e r p r e t a t i o n o f the Magnus salt spectra o f c o u r s e c a r r i e s s t r o n g i m p l i c a t i o n s f o r t h e 46.3 kK s o l u t i o n band o f KpPtClij.: i t should, i f t h i s p i c t u r e i s c o r r e c t , a t l e a s t h a v e a c o m p o n e n t t h a t c a n be a s s o c i a t e d w i t h a p l a t i n u m i o n 5 d 2 —> 6p promotion. S i n c e a t t h e t i m e t h i s p r o p o s a l was made t h e 46.3 kK a b s o r p t i o n was g e n e r a l l y t h o u g h t t o be l i g a n d - t o - m e t a l c h a r g e - t r a n s f e r i n c h a r a c t e r (4,11), a s t u d y aimed a t c h a r a c t e r i z i n g the h i g h - e n e r g y a l l o w e d bands i n K ^ P t C l ^ a n d r e l a t e d compounds was undertaken. 1

3

1

1 1

1

z

Tetrachloroplatinate(II)

Ion

and

Related

Systems

F i g u r e J shows t h e s o l u t i o n s p e c t r a o f K p P t C l 4 and K P d C l 4 . I t w i l l be n o t e d t h a t t h e 46.3 kK b a n d o f KpPtCl2| h a s r o u g h l y t h e same e and, e x c e p t f o r i t s s t r u c t u r e , i s v e r y s i m i l a r i n a p p e a r a n c e t o t h e 35-7 kK KpPdCl/j. b a n d . I t was t h u s r a t h e r g e n e r a l l y a c c e p t e d (11) t h a t t h e s e two b a n d s w e r e c o r r e l a t e d w i t h one a n o t h e r , t h a t t h e 46.3 kK P t C l ^ " b a n d was strongly 2

m

a

x

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

19.

ANEX

Complexes

with

Extended

Chains

289

r e d s h i f t e d when Pd was s u b s t i t u t e d f o r P t , a n d t h a t t h e s e c o n d P d C l ^ " " band had s i m i l a r l y r e d - s h i f t e d from i t s vacuum-ultraviolet location i n PtCl^ "". Moreover, the whole a r r a y o f s t r o n g q u a r t z u l t r a v i o l e t bands i n t h e s e i o n s was t a k e n a s b e i n g c h a r g e - t r a n s f e r i n nature. I n o r d e r t o examine the v a l i d i t y o f t h i s p i c t u r e and, more p a r t i c u l a r l y , t o e x p l o r e t h e p o s s i b i l i t y o f a d —> ρ t r a n s i t i o n b e i n g i n v o l v e d i n t h e 46.5 kK P t C l i j " band, t h e r e f l e c t i o n s p e c t r a o f ^ P d C l i i and ^ P t C l i j . w e r e o b t a i n e d a n d a r e shown i n F i g u r e s 8 a n d 9. I t i s c l e a r t h a t K2PtCl4 c o n t a i n s a s t r o n g h i g h e n e r g y o u t - o f - p l a n e t r a n s i t i o n , w h i l e KoPdCl4 d i s p l a y s no a n a l o g o u s a b s o r p t i o n . I f one m a k e s t h e r e a s o n a b l e assumption t h a t comparabl transition i P t C l i j . ' and P d C l ^ p l a n e P t C l 4 " band jus opposit l y t h o u g h t t o be t h e c a s e - i t s h i f t s t o h i g h e r , n o t p d c l l o w e r , f r e q u e n c i e s on m o v i n g f r o m P t C ^ " 4 · E x a m i n a t i o n o f t h e d —> ρ t r a n s i t i o n s o f P t and Pt and t h e c o m p a r a b l e p a l l a d i u m s p e c i e s r e v e a l s a b l u e s h i f t s i m i l a r t o t h a t i n f e r r e d f o r the t e t r a c h l o r o i o n s a s one m o v e s f r o m p l a t i n u m t o p a l l a d i u m (11), a c o r r e l a t i o n that lends a d d i t i o n a l p l a u s a b i l i t y t o t h e d —> ρ assignment i n PtCl4 ~. One may a l s o n o t e t h a t a t l e a s t two p r e d i c t i o n s f l o w f r o m t h e f o r e ­ going discussion: 1) S i n c e i n ammonia t h e e l e c t r o n s a n a l o g o u s t o those thought to p a r t i c i p a t e i n the lowestenergy l i g a n d - t o - m e t a l c h a r g e - t r a n s f e r band i n P t C l i . 2 " a r e t i e d up i n b o n d i n g , one w o u l d expect t h i s band t o o c c u r a t c o n s i d e r a b l y _ higher energies i n Pt(NH^)4 than i n PtCl4 ", w h i l e t h e 5 d 2 —> 6p~ t r a n s i t i o n , b e i n g con­ f i n e d t o t h e m e t a l , s h o u l d be f o u n d i n r o u g h l y t h e same s p e c t r a l r e g i o n f o r b o t h i o n s . 2) S i n c e t h e s t r o n g o u t - o f - p l a n e t r a n s i t i o n i n t h e s o l u t i o n s p e c t r u m o f P t C l 4 " u n d e r g o e s an a p p r o x i m a t e l y 12 kK s h i f t i n MGS, i t m i g h t be e x p e c t e d t h a t t h e d —> ρ t r a n s i t i o n hypothe­ s i z e d t o o c c u r i n the vacuum u l t r a v i o l e t i n t h e f r e e P d C l 4 ~ i o n c o u l d be s h i f t e d i n t o t h e quartz u l t r a v i o l e t i n the s p e c t r a o f the p a l l a d i u m a n a l o g o f MGS ([PdiNIU)^][PdCl^]). One i n f a c t o b s e r v e s a s t r o n g t r a n s i t i o n i n a q u e o u s s o l u t i o n s o f P ^ N H ^ ^ C l g (11) and r e c e n t c r y s t a l s t u d i e s (15) h a v e shown t h i s t r a n s i t i o n t o have an o u t - o f - p l a n e p o l a r i z a t i o n . L i k e w i s e , the p a l l a d i u m Magnus s a l t shows a m o d e r a t e l y strong outof-plane absorption peaking i n r e f l e c t i o n at 2

2

2

2

2

2

t

o

+

2

?

2 +

z

2

2

f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

290

EXTENDED INTERACTIONS BETWEEN METAL IONS WAVELENGTH

500 490 30t

2

t

mjj

250

30

20 Figure K PtCl,

300

200

40

F R E Q U E N C

7. (

Solution absorption ) and K PdCh ( 2

t

spectra (in 2M HCl) of ) (data from Réf. II)

Journal of the American Chemical Society

Figure 8. Polarized single-crystal reflection spectra of K PdCl, . The II curve obtained when electric vector of the incident light vibrates parallel to the molecular planes; 1 curve obtained when it vibrates perpendicular to the molecular planes (11). 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(

19.

ANEX

Complexes

with

Extended

Chains

291

45.8 kK (14). The f i n d i n g s f o r P t ( N H 2 ) 4 C l are p a r t i c u l a r l y s i g n i f i c a n t r e l a t i v e t o t h e 5a 2 —> 6p assignment, since for Pt(NHj)4 i t i s d i f f i c u l t to p r o p o s e an a l t e r n a t i v e a s s i g n m e n t f o r an o u t - o f - p l a n e b a n d o c c u r r i n g i n t h e 45-50 kK r e g i o n . 2

2 +

Characterization

o f the E x c i t e d S t a t e

H a v i n g r e a c h e d t h e p o i n t w h e r e one h a s developed a r e a s o n a b l y good p i c t u r e o f the s i n g l e - m o l e c u l e o r i g i n o f the c r u c i a l s o l i d - s t a t e a b s o r p t i o n bands i n t h e s e s y s t e m s , t h e n e x t l o g i c a l s t e p i s t o s e e k knowledge r e g a r d i n g the nature of the e x c i t e d s t a t e s a s s o c i a t e d w i t h these bands. I n p a r t i c u l a r , one w o u l d like to distinguis a c t e r i z e the t r a n s i t i o m o l e c u l a r one t h a t i s h i g h l y p e r t u r b e d b y i t s e n v i r o n ment a n d t h o s e t h a t w o u l d i n v o k e v a r y i n g d e g r e e s o f d e r e a l i z a t i o n i n at l e a s t the e x c i t e d s t a t e . As has been p o i n t e d out p r e v i o u s l y (3), mixed c r y s t a l e x p e r i m e n t s o f t h e g e n e r a l t y p e done by Banks a n d B a r n u m (15) and B a s u , Cook, and B e l f o r d (16) c o u l d h a v e a n i m p o r t a n t b e a r i n g on t h i s p r o b l e m . Unf o r t u n a t e l y , t h i s e a r l i e r w o r k on t h e g l y o x i m a t e s , w h i c h was c a r r i e d o u t on c o l l o i d a l s u s p e n s i o n s and NaCl p e l l e t s , appears t o have y i e l d e d s p e c t r a t h a t were n o t p a r t i c u l a r l y w e l l r e s o l v e d . I n any c a s e , these studies r e s u l t e d i n c o n t r a d i c t o r y f i n d i n g s , i n t h a t B a n k s and Barnum r e p o r t e d a s i n g l e i n t e r m e d i a t e band l y i n g between t h o s e o f the pure components i n v a r i o u s b i n a r y m i x t u r e s o f t h e N i ( I I ) , P t ( I I ) , and Pd(II) dimethyIglyoximates, while Basu et a l . reported a s e r i e s of i n t e r m e d i a t e bands i n such s i t u a t i o n s . In l i g h t o f the above o b s e r v a t i o n s , a s e r i e s o f m i x e d - c r y s t a l s t u d i e s i n v o l v i n g s i n g l e - c r y s t a l meas u r e m e n t s was u n d e r t a k e n i n o u r l a b o r a t o r i e s . The r e s u l t s o f one o f t h e s e i n v e s t i g a t i o n s was recently r e p o r t e d (14). I n t h i s work "mixed" Magnus salts of t h e f o r m [ P t ( N H j ) ^ ] [ P d C ^ ] and [ P d ( N H ) 4 ] [ P t C ^ ] were p r e p a r e d and s t u d i e d v i a t h e t e c h n i q u e s o f s p e c u l a r r e f l e c t i o n spectroscopy. The o u t - o f - p l a n e spectrum f o r [Pd(NH-5)4][PtCl4l i s shown i n F i g u r e 10, w h e r e t h e o u t - o f - p l a n e s p e c t r a o f MGS a n d i t s p l a t i n u m a n a l o g a r e a l s o shown f o r c o m p a r i s o n . I t w i l l be n o t e d t h a t t h e m i x e d - c r y s t a l s p e c t r u m p o s s e s s e s a s i n g l e band w i t h a w i d t h c o m p a r a b l e t o t h o s e o f t h e two "pure" s a l t s l o c a t e d a l m o s t midway b e t w e e n them. It also shows an i n t e n s i t y i n t e r m e d i a t e b e t w e e n t h o s e o f t h e pure s a l t s . ( A l l t h r e e c r y s t a l s have e s s e n t i a l l y the same u n i t - c e l l p a r a m e t e r s (17). Thus, the m e t a l - m e t a l T

3

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WAVELENGTH 300

30 F R E Q U E N C Y kK Journal of the American Chemical Society

Figure

9. Pohrized single-crystal reflection and 1 are of same significance k

WAVELENGTH 400 30Ç

30 FREQUENCY

20

spectra of K^tCl,,. as in Figure 8.

mn 25Ç

40

kK

Chemical Physics Letters

Figure 10. Reflection spectra when electric vector of the incident radiation vibrates parallel to the out-of-plane direction of single crystals of [Pt(NH )J [PtClJ ( ), [Pd(NH )A [PtClJ ( ), and [Pd(NH p bands o f K ^ P t C l i i and P ^ N H ^ H C l o a r e observed i n t h i s r e g i o n f o r MGS. * I n view o r t h e h i g h degree o f del o c a l i z a t i o n now known t o e x i s t i n t h e s e s y s t e m s , t h e most r e a s o n a b l e i n t e r p r e t a t i o n o f t h i s f i n d i n g would a p p e a r t o b e t h a t t h e 3 4 . 5 kK MGS-band h a s i t s p a r e n t ­ age i n b o t h t h e c a t i o n i c and a n i o n i c d — > ρ t r a n ­ s i t i o n s , b o t h h a v i n g b e e n r e d - s h i f t e d s h a r p l y i n MGS. For t h i s reason, c u r r e n t p r a c t i c e i n our l a b o r a t o r y i s t o use the t o t a l c o n c e n t r a t i o n o f complex s p e c i e s p r e s e n t , b o t h a n i o n i c a n d c a t i o n i c , i n c o m p u t i n g ev a l u e s f o r MGS-type s y s t e m s . R e c o g n i t i o n i s t h u s made of the f a c t that both metal-containing species are t h o u g h t t o be c o n t r i b u t i n g t o t h e a b s o r p t i v e p r o c e s s i n t h e Magnus salts. The e - v a l u e s and o t h e r i n t e n ­ s i t y measures - such as i n t e g r a t e d i n t e n s i t i e s o b t a i n e d f o r the Magnus s a l t s a r e t h e n d i r e c t l y com­ p a r a b l e t o t h o s e c o m p u t e d i n t h e u s u a l way f o r t h e component i o n s as " i s o l a t e d species. I t w i l l be s e e n t h a t t h i s manner o f c o m p u t i n g c o n c e n t r a t i o n s r e s u l t s i n concentration values that are j u s t twice those b a s e d o n t h e number o f f o r m u l a u n i t s o f t h e M a g n u s s a l t i n q u e s t i o n p e r l i t e r , w h i c h was t h e b a s i s o f t h e c o n c e n t r a t i o n s used and l i s t e d i n R e f e r e n c e 10. 1

z

z

1

f

1 1

f

*The emphasis h e r e i s finds l i t t l e evidence t r a n s i t i o n s o f MGS o r other than the c r u c i a l t h a t the ground s t a t e **Analogous

comments

on t h e e x c i t e d s t a t e , s i n c e o n e i n , f o r instance, the d — > d t r a n s i t i o n s o f t h e NiDMG c r y s t a l g r e e n b a n d t o make o n e f e e l i s extensively delocalized.

apply

to other

Magnus-type

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

salts.

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Comment s h o u l d a l s o be made a t t h i s p o i n t on t h e d i f f e r e n c e s i n t h e l e v e l s o f r e f l e c t i v i t y shown b y t h e MGS c u r v e s o f F i g u r e 2 o f R e f e r e n c e 10 a n d F i g ­ u r e 10 o f t h e c u r r e n t p u b l i c a t i o n . The i m p r o v e m e n t shown i n t h e l a t e r m e a s u r e m e n t s p r i m a r i l y r e f l e c t s i m p r o v e d sample s i z e and q u a l i t y r e s u l t i n g f r o m r e c e n t l y - d e v e l o p e d t e c h n i q u e s o f sample p r e p a r a t i o n . The i n c r e a s e d r e f l e c t i v i t i e s f o r MGS i n t h e m s e l v e s would l e a d t o i n c r e a s e d e-values f o r the KramersK r o n i g a b s o r p t i o n s p e c t r a t h a t r e s u l t f r o m them. This e f f e c t i s , however, o p p o s i t e t o t h a t r e s u l t i n g from the change i n the b a s i s o f computing the c r y s t a l con­ c e n t r a t i o n n o t e d a b o v e , a n d one t h u s o b t a i n s roughly t h e same % f o r the d — > ρ t r a n s i t i o n s of K PtCl4 and P t ( N H ) 4 C l an (12) increased widtn of approximate 2.5-fold increase i n i n t e g r a t e d i n t e n s i t y i n MGS a l l u d e d t o i n an e a r l i e r s e c t i o n o f t h i s p a p e r . A n o t h e r g r o u p o f s u b s t a n c e s whose s t u d y i s p a r ­ t i c u l a r l y i n f o r m a t i v e c o n c e r n i n g the e x t e n t o f del o c a l i z a t i o n i n the c r u c i a l e x c i t e d s t a t e i n c r y s t a l s o f the type being d e a l t w i t h here are the barium s a l t s o f t h e t e t r a c y a n o c o m p l e x e s o f N i ( I I ) , P d ( I I ) , and Pt(II). T h e s e compounds, w h i c h f o r m s i n g l e c r y s t a l s h a v i n g the g e n e r a l f o r m u l a BaM(CN)i|-4H 0, p r e s e n t a s i t u a t i o n t h a t i n many r e s p e c t s p a r a l l e l s t h a t d e ­ s c r i b e d p r e v i o u s l y f o r NiDMG. O n c e a g a i n one h a s a s t a c k i n g o f the complexes i n the s o l i d s t a t e - w i t h c o n s e q u e n t " m e t a l c h a i n " f o r m a t i o n - and t h e develop­ ment o f a s t r o n g , r e l a t i v e l y l o w - e n e r g y band t h a t has no o b v i o u s c o u n t e r p a r t i n t h e f r e e i o n s ( 5 * 1 8 , 1 9 ) . The " s o l i d - s t a t e " b a n d s h e r e a r e a g a i n t h o u g E t z o h a v e a s t h e i r f r e e - i o n p a r e n t a g e r e l a t i v e l y weak b a n d s i n the " c h a r g e - t r a n s f e r " r e g i o n o f the s o l u t i o n absorp­ t i o n spectrum (18). One o f t h e " ~ T n t e r e s t i n g a s p e c t s o f t h e s o l i d s t a t e s t u d i e s on t h e s e c o m p o u n d s i s t h e f a c t t h a t one c a n p r e p a r e m i x e d c r y s t a l s o f t h e m i n w h i c h two dif­ f e r e n t c o m p l e x s p e c i e s a p p e a r i n t h e same c r y s t a l . One can, f o r i n s t a n c e , grow c r y s t a l s o f t h e f o l l o w i n g t y p e : a

5

x

p

2

2

Ba[Ni(CN) ] [Pt(CN) l ^ -4H o' 4

x

4

1

x

2

f

(5)

^Isomorphism o f these c r y s t a l s w i t h the s i n g l e - a n i o n c r y s t a l s h a s n o t b e e n a s c e r t a i n e d by d i r e c t e x p e r i m e n t , but i s i n f e r r e d from the g r a d u a l gradation of o p t i c a l p r o p e r t i e s a s one v a r i e s t h e c o m p o s i t i o n o f t h e m i x e d crystals.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

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Chains

295

f

While the mixed-Magnus s t u d i e s had the a d v a n t a g e o f d e f i n i t e i o n i c d i s t r i b u t i o n i n t h e m e t a l s t a c k s (one knows t h a t i n M a g n u s s a l t s t h e a n i o n i c and c a t i o n i c s p e c i e s a l t e r n a t e a l o n g t h e c h a i n ) , one i s l i m i t e d , a t l e a s t w i t h the c r y s t a l s prepared t o date, t o a 1:1 r a t i o o f t h e two m e t a l s i n v o l v e d . In the p r e s e n t s i t u a t i o n t h e a d v a n t a g e s and d i s a d v a n t a g e s a r e j u s t r e v e r s e d r e l a t i v e t o the Magnus salts: one c a n v a r y composition, but the sequencing a l o n g the c h a i n i s n o l o n g e r known. I f one p r e p a r e s c r y s t a l s w i t h a c o m p o s i t i o n corresponding to χ i n Equation 5 having a value of approximately 0.5, the s p e c t r a l r e s u l t s are c o m p l e t e l y a n a l o g o u s t o t h o s e o b s e r v e d f o r the mixed-Magnus c a s e : a s i n g l e b a n d whos between t h o s e o f BaNi(CN) (18). The u n i q u e r e s u l t s o f t h i s s t u d y a r e , h o w e v e r , i l l u s t r a t e d b y F i g u r e 11, where t h e r e i s p l o t t e d t h e p o s i t i o n o f the o u t - o f - p l a n e r e f l e c t i o n band as a function of P t ( C N H ~ content f o r a series of B a P t ( C N ) h - B a N i ( CN ) 4 m i x e d c r y s t a l s i n w h i c h t h e Pt(CN)4^~ c o n t e n t i s v a r i e d over r a t h e r wide l i m i t s . R e g a r d l e s s o f t h e a m o u n t o f P t ( C N ) 4 ~ t h a t was intro­ d u c e d i n t o t h e B a N i ( C N ) 4 c r y s t a l , o n l y one r e f l e c t i o n b a n d was f o u n d , a n d f o r e v e n t h e l o w e s t l e v e l s o f c o n c e n t r a t i o n o f one s p e c i e s i n t h e p r e s e n c e o f t h e o t h e r , t h i s b a n d was a s s u m i n g " i n t e r m e d i a t e " c h a r a c ­ teristics. T h e b e h a v i o r shown i n F i g u r e 11, which i s p a r a l ­ l e l e d by o t h e r s p e c t r a l c h a r a c t e r i s t i c s o f t h e bands i n question, presents very convincing evidence f o r e x t e n s i v e d e r e a l i z a t i o n i n the e x c i t e d s t a t e respon­ s i b l e f o r the s t r o n g o u t - o f - p l a n e band i n t h e s e c r y s ­ tals. The i m p l i c a t i o n h e r e i s t h a t i f , a s a p p e a r s t o b e t h e c a s e , t h e e f f e c t s o f s u b s t i t u t i o n became a p p a r ­ e n t a f t e r 5% " i m p u r i t y " i s i n t r o d u c e d , t h e n one w o u l d h a v e t o h a v e a v e r a g i n g o v e r a r a t h e r l a r g e number o f c e n t e r s i n the r e l e v a n t e x c i t e d s t a t e . Otherwise, one m i g h t e x p e c t t h e d e v e l o p m e n t o f one o r more " i n t e r ­ m e d i a t e " bands, a l o n g w i t h a band c o r r e s p o n d i n g t o the m a j o r component, i n the e a r l i e r s t a g e s o f s u b s t i t u ­ f

1

2

2

tion

(5,16).

General Discussion The o v e r a l l p i c t u r e t h a t e m e r g e s h e r e i s one o f a s t r o n g p a r a l l e l i s m i n t h e o p t i c a l phenomena o b s e r v e d f o r a l l of the c l o s e l y stacked c r y s t a l s d i s c u s s e d . Thus, i n the g l y o x i m a t e s , the Magnus s a l t s , and t h e 1

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Figure 11. a function

Variation of position of the out-of-plane reflectivity of BaPt(CN), · 4Η*0 content in mixed crystals BatNifCNhhlPtfCNhl,.* · 411 JO. t

maximum as of the form

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

19.

ANEX

Complexes

with

Extended

Chains

297

c y a n i d e s , one f i n d s f o r t h e m e t a l - s e p a r a t i o n d e p e n d e n t o u t - o f - p l a n e band: 1) S i m i l a r b e h a v i o r w i t h r e s p e c t t o r e l a t i v e p o s i t i o n as a f u n c t i o n o f the c e n t r a l m e t a l ion involved (l4,l6,l8). 2) S i m i l a r b e h a v i o r wTtYTrespect t o i n c r e a s i n g metal-metal separation (3,10,l8). 3) S i m i l a r b e h a v i o r when m i x e c T c r y s t a l s a r e studied (l4,15 l8).* These o b s e r v a t i o n s p r o v i d e a c o n v i n c i n g case f o r t h e p r o p o s i t i o n t h a t one i s d e a l i n g w i t h b a s i c a l l y t h e same k i n d o f t r a n s i t i o n i n e a c h i n s t a n c e . I f one a c c e p t s the v a r i o u s arguments p r e s e n t e d i n the f o r e ­ g o i n g s e c t i o n s , t h i s t r a n s i t i o n becomes i d e n t i f i e d as a highly perturbed A s was r e m a r k e Magnus s a l t s t u d i e s , i t i s t e m p t i n g t o v i e w t h i s as a s i t u a t i o n i n w h i c h one i s e s s e n t i a l l y i n v o l v e d w i t h a c h a i n o f i n t e r a c t i n g m e t a l atoms and t o d e e m p h a s i z e the r o l e of l i g a n d s i n attempting t o understand the phenomena b e i n g d e a l t w i t h . T h a t t h i s w o u l d be an o v e r s i m p l i f i c a t i o n b e c o m e s c l e a r when one r e c o g n i z e s , f o r i n s t a n c e , t h a t Magnus green s a l t , with a Pt-Pt d i s t a n c e o f 3.25 A (17). has i t s s t r o n g o u t - o f - p l a n e a b s o r p t i o n l o c a t e d aT; 34.5 kK ( 1 0 ) , w h i l e t h e compar­ able absorption i n B a P t ( C N ) h · 4 H 0 , which possesses a P t - P t s e p a r a t i o n o f 3.27 fi, o c c u r s a t 22.6 kK ( l 8 , 1 9 ) . S i m i l a r l y , the s t r o n g o u t - o f - p l a n e a b s o r p t i o n band~Tor β-NiEMG o c c u r s a t 20.5 kK (3) a n d t h e a n a l o g o u s b a n d i n B a N i ( C N ) 4 - 4 H 0 i s o b s e r v e d a t 27.2 kK ( l o ) , i n spite of approximately equal metal-metal separations (3,21). F i n a l l y , one may n o t e that the orthorïïombic f o r m o f N i ( I I ) N - m e t h y l s a l i c y l a l d i m i n a t e , w h o s e s t r u c t u r e i s s i m i l a r t o NiDMG a n d h a s a m e t a l m e t a l s p a c i n g o f 3.30 A ( 2 2 ) , shows no b a n d analogous t o t h e NiDMG g r e e n b a n d tïïroughout t h e v i s i b l e a n d q u a r t z u l t r a v i o l e t (23). Thus, the n a t u r e o f the l i g a n d a p p e a r s t o p l a y an i m p o r t a n t r o l e i n t h e e n e r g e t i c s and e v e n t h e f o r m a t i o n o f t h e o u t - o f - p l a n e band of s p e c i a l i n t e r e s t here. The s p e c i f i c n a t u r e o f t h e s o l i d s t a t e p e r t u r b a t i o n o p e r a t i v e i n t h e s e s y s t e m s i s as y e t n o t det e r m i n e d w i t h any c e r t a i n t y . E a r l y s p e c u l a t i o n s on 5

1

1

p

2

* P r e l i m i n a r y s t u d i e s on s i n g l e - c r y s t a l s o f t h e m i x e d d i m e t h y l g l y o x i m a t e s (20) show t h a t t h e b e h a v i o r h e r e p a r a l l e l s t h a t found T o r the mixed Magnus s a l t s and t h e P t ( I I ) and N i ( I I ) c y a n i d e m i x e d c r y s t a l w i t h χ e q u a l a p p r o x i m a t e l y 0.5 i n E q u a t i o n 5. f

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METAL

IONS

t h e e l e c t r o n i c s t r u c t u r e o f t h e s e compounds (£4) i n v o k e d band f o r m a t i o n i n v o l v i n g the h i g h e s t T i l l e d d 2 and t h e l o w e s t empty p o r b i t a l s on t h e m e t a l i o n s m a k i n g up t h e c h a i n s , a n d some o f t h e l a t e s t w o r k i n t h i s a r e a c o n t i n u e s t h e s e i d e a s (25). Alternate pro­ p o s a l s ( 5 , 6 , 1 6 , 2 6 ) h a v e b e e n made i n w h i c h e l e c t r o ­ s t a t i c p e r t u r b a t i o n s , e x c i t o n i c i n t e r a c t i o n s , and c h a r g e - t r a n s f e r e f f e c t s a r e i n v o k e d , a n d i n some c a s e s the d — > ρ assignment t o a g r e a t e r or l e s s e r extent challenged. The e x p e r i m e n t a l i n f o r m a t i o n now on h a n d makes i t u n l i k e l y t h a t an e l e m e n t a r y a p p l i c a t i o n o f any o f t h e s e a p p r o a c h e s w i l l a d e q u a t e l y a c c o u n t f o r a l l t h e o b s e r v a t i o n s , a l t h o u g h e a c h o f them c a n be d i s c u s s e d i n terms o f a p e r t u r b e d d > ρ transition. F o r i n s t a n c e , band (25) r a t e d 2 b a n d s and a n d c a t i o n i c s p e c i e s , a n d t h u s m o r e t h a n one e l e c t r o n ic transition. E v e n i f one i n v o k e s l a c k o f s p e c t r o ­ s c o p i c r e s o l v i n g power as t h e r e a s o n f o r t h e f a i l u r e t o o b s e r v e m o r e t h a n one t r a n s i t i o n , s u c h a r g u m e n t s w i l l not apply t o the m i x e d - c r y s t a l experiments. S i m i l a r l y , i n a d d i t i o n t o h a v i n g t o i n v o k e i n a some­ w h a t a d h o c m a n n e r some e x t r e m e l y l a r g e second-order e f f e c t s ' t o e x p l a i n the i n t e n s i t y b e h a v i o r i n the g l y o x i m a t e s and c y a n i d e s , i t does n o t a p p e a r t h a t exciton theory w i l l , at least i n a straight-forward manner, a c c o u n t f o r t h e m i x e d - c r y s t a l r e s u l t s . Com­ ments s i m i l a r t o those r e g a r d i n g the e x c i t o n mechanism apply to that of charge-transfer. The f o r e g o i n g r e m a r k s a r e n o t m e a n t t o i m p l y t h a t e x c i t o n e f f e c t s , c h a r g e - t r a n s f e r , and/or band forma­ t i o n w i l l not f i g u r e i n the f i n a l e x p l a n a t i o n o f the o p t i c a l p r o p e r t i e s o f these systems. At this point a l l one c a n s a y i s t h a t n o n e o f t h e m o b v i o u s l y a n d c l e a r l y e x p l a i n s t h e o b s e r v a t i o n s , and more s o p h i s t i ­ c a t e d a p p r o a c h e s a p p e a r t o be r e q u i r e d . W h e t h e r one or a c o m b i n a t i o n o f the o l d approaches, o r a t o t a l l y new one, w i l l p r o v e a d e q u a t e r e m a i n s t o b e s e e n . In a n y c a s e , a s u f f i c i e n t amount o f e x p e r i m e n t a l e v i d e n c e now e x i s t s t o p r o v i d e r a t h e r c r i t i c a l q u a l i t a t i v e a n d q u a n t i t a t i v e t e s t s f o r any p r o p o s a l s t h a t a r e p u t forward. z

z

2

Acknowledgment : The a u t h o r w i s h e s t o a c k n o w l e d g e t h e e f f o r t s o f t h e many u n d e r g r a d u a t e s , g r a d u a t e s t u d e n t s , a n d p o s t ­ d o c t o r a l a s s o c i a t e s who h a v e b e e n i n l a r g e p a r t r e s p o n s i b l e f o r the account t h a t has been g i v e n here.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

19. ANEx

Complexes

with

Extended

Chains

1

299

A number o f t h e s e w o r k e r s names a p p e a r i n t h e r e f e r e n c e s t h a t have been c i t e d . O t h e r s , who made c o n t r i b u t i o n s t o t h e development o f t h e equipment and t e c h n i q u e s u s e d i n t h i s work i n t h e c o n t e x t o f o t h e r types o f studies, a l s o deserve a share o f the c r e d i t . W o r t h y o f s p e c i a l n o t e among t h i s l a t t e r g r o u p a r e s e v e r a l o f my e a r l y g r a d u a t e s t u d e n t s : D r s . Α . V . F r a t i n i , S. C. N e e l y , J . B e r n s t e i n , a n d C. J . E c k h a r d t .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS BETWEEN

300

M E T A L IONS

Literature Cited 1. 2.

Anex, B. G., M o l . C r y s t . (1966) 1, 1. S t e w a r t , R. F., D a v i d s o n , N., J. Chem. Phys.

3.

Anex, B. G., Krist, F. K., J. Amer. Chem. Soc.

4.

Gray, H. B., B a l l h a u s e n , C. J., J. Amer. Chem.

(1965) 39, 255.

5. 6.

(1967)

89,

(1963)

Soc.

6114. 85, 26Ο.

Day, P., I n o r g . Chim. A c t a , Rev. (1969) 3, 8l. O h a s h i , Y., H a n a z a k i , I., Nagakura, S., I n o r g . Chem. (197Ο) 9, 2551. 7. Day, P., O r c h a r d , A. F., Thomson, A. J., Williams, R. J. 1973. 8. M a r t i n , D. S., Jr., Rush, R. Μ., K r o e n i n g , R. F., Fanwick, P. Ε . , I n o r g . Chem. (1973) 12, 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

301.

Day, P., O r c h a r d , A. F., Thomson, A. J., Williams, R. J. P., J. Chem. Phys. (1965) 4 3 , 3763.

Anex, B. G., Ross, M. E., Hedgcock, M. W., J. Chem. Phys. (1967) 46, 1090. Anex, B. G., T a k e u c h i , N., J. Amer. Chem. Soc., accepted for publication. Anex, B. G., F o s t e r , S. I., F u c a l o r o , A. F., u n p u b l i s h e d work. Anex, B. G., F o s t e r , S. I., u n p u b l i s h e d work. Anex, B. G., F o s t e r , S. I., F u c a l o r o , A. F., Chem. Phys. Lett. (1973) 18, 126. Banks, C. V., Barnum, D. W., J. Amer. Chem. Soc.

(1958) 80, 4767.

Basu, G., Cook, G. M., Belford, R. L., I n o r g . Chem. (1964) 3, 1361. Miller, J. R., J. Chem. Soc. (1961), 4452. Anex, B. G., Musselman, R. L., u n p u b l i s h e d work. M o n c u i t , C., P o u l e t , H., J. Phys. Radium (1962) 23,

353.

22. 23. 24. 25.

Anex, B. G., N a r a i n , G., u n p u b l i s h e d work. L a r s e n , F. K., Hazell, R. G., Rasmussen, S. Ε., A c t a Chem. Scand. (1969) 23, 6 1 . F e r g u s o n , J., J. Chem. Phys. (1961) 34, 611. Anex, B. G., F u r t a d o , L. T., u n p u b l i s h e d work. R u n d l e , R. E., J. Phys. Chem. (1957) 6l, 45. I n t e r r a n t e , L. V., Messmer, R. P., I n o r g . Chem.

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33,

1174. 1625.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

20 T h e Single Crystal Polarized Reflectance Spectra and Electronic Structures o f Rh(CO) acac and 2

THOMAS A . DESSENT a n d RICHARD D u k e U n i v e r s i t y , D u r h a m , N . C . 27706 SALLY

M.

A.

Ir(CO) acac 2

PALMER

HORNER

M e r e d i t h C o l l e g e , R a l e i g h , N . C . 27602

Introduction 8

Among s q u a r e planar d c o m p l e x e s it m i g h t be e x pected that those o f the lowest formal metal valence (in t h e a b s e n c e o f "partial oxidation"(1)) w o u l d exhibit t h e most striking evidence o f extended metal-metal interaction. This prospect was b o r n e o u t in 1966 b y t h e w o r k o f Pitt a n d others (2), who first documented t h e anisotropic electrical conductivity of dicarbonyl acetylacetonato-rhodium(I) and -iridium(I). As illustrated in Figure 1, t h e s e c o m p o u n d s crystallize ( a s d o many o f their a n a l o g s discussed in this Symposium) with the planar molecules stacked parallel t o e a c h other in chains a n d short m e t a l - m e t a l d i s t a n c e s . The compounds w e r e first p r e p a r e d b y Bonati a n d Wilkinson (3), and t h e crystal structure work by B a i l e y , et al (4), h a s shown t h e two structures t o be i s o m o r p h o u s , with a 3.26Å M-M separation for t h e r h o d i u m compound and a 3.20Å separation f o r the iridium c o m p o u n d . The s p a c e g r o u p is P1 with Ζ = 2. The molecular sym­ metry is C , although t h e site s y m m e t r y is, of c o u r s e , only C. Pitt found the dc electrical conductivity of single crystals t o be 100 t i m e s greater along t h e M-M direction than perpendicular to it for t h e r h o d i u m com­ p o u n d a n d 500 t i m e s greater for the iridium compound (2). He s u g g e s t e d t h a t we might try t o d e t e r m i n e t h e polarized single crystal electronic spectra as an adjunct to the electrical m e a s u r e m e n t s , b u t it was obvious a t a glance, from the lustrous, metallic appear­ ance o f the crystals, that absorptivities in excess of 10 were involved, which would preclude consideration of single crystal absorption techniques. However, t h e s u c c e s s o f Anex (5) in deriving polarized single cry­ stal absorption s p e c t r a from specular reflectance mea­ surements s u g g e s t e d t h a t this m i g h t be a useful tech2v

1

3

301

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

302

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

nique t o t r y i n t h i s i n s t a n c e . We h a v e c o n s t r u c t e d e q u i p m e n t f o r t h i s p u r p o s e a n d t h e r e s u l t s o f o u r mea­ s u r e m e n t s on t h e s e r h o d i u m a n d i r i d i u m c o m p l e x e s f o r m the b a s i s f o r t h i s p a p e r . Single C r y s t a l Micro f o r the Cary-14R

Specular

Reflectance

Attachment

F i g u r e 2A shows a s c h e m a t i c d i a g r a m o f o u r p o l a r ­ i z e d micro s p e c u l a r r e f l e c t a n c e attachment f o rthe Cary l 4 R spectrophotometer. We c h o s e t o b a s e t h e e q u i p m e n t on a h i g h q u a l i t y s p e c t r o p h o t o m e t e r rather t h a n s t a r t i n g " f r o m s c r a t c h " , i n o r d e r t o t a k e .advan­ tage o f the i n h e r e n t l o n g term s t a b i l i t y o f the double beam i n s t r u m e n t a n cromator. The a t t a c h m e n compartment o f t h e Cary ; t s not a t t a c h e d t o the compartment, b u t i s p l a c e d w i t h i n i t and a l i g n e d by screws v e r t i c a l l y and h o r i z o n t a l l y . Once a l i g n e d , t h e a c c e s s o r y c a n b e t a k e n ou t a n d r e p l a c e d w i t h o u t f u r t h e r alignment. As shown i n F i g u r e 2A, t h e m o n o c h r o m a t i c l i g h t beam e n t e r s f r o m t h e s i g n a l g e n e r a t o r c o m p a r t m e n t t h r o u g h a c a l c i t e Glan-Thompson p o l a r i z e r (P) i n the l e f t w a l l o f sample compartment, passes t h r o u g h a q u a r t z s u b s t r a t e beam s p l i t t e r ( B S ) a n d i s f o c u s e d o n t h e s u r f a c e o f t h e s a m p l e ( S ) b y a 36X C a s s e g r a i n m i c r o s c o p e o b j e c t i v e ( 0 ) . The s a m p l e i s a l i g n e d b y a g o n i o m e t e r ( G ; S O a s t o r e f l e c t t h e beam b a c k through the o b j e c t i v e . The l i g h t t h u s i m p i n g e s o n t h e c r y s t a l at nominally normal i n c i d e n c e . The r e f l e c t e d i n t e n s i t y s t r i k e s t h e beam s p l i t t e r a n d t h a t w h i c h i s r e f l e c t e d i s d i r e c t e d b y p l a n e m i r r o r s ( M l , M2, M3) b a c k i n t o t h e normal path f o r the i n s t r u m e n t and thus t o the d e t e c ­ tor. C o n c u r r e n t l y t h e r e f e r e n c e beam i s a t t e n u a t e d b y n e u t r a l d e n s i t y f i l t e r s and an i r i s diaphram so as t o b a l a n c e t h e beams w i t h i n t h e r a n g e o f t h e s l i d e w i r e . In o r d e r t o a v o i d back r e f l e c t i o n from the c e n t r a l m i r r o r o f the Cassegrain o p t i c s , the o b j e c t i v e i s used i n an o f f a x i s o r i e n t a t i o n , as i l l u s t r a t e d i n F i g u r e 2B. The d e s i g n o f t h i s e q u i p m e n t i s v e r y s i m i l a r t o t h a t u s e d p r e v i o u s l y b y Anex (5_) e x c e p t t h a t i t i s a n e a s i l y i n t e r c h a n g e a b l e a c c e s s o r y t o an e x i s t i n g i n ­ strument. I t i s d e s c r i b e d more c o m p l e t e l y e l s e w h e r e (6). The

Electronic

Structure

of Planar

d^ Complexes

For a planar C d^ complex the o r i e n t a t i o n o f the axes g i v e n i n F i g u r e 1 l e a d s t o the h i g h e s t f i l l e d d o r b i t a l b e i n g t h e d 2 _ 2 (&i i n C p ) a n d t h e l o w e s t u n f i l l e d , the d (ο "ΐη C ) · T h i s o r i e n t a t i o n em2 v

Y

x

y

2

Y

2 v

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

DESSENT

E T AL.

Single

Crystal

Spectra

Β Figure 2. Single crystal microspecular reflectance attachment for the Cary 14R spectrophotometer. (A) Incident ( ) and reflected ( ) light paths. (B) Orientation of light path with respect to objective axis.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

304

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

p h a s i z e s the importance of the metal-metal i n t e r a c t i o n a n d i s t h e same a s t h a t u s e d b y M a r t i n (7_) r e c e n t l y i n h i s a n a l y s i s o f the s p e c t r u m o f the a n a l o g o u s Pt(en)Cl2. I n s u c h a m o l e c u l a r s y s t e m one m i g h t e x p e c t t o o b s e r v e s e v e r a l t y p e s o f t r a n s i t i o n s , i n c l u d i n g ones l o c a l i z e d on t h e m e t a l , o t h e r s i n v o l v i n g t r a n s f e r o f e l e c t r o n d e n s i t y f r o m the m e t a l t o the l i g a n d s and v i s a v e r s a , a n d s t i l l o t h e r s l o c a l i z e d on t h e l i g a n d s . Some o f t h e s e a r e i l l u s t r a t e d i n F i g u r e 3 a l o n g w i t h t h e symmetry p r o p e r t i e s o f the e x c i t e d s t a t e s and the molec u l a r a x i s a l o n g w h i c h e a c h w o u l d be a l l o w e d . The g r o u n d s t a t e i s o f c o u r s e ^A^ i n t h e s e d i a m a g n e t i c compounds. Results

and

Discussio

Solutions of Rh(C0) Ir(C0)oaca y f a i n t l y c o l o r e d , i n marked c o n t r a s t t o the l u s t e r o u s l y opaque s o l i d s . The s p e c t r a o f t h e two compounds i n c h l o r o f o r m s o l u t i o n a r e shown i n F i g u r e 4 . The r e s o l ution i s not very i n s p i r i n g , but i n both spectra e s s e n t i a l l y t h e same p a t t e r n i s o b s e r v e d ; t h a t i s , t h r e e s t r o n g bands, e a c h accompanied by a weaker shoulder. A l t h o u g h i t i s p o s s i b l e t h a t some l i g a n d l i g a n d b a n d s a r e i n v o l v e d h e r e — p a r t i c u l a r l y i n t h e 33 kK r e g i o n { & ) — t h e s e b a n d s m i g h t o t h e r w i s e l o g i c a l l y be a s s i g n e d t o e i t h e r d - d o r c h a r g e t r a n s f e r t r a n s i tions. I n terms o f metal l o c a l i z e d t r a n s i t i o n s the t h r e e s t r o n g e r b a n d s w o u l d be t h e t h r e e s p i n - a l l o w e d , symmetry-a11owed bands and the s h o u l d e r s , t h e s p i n forbidden counterparts. H o w e v e r , o b s e r v a t i o n s on P t ( l l ) complexes would s u g g e s t t h a t the s p i n - f o r b i d d e n b a n d s s h o u l d be e x p e c t e d a t c o n s i d e r a b l y l o w e r e n e r g y than those observed here. C h a r g e t r a n s f e r , on t h e o t h e r h a n d , a p p e a r s a much more l i k e l y e x p l a n a t i o n — a t l e a s t f o r t h e more p r o m i n e n t f e a t u r e s a t 3 0 . 5 and 3 9 . 0 kK f o r t h e r h o d i u m c o m p l e x a n d 2 9 . 2 and 39.2 kK f o r the i r i d i u m complex. ( T h e r e g i o n a r o u n d 33 kK l i k e l y i n v o l v e s an acac t r a n s i t i o n , though the d i f f e r e n c e i n a b s o r p t i v i t y a n d b a n d s h a p e b e t w e e n t h e two compounds may i n d i c a t e a t h i r d p r o m i n e n t c h a r g e t r a n s f e r t r a n s i t i o n here a l s o . ) The l a r g e r s i z e o f t h e i r i d i u m would p r e d i c t lower energy charge t r a n s f e r than i n t h e r h o d i u m a n a l o g , w h e r e a s t h e d-d t r a n s i t i o n s w o u l d be e x p e c t e d a t h i g h e r e n e r g y . Thus, the r e l a t i v e l y low e n e r g y o f the i r i d i u m bands f a v o r s the charge t r a n s f e r assignment. These t e n t a t i v e a s s i g n ments a r e s u m m a r i z e d i n T a b l e 1. B e f o r e we c o m p l e t e d o u r s p e c u l a r r e f l e c t a n c e a c c e s s o r y we a t t e m p t e d t o q u a n t i f y t h e i n t e n s e a b s o r p 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

20.

DESSENT

E T AL.

Single

Crystal

Spectra

Excited-state Symmetry

One-electron Orbitale

3,l

d

Λ

V d

3,1

+ d

3.1ο

x y

B

* w'Ca,) χ -y d -w (a ) χ -y 2

Figure 3. Oneelectron transitions in M(CO) acac. + applies only to sin­

2

2

a

2

° 2 2 χ -y -

2 v

forbidde

* d xy

xz

Polarizat in C

n

d • d yz xy d

305

-

forbiddei

2

2

**d d-Λιπ

t l o n o f t h e s e compound to the m e t a l l i c l u s t e r m u l l s and KBr p e l l e t s . The m u l l s p e c t r a w e r e o f r a t h e r p o o r q u a l i t y and, a l t h o u g h the KBr d i s k s d i d , i n f a c t , r e v e a l a s t r o n g v i s i b l e b a n d i n b o t h compounds b e t w e e n 14 a n d 20 kK, t h e r e s u l t s f r o m r u n t o r u n w e r e n o t r e p o r d u c i b l e and gave e v i d e n c e o f d e c o m p o s i t i o n d u r i n g p r e p a r a t i o n — p a r t i c u l a r when t h e s a m p l e s w e r e g r o u n d i n the p r e s e n c e o f oxygen. Furthermore, the p o l a r i z a t i o n p r o p e r t i e s o f t h e s o l i d s t a t e band o r bands were o f course not a v a i l a b l e from these n o n o r i e n t e d samples. F r o m m a t e r i a l g e n e r o u s l y s u p p l i e d b y P i t t we w e r e a b l e t o g r o w h i g h q u a l i t y s i n g l e c r y s t a l s o f b o t h com­ p o u n d s w i t h f a c e s o f c a . 1 mm s q u a r e . The rhodium c r y s t a l s were grown by s l o w s u b l i m a t i o n a t r e d u c e d p r e s s u r e and the i r i d i u m c r y s t a l s , by s l o w e v a p o r a t i o n of acetone s o l u t i o n s . I n F i g u r e 5 i s shown t h e p o l a r i z e d s p e c u l a r r e ­ f l e c t a n c e e l e c t r o n i c spectrum of Rh(C0) acac measured i n the C a r y l4R. The m o s t p r o m i n e n t f e a t u r e i s t h e i n t e n s e ζ - p o l a r i z e d b a n d a t 19.1 kK. I n F i g u r e 6 we see t h e a n a l o g o u s d a t a f o r the i r i d i u m compound. Again the most p r o m i n e n t f e a t u r e o f the s p e c t r u m i s t h e z p o l a r i z e d band i n the v i s i b l e . I t i s a t lower energy than i n the rhodium a n a l o g — s o low i n f a c t t h a t i t i s the l o w e s t e n e r g y band o b s e r v a b l e w i t h i n the range o f t h e GaAS d e t e c t o r . The d a t a w e r e o b t a i n e d b y s u b s t r a c t i n g t h e a p p a r ­ e n t a b s o r p t i o n o f the i n s t r u m e n t w i t h the r e f e r e n c e m i r r o r i n the sample p o s i t i o n ( A f ) from t h a t w i t h the c r y s t a l i n p o s i t i o n ( A y t ) , w i t h a l l o t h e r c o n ­ d i t i o n s t h e same. The r e f l e c t a n c e R o f t h e c r y s t a l i s then g i v e n by 2

r e

c r

R

c r y s t

(a))

= R

r 8 f

(u))xl0"

S

( A

cryst-A

r e f

)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

( l

)

DESSENT

Single

E T AL.

Crystal

Spectra

Rh(CO) acac 2

Specular Reflectance ζ •20

X

y

oc

.10

0 Figure

5.

40 Polarized

single

30 crystal specular Rh(CO) acac

20 reflectance

spectrum

of

2

lr(CO) acac 2

Specular Reflectance xy

10

—

1

40

30

kK Figure

6.

Polarized

single

crystal specular Ir(CO) acac

1

20 reflectance

spectrum

of

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

308

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

R f ( u ) ) was d e t e r m i n e d b y P l a n a r O p t i c s o v e r t h e r a n g e 5-50 kK. D a t a p o i n t s were t a k e n m a n u a l l y f r o m the C a r y r e c o r d e r c h a r t a n d f e d i n t o a PDP-15 c o m p u t e r w h i c h c a l c u l a t e d the R t ( u > ) and p l o t t e d r e f l e c t a n c e v s . e n e r g y s p e c t r a ( p r o g r a m s PSRC, S R I , P I X R a n d P L O T R ) . A v a l u e s u s e d w e r e t h e a v e r a g e o f 3-5 r u n s on d i f f e r e n t crystals. The s t a n d a r d d e v i a t i o n o f t h e r a w d a t a was n o r m a l l y < 5$ e x c e p t i n t h e h i g h e n e r g y r e g i o n ( < 4 0 k K ) . The r e f l e c t a n c e d a t a w e r e u s e d a s i n p u t t o a p r o ­ g r a m (KRAMER) w h i c h c a l c u l a t e d t h e p h a s e s h i f t a n d from t h i s t h e o t h e r n e c e s s a r y o p t i c a l c o n s t a n t s n, k, a n d e,. The m e t h o d s u s e d a r e s i m i l a r t o t h o s e r e p o r t e d b y Anex (5_) a n d b y S t e r n ( £ ) a n d o t h e r s a n d a m o u n t t o a Kramers-Kronig analysis. In such a c a l c u l a t i o n the phase s h i f t θ depend o f a) f r o m -°° t o + » r e f l e c t a n c e a t e n e r g i e s h i g h e r and l o w e r t h a n the r e g i o n o f o b s e r v a t i o n ( 1 1 - 4 0 kK) i s n e c e s s a r y . Sev­ e r a l p r o c e d u r e s h a v e b e e n u s e d t o make t h i s e x t r a p o ­ l a t i o n , i n c l u d i n g s e t t i n g the r e f l e c t a n c e o u t s i d e the measurement i n t e r v a l e q u a l t o an a d j u s t a b l e c o n s t a n t (10) o r an e x p o n e n t i a l f u n c t i o n ( £ ) , o r g e n e r a t i n g a dummy b a n d i n one o r b o t h o f t h e o u t l y i n g r e g i o n s (5.). We f o u n d a m o d i f i c a t i o n o f S t e r n ' s e x p o n e n t i a l m e t h o d (£) m o s t s a t i s f a c t o r y . I n t h i s method r e

c r v s

R

= R

(ω/ω·^

x

u>
u)

2

(2b)

2

w h e r e R^ a n d m r e f e r t o t h e r e f l e c t a n c e a n d e n e r g y a t t h e l o w e r l i m i t o f t h e m e a s u r e m e n t i n t e r v a l , a n d R2 and t o the c o r r e s p o n d i n g v a l u e s a t the upper l i m i t . The v a l u e s o f t h e p h a s e s h i f t f o r ω b e l o w t h e m e a s u r e ­ ment i n t e r v a l ( θ·^), i n t h e i n t e r v a l ( θ ) , a n d a b o v e i t (Θ3) are then 2

R(U) )

œ

0

1

,

ω

,

Μ-Ο)

2

°

π

R

Γ 2 ω

0

= ~

J

u 3

) _ 0

l 2

, π

n

0

+

ω

ω

*

ι

p π

1

1

0

η=0

(®ι/ p-type semi­ conductor -* degenerate semiconductor - 1-D metal. We contend that the f i n a l transformation w i l l occur ( p o s s i b l y abruptly) at x = gi moment we neglect the i n f l u e n c e of d i s o r d e r , tf

x

e

m

a

F

o

r

ft

t n e

c

the very l i k e l y p o s s i b i l i t y of one or more m i s c i b i l i t y gaps (6 5 0 i n F i g . 2) and other perturbations which may turn o f f the m e t a l l i c conduction (e.g. P e i r e l s d i s t o r t i o n ) , and confine our­ selves instead to the f i r s t order problem: What are the s o l i d state factors s t a b i l i z i n e

f o r g e n e r a l i t y , a f i n i t e homogeneity range, γ * 0, f o r the 1-D metal phase i n F i g . 2. F i g . 2 would transform i n t o the s p e c i a l case of F i g . 1 i f γ = 0 and χ . = δ = 0.15. magic In the present study we have chosen II as Magnus green s a l t (MGS), P t ( N H ) P t C l , and IV = K P t ( C N ) Y (Y - CI,Br) and K«PtCl . The t e t r a g o n a l Magnus s a l t s t r u c t u r e i s shown i n F i g . 3. 1

3

4

4

2

4

2

6

II

I t i s c h a r a c t e r i z e d by the s t a c k i n g of square co-planar cations and P t C l " anions at a 3.24 4

Pt(NH^)

4

A separation along the c r y s -

t a l l o g r a p h i c c - a x i s , and the r e l a t i v e l y long i n t e r c h a i n Pt-Pt ° 6 distance of 6.35 A along the d i r e c t i o n . Previous studies 7-9 suggested that the e l e c t r i c a l and o p t i c a l p r o p e r t i e s of MGS could be described by M i l l e r ' s model i n which f i l l e d 5d 2 metal ζ

bands are separated by £ 0.6 eV from empty 6p bands. Recent band c a l c u l a t i o n s ^ , c o n d u c t i v i t y ' " ^ and i n f r a r e d measurements ^ suggest that the 5d 2-6p gap i s ^ 4.5 eV and that the measured 8 9 ïl 12 —6 —2 —1 —1 conductivities, ' ' ' which range from 10 to 10 Ω -cm , 14 may be impurity dominated. In our EPR studies of MGS we have suggested that the s i g n a l i s due to a s e l f - t r a p p e d 5d 2 hole whose energy l e v e l may l i e ^ 0.6 eV above the top of the 5d 2 z

1

1 1

1

Z

z

z

valence bands. Such states are formally equivalent to those i n ­ troduced by the s u b s t i t u t i o n of P t ( I I I ) f o r P t ( I I ) i n the P t ( I I ) l a t t i c e and are introduced i n t o MGS due to the presence of Pt(IV) complexes during the preparation of the compound. This s i t u a ­ t i o n i s depicted i n the band model of F i g . 4, which may be con­ s i d e r e d our s t a r t i n g point and equivalent to a s o l i d s o l u t i o n 14 -4 with a very small value of χ (-10 ) i n the phase diagram of F i g . 2. In the present work we have found i t p o s s i b l e to i n ­ crease χ to f a i r l y l a r g e values and report the e f f e c t of such

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

22.

SCOTT

ET

Magnus

AL.

Green

Salt

335

β /-ID METAL

Br

—

ID

METAL + 12

1 SOLID ' Ι § SOLUTIONS ρ - 8 —

• wÊÊÊÊÊÊË Χ

magic

Figure 2. Hypothetical phase diagram between a Pt(H) and Pt(IV) compound illustrating the approach taken here for achiev­ ing the 1-D metallic state. Parameter δ is width of two-phase region, and γ is assumed homogeneity range for the 1-D metallic phase. In principle, many intermediate phases all separated by small immiscibility gaps could exist in such a system.

Figure PtCh, ' 2

3. Stacking ofPt(NH,) and ions in Magnus' green salt (6) /t

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2+

336

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

systematic v a r i a t i o n s i n the t o t a l e l e c t r o n concentration on the p h y s i c a l p r o p e r t i e s of the system. I t i s our contention, then, that there w i l l be a s p e c i a l χ » χ . f o r which MGS w i l l magic behave as a 1-D "metal". The approach to x sys­ tem, and i t s i m p l i c a t i o n s f o r the e n t i r e c l a s s of p a r t i a l l y o x i ­ d i z e d metal chain compounds, i s thus the main subject of t h i s work. r

i n

t n e

M

G

S

m a g i c

M a t e r i a l s Preparation A l l platinum s a l t s used i n t h i s study were prepared from 99.99% pure platinum metal by standard conversion techniques as 1 5

described by B r a u e r .

Samples of K P t C l 2

I ^ P t C l ^ and

6 >

e

K P t ( C N ) 3 H 0 were a l s 2

4

2

Mass.) f o r purposes of comparison. The r e s u l t s reported h e r e i n were found to be independent of m a t e r i a l s source. The Magnus s a l t s o l i d s o l u t i o n s (MGS(SS)) were prepared ac­ cording to the f o l l o w i n g r e a c t i o n scheme (1)

(a)Pt(II)(NH ) 3

+ + 4

+ (b)Pt(II)Cl

= 4

+ (c)Pt(IV)L Y 4

= 2

+ MGS(SS),

where the tetrammine platinum(II) complex was present as the CI or CIO. s a l t , K P t C l , was the anion source, and the molar r a t i o 4 2 4 (a):(b) = 1. S u c c e s s f u l i n c o r p o r a t i o n of higher valence Pt i n t o MGS was found to occur_un Cl . 4

q

q

The maximum values of q obtained f o r the v a r i o u s systems i s shown i n Table I I . I t should be noted that the parameter q as used i n the formulae f o r MGS(SS) systems i s a measure o f the de­ gree of s u b s t i t u t i o n of MGS a n i o n i c s i t e s by P t ( I I I ) . I t i s of course r e l a t e d , but not e q u i v a l e n t t o , the composition parameter χ of F i g s . 1 and 2. For comparison, the p a r t i a l l y o x i d i z e d Pt(+2.3) s t a t e occurs at χ = 0.15 i n F i g s . 1 and 2 and would be equivalent to q = 0.6 i n Formulae (3) and (4) i f an "averaged" o x i d a t i o n s t a t e were a p p r o p r i a t e f o r the MGS s o l i d s o l u t i o n s . This p o i n t w i l l be considered again i n the subsequent d i s c u s s i o n . I f the top of the 5d 2 valence bands are mostly of a n i o n i c z

( P t ( I I ) C l ~ ) c h a r a c t e r , as shown i n F i g . 4, then the 5d 2 hole i s 4

z

l o c a t e d on the P t C l (5)

4

group

Pt (II) ( N H ) ( P t (II) C l 3

4

4

) ^ (Pt (II) (CN) ) (Pt ( I I I ) C l ) Y . 4

4

q

q

E.P.R. powder data f o r s o l i d s o l u t i o n s of the type (4) gave the f o l l o w i n g g-tensor parameters: g., = 1.94 and g = 2.50. 14 These values are i d e n t i c a l to those measured f o r MGS(SS) i n which K P t ( C N ) C l or K P t ( C N ) B r i s the oxidant; Thus, EPR suggests that the P t ( I I I ) s i t e i s independent of o x i d i z i n g agent. Whether the 5d 2 hole r e s i d e s p r i m a r i l y on ( P t C l ) or P t ( N H ) 2

4

2

2

4

2

g

4

3

4

groups depends on the r e l a t i v e energies of the c a t i o n and anion bands, of course, and we choose the former s i t e to be c o n s i s t e n t with the assignments i n M i l l e r ' s o r i g i n a l model.^ Our formulation of Magnus s a l t s o l i d s o l u t i o n systems places charge compensating h a l i d e ions i n i n t e r s t i t i a l s i t e s i n the s t r u c t u r e . Our main evidence f o r t h i s c o n s i s t s of i n f r a r e d data and x-ray d i f f r a c t i o n measurements of the l a t t i c e constants of MGS(SS). The f a r i n f r a r e d spectrum of the K P t ( C N ) B r - o x i d i z e d 2

4

2

MGS product with q = 0.2 shows no evidence f o r a Pt-Br s t r e t c h band. Such a band would be expected f o r s u b s t i t u t i o n along the chain, or f o r charge compensation through replacement of one of the l i g a n d s w i t h i n the Pt square c o o r d i n a t i o n plane (e.g., r e - _^ placement of NH ). The changes i n i n f r a r e d p a t t e r n above 700 cm 3

with i n c r e a s i n g q values are shown i n F i g . 6. These s p e c t r a were taken f o r constant s o l i d c o n c e n t r a t i o n i n the KBr matrix, and show only the i n t r o d u c t i o n of one new band due to the C Ξ Ν 1

s t r e t c h i n g vibrâtion_near 2200 cm" . This i s c o n s i s t e n t with the replacement of P t C l ~ anions i n the MGS s t r u c t u r e with Pt(CN) ~~ 4

4

as determined by chemical a n a l y s i s . Unit c e l l data as a f u n c t i o n o f q f o r the K P t ( C N ) B r 2

4

2

oxi-

dized s o l i d s o l u t i o n s , shown i n F i g . 7, i n d i c a t e s expansion of

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

340

EXTENDED

INTERACTIONS

BETWEEN

METAL

TABLE I I Solid Solution Limits Dopant ( O x i d i z i n g Agent)

K Ptci 2

0.03

6

K Pt(CN) Cl 2

4

K Pt(CN) Br 2

4

0.08

2

0.20

2

See Eqs. ( 4 ) , ( 5 ) . WAVELENGTH (MICRONS) 3

3000

4

5

6

2000

7

1500

8

9

1200 cm

Figure

6. Infrared KgPtfCN^Brj.

10

II

1000 900

12

13

800

14

15

700

-I

spectra for MGS(SS) formed by partial oxidation Composition parameter q is defined in Equation 5.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

with

IONS

22.

seoir E TAL.

the MGS the C

q

Magnus

Green

Salt

341

c e l l along the t e t r a g o n a l a - a x i s and c o n t r a c t i o n along Q

stacking d i r e c t i o n .

Expansion

transverse to the l i n e a r

chains i s p l a u s i b l e f o r Br" i n t e r s t i t i a l s combined with the i n ­ t r o d u c t i o n of P t ( C N ) groups i n a n i o n i c l a t t i c e s i t e s , whereas 4

c o n t r a c t i o n along C i s c o n s i s t e n t with the attendant i n t r o d u c ­ t i o n of the s m a l l e r P t ( I I I ) ions i n the c h a i n s . In t h i s connec­ t i o n i t i s r e l e v a n t that a l l the p a r t i a l l y o x i d i z e d Pt s a l t s shown i n Table I have very short Pt-Pt spacings. Thus, our ob­ s e r v a t i o n of a decrease i n C f o r MGS(SS) i s e n t i r e l y expected Q

q

with p a r t i a l o x i d a t i o n ( i n c r e a s i n g q) o f t h i s phase. However, u n l i k e the m e t a l l i c - l i k e s a l t s o f Table I , we expect two d i f f e r ­ ent types of platinum s i t e s along the c h a i n s : a m a j o r i t y con­ s i s t i n g o f the P t ( I I ) f a c t , the expected d i s o r d e t h i s arrangement i s c l e a r l y observed i n the x-ray d i f f r a c t i o n as a l o s s and broadening of (00il) r e f l e c t i o n s with i n c r e a s i n g q. E l e c t r i c a l and O p t i c a l P r o p e r t i e s . The formation o f mixed valence s o l i d s o l u t i o n s through the i n t r o d u c t i o n of P t ( I I I ) has profound e f f e c t s on the p r o p e r t i e s of MGS. Measurements of the d.c. e l e c t r i c a l c o n d u c t i v i t y of powder compactions shown i n Table I I I show a four order of magnitude change i n σ as the com­ p o s i t i o n was changed from o s t e n s i b l y pure MGS (undoped, q = 0) to a s o l i d s o l u t i o n c o n t a i n i n g the maximum degree of s u b s t i t u t i o n , q = 0.20. Moreover, the h o l e i o n i z a t i o n e n e r g i e s , E determined &9

from the a c t i v a t i o n energies f o r e l e c t r i c a l conduction become s m a l l e r with i n c r e a s i n g q. Although the measurements were per­ formed on compacted powders, these general trends are c l e a r l y evident and q u i t e s i m i l a r to the behavior of doped three-dimen­ s i o n a l semi-conductors as the impurity concentrations are i n ­ creased · The o p t i c a l spectrum i n the near i n f r a r e d r e g i o n a l s o r e ­ v e a l s some i n t e r e s t i n g f e a t u r e s . These measurements were c a r r i e d out on samples d i s p e r s e d i n KBr p e l l e t s . F i g . 8 d e p i c t s the spec­ trum f o r two d i f f e r e n t "pure" (q = 0) MGS samples. Sample A was measured i n two separate pressings of the same p e l l e t to d e t e r ­ mine the v a r i a t i o n i n o p t i c a l d e n s i t y to be expected due to v a r ­ i a t i o n s i n sample preparatory technique. The s p e c t r a were mea­ sured with no r e f e r e n c e i n the standard compartment of the spec­ trometer. We a s c r i b e the absorption r i s e i n the 1-2.4μ r e g i o n of the spectrum mostly to s c a t t e r i n g , as there do not appear to be any d i s t i n c t bands i n t h i s range. On the other hand, F i g . 9 shows the a b s o r p t i o n i n the MGS(SS) system, prepared with K P t ( C N ) B r oxidant, f o r the range of compositions q = 0.05 to 2

4

2

0.2. The new band appearing i n t h i s frequency r e g i o n i s i n t e n s e , and d i r e c t l y p r o p o r t i o n a l to the c o n c e n t r a t i o n of P t ( I I I ) centers i n the sample as determined by chemical a n a l y s i s based on carbon

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

342

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

9.04

5 10 MOLE % Pt INTRODUCED

Figure 7. Lattice constants vs. mole % platinum introduced for MGS(SS) prepared by K Pt(CN)j,Br . Constants obtained by least-squares refinement of Guinier camera powder data. 2

3.00|

1

Q00 4000 1

1

1

1

8000

1

1

1

1

1

1

12000

1

1

ι

1

16000

r

1

1

20000

1

24000

WAVELENGTH, Â Figure 8. Spectra of nominally pure MGS in the near IR. Curves A correspond to spectra obtained in two separate pressings of the same pellet; curve Β obtained from a separate MGS preparation made up to the same concentration in pellet as A.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

22.

SCOTT

ET

Magnus

AL.

Green

Salt

343

(CN) content. Excluding the q = 0.05 samples, whose chemical a n a l y s i s i s l e a s t r e l i a b l e , we f i n d an absorption c o e f f i c i e n t α = ( 6 . 0 + 0 . 3 ) χ 10 1/mol-cm, assuming the absorption i s en­ t i r e l y p o l a r i z e d along the c-axis of the c r y s t a l l i t e s ( i . e . , α » α ). (|

χ

In a d d i t i o n to the intense absorption peaking near 1.3μ, another important feature of F i g . 9 i s the gradual s h i f t of ab­ s o r p t i o n edge to lower energies with i n c r e a s i n g q. This i s sug­ gested by the general increase i n o p t i c a l d e n s i t y at 2.4μ as q i n c r e a s e s . Although we can not o b t a i n an accurate absorption edge from the p e l l e t measurements, t h i s q u a l i t a t i v e trend i s con­ s i s t e n t with the lowering of a c t i v a t i o n energy f o r e l e c t r i c a l conduction with q as shown i n Table I I I . I t i s r e l e v a n t to point out that e a r l y measurements of the IR spectrum of o s t e n s i b l the near IR region shown i n F i g . 9. Moreover, Collman e t . a l . reported photocurrent i n MGS with e x c i t a t i o n between 0.74-2.5μ. 13 Other measurements, i n c l u d i n g those on much purer MGS samples , 18 cast doubt on the presence of t h i s absorption, but i t i s c l e a r that samples prepared i n the presence of Pt(IV) complexes d e f i ­ n i t e l y c o n t a i n the broad IR peak. We have i n f a c t found the iden­ t i c a l IR band i n MGS(SS) samples produced by K P t ( C N ) C l and K P t C l ^ o x i d a t i o n . However, we i n t e r p r e t the absorption as due to t r a n s i t i o n s from the 5d 2 valence band to the 5d 2 bound ζ ζ s t a t e s , r a t h e r than to an interband t r a n s i t i o n . ' 2

4

2

2

?

8

Discussion I t i s worthwhile to review some of the p r o p e r t i e s of the mixed valence MGS s o l i d s o l u t i o n s before examining t h e i r r e ­ l a t i o n s h i p to the c l a s s of p a r t i a l l y o x i d i z e d platjinum s a l t s of Table I such as KCP. P r e v i o u s l y , we found from EPR measure14 ments that " e x t r i n s i c " MGS c r y s t a l s u s u a l l y c o n t a i n small con­ c e n t r a t i o n s of bound 5d 2 holes ( P t ( I I I ) ) l y i n g ^ 0.6 eV above the 5d 2 o r b i t a l s . The P t ( I I I ) holes whose occurence i s due to the presence of Pt(IV) complexes during sample p r e p a r a t i o n , have a f a i r l y wide extent-over at l e a s t s e v e r a l metal chain l a t t i c e 14 sites . In these s t u d i e s we have been able to s i g n i f i c a n t l y i n ­ crease the concentration of P t ( I I I ) centers by p a r t i a l o x i d a t i o n with Pt(IV) complexes, r e s u l t i n g i n as many as one out of ten P t ( I I ) chain s i t e s replaced by P t ( I I I ) f o r the case of o x i d a t i o n with K P t ( C N ) B r (Table I I , q - 0.2). Concomitant with the i n ­ c r e a s i n g P t ( I I I ) concentration i s an increase i n d.c. c o n d u c t i v i t y and the appearance of a strong IR t r a n s i t i o n due to 5d 2 band to z

2

4

2

z

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS

344

BETWEEN

METAL

TABLE I I I E l e c t r i c a l Data on Powder Compactions ((IV)

a

= K Pt(CN) Br ) 2

R T

4

, (ohm-cm)"

2

Ε

1

q

, eV a'

0.0

3.7 χ 1 0 ~

7

0.66

0.12

2.5 χ 1 0 "

4

0.27

0.20

6.3 χ 1 0 ~

3

0.15

>gl 4000

ι

ι 8000

ι

ι

I

ι

12000

16000

1

1 20000

1 — -J 24000

W/VELENGTH, Â

Figure 9. Near IR spectra for MGS(SS) prepared from Br . Composition parameter q defined in Equation results were obtained for partial oxidation of MGS with CL and K PtCl . 2

z

K Pt(CN) Similar K>Pt(CN) É

5.

r

r

r>

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

22.

SCOTT

ET

Magnus

AL.

5d 2 h o l e e x c i t a t i o n . z

Green

Salt

345

A c t i v a t i o n energies obtained from the d.c.

c o n d u c t i v i t y measurements i n d i c a t e a decrease i n Ε , the h o l e i o n ­ i z a t i o n energy. In what f o l l o w s we w i l l consider the p o t e n t i a l the MGS system has f o r e x h i b i t i n g a t r a n s i t i o n to a "1-D metal" at dopant l e v e l s higher than we have been p r e s e n t l y able to achieve and f u r t h e r i m p l i c a t i o n s concerning the s t a b i l i z a t i o n of known 1-D metals such as KCP at s p e c i a l s t o i c h i o m e t r i e s . By c o n s i d e r i n g the low h o l e c o n c e n t r a t i o n l i m i t , one can more c l e a r l y understand j u s t what might be r e s p o n s i b l e f o r a t r a n s i t i o n to m e t a l l i c behavior at higher hole c o n c e n t r a t i o n . I f we e n v i s i o n a f u l l band (the MGS l i m i t ) and introduce a few l o c a l c a r r i e r s by s u b s t i t u t i o n , then the c a r r i e r , i n t h i s case a d 2 h o l e , w i l l not ζ * be f r e e l y mobile, but w i l l i n s t e a d be r a t h e r t i g h t l y bound to i t s site. This b i n d i n g ma between the P t ( I I I ) hol s u l t i n the formation of an e x c i t o n i c or quasibound p a r t i c l e - h o l e 19 s t a t e r a t h e r than i n a f r e e c a r r i e r . This coulomb p a r t i c l e - h o l e a t t r a c t i o n can however be reduced i n s e v e r a l ways. As the concen­ t r a t i o n of dopant and d e n s i t y o f c a r r i e r s i n c r e a s e s , the s i n g l e c a r r i e r p o t e n t i a l r e s u l t i n g from the charge compensators w i l l tend to be screened. This mechanism, analogous to the Mott mechanism i n higher dimensional i s o t r o p i c conductors, should l e a d to a semi­ conductor-metal t r a n s i t i o n . A l s o , with an i n c r e a s e i n doping, the d e n s i t y of charge compensators which an i n d i v i d u a l d 2 hole sees w i l l become e f f e c t i v e l y constant, i . e . independent of the s i t e on which the hole f i n d s i t s e l f . This w i l l occur s i n c e the o v e r l a p ­ ping coulomb t a i l s from the impurity or compensator d i s t r i b u t i o n w i l l average out the one e l e c t r o n (hole) p o t e n t i a l f l u c t u a t i o n s more e f f e c t i v e l y at high c o n c e n t r a t i o n s . For the case of the KCP l i m i t , there are ^ 0.3 h a l i d e ions f o r each P t . We suggest that f o r KCP i t i s t h i s s t o i c h i o m e t r i c r a t i o , corresponding to the composition g ^ 0.15 i n our phase diagram ( F i g . 1) at which the l o c a l e x c i t o n s t a t e goes over to the m e t a l l i c s t a t e . The f a c t that t h i s r a t i o occurs c o n s i d e r a b l y below a P t ( I I I ) / P t ( I I ) r a t i o of u n i t y comes about because the h o l e s , w h i l e l o c a l i z e d , are i n extended e i g e n s t a t e s which reach s e v e r a l Pt s i t e s along the c h a i n . Such i n t e r p r e t a t i o n can be i n f e r r e d from the EPR spec­ t r u m * ^ as w e l l as the l a t t i c e r e g u l a r i t y . This i n d i c a t e s that the hole-anion i n t e r a c t i o n reduced by h o l e - h o l e i n t e r a c t i o n s and smoothed due to i n c r e a s e d h a l i d e c o n c e n t r a t i o n could r e s u l t , at s u f f i c i e n t l y high dopant l e v e l , i n a s i n g l e hole wave f u n c t i o n with a coherence length s u f f i c i e n t l y l a r g e to r e s u l t i n m e t a l l i c l i k e behavior. I t i s i n t e r e s t i n g to note that f o l l o w i n g M o t t s o r i g i n a l a r ­ gument, the dopant d e n s i t y ^ 0.X5 i s i n near agreement with 9

z

x

m

2

a

i

c

1

1 , 3

1

x

m

a

g

i

c

1

the o r i g i n a l Mott c r i t e r i o n f o r the t r a n s i t i o n . Following Mott s o r i g i n a l argument, based on Thomas-Fermi screening of the

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

346

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

c a r r i e r s , the semiconductor -> metal t r a n s i t i o n f o r impurity con­ d u c t i o n i n a three dimensional system should occur approximately when n ^ /

3

a^ *υ .25.

As Mott and Twose p o i n t o u t , ^

1

a sharp break

i n the c o n d u c t i v i t y of both η-type and p-type germanium occurs i n 1/3 f a c t f o r n^ a^ ^ .3. In these expressions, i s the c a r r i e r d e n s i t y and a^ i s the e f f e c t i v e s i t e r a d i u s . I f we take a as 1/3 one-half of the Pt-Pt spacing along the c h a i n , then n o f the band: k

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

360

U

= - Ζα -

B

ψ

sin

φ

(3)

We note t h a t both α and β a r e f u n c t i o n s o f R and Z, and t h a t the p r i n c i p a l dependence o f β on these parameters i s s u r e l y expo­ n e n t i a l . Taking t h e r a d i a l screening constant f o r t h e wavefunction I i> t o have roughly the S l a t e r form (20), we w r i t e t h i s dependence a s

β - 3 exp [ - ( y - n Z ) R ]

(4)

o

where βο, μ, and η « μ a r e constants. I n c o n t r a s t , the v a r i a t i o n o f t h e s m a l l (19) energy α w i t h each o f t h e parameters R and Ζ i s a t most a paver-law dependence, comparable w i t h those o f t y p i c a l c o n t r i b u t i o n s t o the Madelung energy U . T h i s o b s e r v a t i o n has s p e c i a l s i g n i f i c a n c e f o r t h e f i l l e d band case Z=2, where U Under these circumstances % r a t h e r than U . We thereby account f o r t h e wide v a r i a t i o n i n R among the d i v a l e n t parent cxmpounds (1), i n p a r t i a l answer t o q u e s t i o n (b). With p a r t i a l o x i d a t i o n o f these m a t e r i a l s t o form Krogmann s a l t s , t h e s t r o n g l y R- and Z - dépendait second term i n Equation 3 e n t e r s U , and the s i t u a t i o n i s a l t e r e d d r a s t i c a l l y . On t h e b a s i s o f o u r d i s c u s s i o n i n t h e preceding s e c t i o n , i t i s n o t unreasonable t o regard t h e i n t e r c h a i n channels a s a "charge s i n k " , capable o f accormodating a s many counterions a s are necessary t o balance any Ζ i n t h e range o f i n t e r e s t . To t h e extent t h a t t h i s i s t r u e , t h e c h a i n s c o n s t i t u t e a c h e m i c a l l y unique system i n which the number o f bonding e l e c t r o n s may be regarded a s a v a r i a t i o n a l parameter. I f , as suggested by Krogmann (1), Ζ i s determined by U , then f o r g i v e n R the observed Ζ must correspond t o a minimum i n 1%: M

B

B

B

R

s i n -γ)

(5)

When Equations 4 and 5 a r e combined, a t r a n s c e n d e n t a l equa­ t i o n i s obtained r e l a t i n g R and Z. A t y p i c a l s o l u t i o n i s shown i n F i g u r e 1. Here we have simply determined μ and η a c c o r d i n g t o S l a t e r ' s r u l e s (20), and adjusted t h e r ^ t i o βς/α s o as t o reproduce the experimental v a l u e Z=1.7 f o r R=2.88A. More e l a b o r a t e f i t t i n g procedures a r e c e r t a i n l y a v a i l a b l e , but we have found no p h y s i c a l ­ l y reasonable c h o i c e o f parameters which s u b s t a n t i a l l y a l t e r s Figure 1 o r our f i n a l conclusions. Under t h e c o n d i t i o n imposed by Equation 5, we a r e now i n a p o s i t i o n t o c a l c u l a t e U from Equations 3 and 4. The r e s u l t i s B

π Λν - V ~ a

7 + ù

sin(ïïZ/2) (π/2)cos (πΖ/2) + (3£ηβ/9Ζ) sin(πΖ/2)

l 0 j

Equation 6 i s p l o t t e d i n F i g u r e 2, which represents t h e v a r i a t i o n o f U w i t h R under t h e c o n s t r a i n t t h a t each R, Ζ i s s e l f c o n s i s t e n t l y t o be r e a d j u s t e d so a s t o minimize U . Q u a l i t a t i v e l y , the c a l c u l a t i o n i s c o n s i s t e n t w i t h t h e B

B

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

BLOCH

AND

WEiSMAN

Krogmann

Salts

Figure 1. Variation of conduction-band occupation, Z, with lattice constant, R, for a one-dimensional tight-binding system under the constraint of Equation 5

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

INTERACTIONS

BETWEEN

METAL

R(A)

,2.0 I

I

I.I

2.6 2.7 I

I

12

2.85

2.8 i

1.3

1.4

l

2.88

29

l

1.5

1.6

ι

1.7

1.8

1.9

2.0

Ζ

Figure 2. Variation of band-structure stabilization U with R and Ζ for a one-dimensional tight-binding system under the constraint of Equation 5. At the experimental values Ζ — 1.7 and R = 2.88 A , U (Z)-Ui*(2) is much too small to account for the stability of Pt 2.3+. B

B

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

24.

BLOCH

AND

WEISMAN

Krogmann

363

Salts

d e s c r i p t i o n o f f e r e d by Krogmann (1). F o r l a r g e R, we have Z=2 and ϋβ/α independent o f R. With decreasing R, the bandwidth 4β i s increased, u n t i l the h i g h e s t - l y i n g s t a t e s i n the band become a n t i bonding w i t h r e s p e c t t o E ° . The band-structure energy 1% now f a v o r s p a r t i a l o x i d a t i o n o f the chains, and the parameters Ζ and R a r e coupled according t o F i g u r e 1. With p r o g r e s s i v e o x i d a t i o n and shrinkage o f the l a t t i c e , energy 1% i s gained a s shewn i n F i g u r e 2. U l t i m a t e l y , however, the compression and hence the o x i d a t i o n must o f course be l i m i t e d by the r e p u l s i v e term U R i n Equation 1, which r i s e s r a p i d l y f o r R s m a l l and decreasing. The r e s u l t i s a minimum i n the sum U + U a t seme s h o r t R and f i x e d o x i d a t i o n s t a t e 1d y >d * which i s the same as that p r e v i o u s l y proposed by Mason and Oray (22) but u n l i k e that deduced by Piepho, Shatz and McCaffery (PSM) (3) on the b a s i s of s p e c t r a l and magnetic c i r c u l a r d i c h r o i s m data. The same energy l e v e l order was obtained i n our e a r l i e r s t u d i e s of the M C l ^ " (M • Pd,Pt) complexes (18) where again the highest occupied energy l e v e l was the b (d ). The major differences with respec the r e l a t i v e p o s i t i o n 3a (M p +L) o r b i t a l energy l e v e l s which are reversed i n the two types of complexes. This 3 a o r b i t a l i s found to be mainly l o c a l i z e d on the n i t r o g e n atoms of the CN" groups and i s c o n s i d e r a b l y c l o s e r i n energy to the occupied d - l i k e MO's than i n the M C l ^ " case, which should l e a d to r e l a t i v e l y low energy e l e c t r o n i c t r a n s i t i o n s w i t h a p p r e c i a b l e M -*· L charge t r a n s f e r c h a r a c t e r . In a manner analogous to that p r e v i o u s l y reported f o r the MCl^ ~ complexes (18), t r a n s i t i o n s t a t e c a l c u l a t i o n s were c a r r i e d out on the Pt(CN) 2"" i o n l e a d i n g to c a l c u l a t e d e l e c t r o n i c t r a n s i t i o n energies which are i n good agreement w i t h both spec­ t r a l and magnetic c i r c u l a r dichroism experiments as w e l l as p h o t o e l e c t r o n data. The comparison w i t h the s p e c t r a l data i s shown i n Table I I . Following PSM (3), the lowest energy v i s i b l e a b s o r p t i o n band has been assigned to a Ey' s t a t e (using double group n o t a t i o n ) produced by mixing of the u s u a l " s i n g l e t " and " t r i p l e t " e x c i t e d s t a t e s of the P t i C N ) ^ ~ i o n under the i n f l u e n c e of s p i n o r b i t c o u p l i n g ( I t must be emphasized that s p i n o r b i t c o u p l i n g has not been e x p l i c i t l y included i n t h i s c a l c u l a t i o n , o n l y i t s e f f e c t on the number and symmetry of the e x c i t e d s t a t e s has been considered). Here, however, the o r i g i n of t h i s t r a n s i t i o n i s i d e n t i f i e d as the 2b l e v e l r a t h e r than the a ( d 2 ) as was assumed by PSM. Also the shoulder a t 5.1 eV has been assigned to a t r a n s i t i o n from one of the l i g a n d π-levels ( l a ) to the 3 a « Otherwise the a s s i g n ­ ments are i n b a s i c agreement with those proposed p r e v i o u s l y and are e n t i r e l y c o n s i s t e n t with the a v a i l a b l e magnetic c i r c u l a r d i c h r o i s m and s p e c t r a l data. Furthermore, the r e s u l t s are a l s o i n good q u a l i t a t i v e agreement w i t h the photoelectron data f o r the P t ( C N ) ^ 2 - complex (7,23) suggesting t h a t , indeed, a s a t i s f a c t o r y d e s c r i p t i o n f o r the e l e c t r o n i c s t r u c t u r e of t h i s i o n has been obtained. As i s evident from F i g u r e 2, combining two such P t ( C N ) ^ u n i t s a t 2.89 A s e p a r a t i o n i n the staggered c o n f i g u r a t i o n found z

l g

X

x z

Z

2

2 g

2 U

x y

2

2 U

2

z

J|

2

2

l g

2

z

2 u

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(10,700)

39.2

(22,100)

15

1

l a

l a

5 a

5 e

41.1

43.6

50.8

52.4

*

2 b

39.5

2 e

2 e

38.7

38.7

2 b

37.9

3

Transition State Calculation cm" χ 10"

« *

( E

2«

2u

( A

( A

( A

( B

lu

A

)

E

)

2u

lu

)

)

2« 2U, u

2u

lg u>

3 a

3 a

3 a

3 a

(

2u V

( E

(

2u V

2u «» 2u

3 a

3 a

3 a

6 a

lg *

2g -

2g *

2g *

g *

g *

2g *

Transition

)

b

a

e

b

a

a

(

e

2g * u

a

( E

u

( A

V

)

lg - 2u 2u

(E

2 u

g * 2u u>

2g -

)

Mason and Gray*

e

a

°2g

g -

a

a

2 u

(Ε

(

2u

( B

(

)

V

Α

lu

2u V a

&

(E

a

g * 2α ύ, 2ύ>

a

l g * 2u u>

lg *

e

a

Piepho, Shatz and McCaffery c

·

* Several additional transitions (3 symmetry forbidden, singlet and 6 t r i p l e t ) are calculated to occur i n this region. a) Ref. [3]· b) Energies of f u l l y allowed charge transfer transitions are underlined; c) Primes on symmetry labels denote a double group state derived from spin orbit s p l i t t i n g of the appropriate t r i p l e t excited state. Only the resultant symmetry allowed transition states are noted; d) Ref. 122J

46.1

41.3 (^l,900sh)

(1,480)

1

35.8

3

Experimental Absorption Peak Maxima cm" χ 10-3 (extinction coeff)

4

2Table I I . Experimental and T h e o r e t i c a l T r a n s i t i o n Energies f o r T r a n s i t i o n s i n P t ( C N )

EXTENDED

390

INTERACTIONS BETWEEN M E T A L IONS

i n the u n i t c e l l o f the KCP complexes r e s u l t s i n s u b s t a n t i a l i n t e r a c t i o n among s e v e r a l o f the o r b i t a l s l e a d i n g to "bonding" and "antibonding" o r b i t a l p a i r s separated by as much as 2,65 eV, The l a r g e s t i n t e r a c t i o n i s c l e a r l y among the a o r b i t a l s and i n p a r t i c u l a r between the 5 a o r b i t a l s which our c a l c u l a t i o n s i n d i c a t e a r e l a r g e l y based on the metal and have a h i g h concent r a t i o n o f e l e c t r o n d e n s i t y along the metal c h a i n a x i s . In the case o f the 5 a o r b i t a l t h i s interaction i s s u f f i c i e n t to r a i s e an antibonding a o r b i t a l such that i t becomes the highest occupied l e v e l i n the dimer u n i t . Large i n t e r a c t i o n s a r e a l s o i n d i c a t e d among s e v e r a l o f the a l e v e l s - p a r t i c u l a r l y the lowe s t unoccupied 3 a - as w e l l as among c e r t a i n o f the e l e v e l s . In g e n e r a l , w i t h i n each type o f o r b i t a l there seems to be a rough c o r r e l a t i o n between the amount of o r b i t a l i n t e r a c t i o n and the proportion of electro densit th metal would b expected due t o the r e l a t i v e l y staggered arrangement in-plane b and b o r b i t a l s s h o w e s s e n t i a l l y no i n t e r a c t i o n a t t h i s metal-metal s e p a r a t i o n (2.89 A) c a r r y i n g over as e s s e n t i a l l y u n s p l i t b , b p a i r s o f l e v e l s . I n a d d i t i o n t o these d i r e c t o r b i t a l i n t e r a c t i o n s there i s a general r a i s i n g o f the energy o f the o r b i t a l s i n the dimer w i t h respect t o those i n the f r e e i o n , presumably r e f l e c t i n g the increased s h i e l d i n g i n the c l o s e l y spaced dimer u n i t . l

g

l g

J g

x

2

u

2 U

J g

2 g

e

2

These observations have some important i m p l i c a t i o n s w i t h regard to s o l i d s t a t e i n t e r a c t i o n s i n Pt(CN) ~ complexes. In p a r t i c u l a r , the ordering of the energy bands and the p o s i t i o n o f the Fermi l e v e l i n these s o l i d s should be a v e r y s e n s i t i v e f u n c t i o n o f the intermolecular s e p a r a t i o n and i n the case o f the KCP complexes, where t h i s s e p a r a t i o n i s q u i t e short, the h i g h e s t occupied band i s indeed l i k e l y to be a n e a r l y f r e e - e l e c t r o n l i k e d 2 + s band as has been suggested p r e v i o u s l y (24). However, as these r e s u l t s show, i t does not f o l l o w that the d 2 - l i k e o r b i t a l i s a l s o the highest l e v e l i n the f r e e i o n and, indeed, f o r many of the s o l i d P t ( C N ) 2 - complexes which d i s p l a y unusual s o l i d s t a t e s p e c t r a l p r o p e r t i e s buf where the metal-metal separations a r e i n the range 3.63 - 3.09 A (6), the Fermi l e v e l may w e l l l i e w i t h i n q u i t e narrow " d " or d " type bands r a t h e r than a wide d 2 - 1 i k e band. ' More d e f i n i t i v e answers t o these questions and t o the c e n t r a l question regarding the e l e c t r o n i c s t r u c t u r e o f the KCP complexes a r e c u r r e n t l y being sought i n f u r t h e r SCF-Xa-SW c a l c u l a t i o n s on [Pt(CN). ~ ] u n i t s as w e l l as through the use o f band s t r u c t u r e c a l c u l a t i o n methods. n

z

z

|f

, f

x y

x z > y z

z

n

m

Acknowledgment s T h i s r e s e a r c h was supported i n part by the A i r Force O f f i c e of S c i e n t i f i c Research (AFSC), United States A i r Force, under Contract F44620-71-0129.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

27.

INTERRANTE AND MESSMER

Solid

State

Interactions

391

Literature Cited 1. Z e l l e r , H. R., Advances in S o l i d State Physics (1973), 13, 31. 2· Krogmann, Κ., Angew. Chem. I n t e r n a t . E d i t . , (1969), 8, 35. 3. Piepho, S. Β., Shatz, P. Ν., and McCaffery, A. J., J. Am. Chem. Soc., (1969), 91, 5994. 4. Minot, M. J . and P e r l s t e i n , J . Η., Phys. Rev. L e t t e r s (1971), 26, 371. 5. Mehran, F. and Scott, Β. Α., Phys. Rev. L e t t e r s (1973), 31, 1347. 6. Moreau-Colin, M. L., S t r u c t . Bond. (1972), 10, 167 and references t h e r e i n . 7. I n t e r r a n t e , L. V. and Messmer, R. P., Chem. Phys. L e t t e r s (1974), 26, 225. 8. S l a t e r , J. C. and Johnson 9. Johnson, Κ. H. and 831. 10. S l a t e r , J. C., "Advances in Quantum Chemistry", V o l . 6, P.O. Loẅdin, ed., p. 1, Academic Press, (1973). 11. Johnson, Κ. Η., "Advances in Quantum Chemistry", V o l . 7, P.O. Loẅdin, ed., p. 143, Academic Press, (1973). 12. Kohn, W. and Rostoker, Ν., Phys. Rev. (1954), 94, 1111. 13. Rösch, N., Klemperer, W. G. and Johnson, Κ. H., Chem. Phys. L e t t e r s (1973), 23, 149. 14. Herman, F., Williams, A. R. and Johnson, Κ. Η., t o be published. 15. Watson, R. E., Phys. Rev. (1958), 111, 1108. 16. Schwarz, Κ., Phys. Rev. (1972), B5, 2466; Schwarz, Κ., t o be published. 17. Messmer, R. P., I n t e r n . J. Quantum Chem. (1973), 7S, 371. 18. Messmer, R. P., I n t e r r a n t e , L. V. and Johnson, Κ. Η., J. Am. Chem. Soc. (1974), 96, 3847. 19. Rösch, Ν., Messmer, R. P. and Johnson, Κ. Η., J. Am. Chem. Soc. (1974), 96, 3855. 20. Norman, J. G., Jr., K o l a r i , H. J., J. C. S. Chem. Comm. (1974) 303. 21. Rösch, N. and Johnson, Κ. Η., Chem. Phys. L e t t e r s (1974), 24, 179. 22. Mason, W. R. and Gray, Η. Β., J. Am. Chem. Soc. (1968), 90, 5721. 23. Bastas, R., Cahen, D., L e s t e r , J . Ε., and Rajaram, J., Chem. Phys. L e t t e r s (1973), 22, 489. 24. Z e l l e r , H. R., t h i s volume

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28 One-Dimensional and Pseudo-One-Dimensional Molecular Crystals ULRICH IBM

T.

MUELLER-WESTERHOFF

and

FRIEDRICH

HEINRICH

Research L a b o r a t o r y , San Jose, C a l i f . 95193

In t h i s concludin new systems which we are l o o k i n g at i n order to o b t a i n some clarification concerning the P e i e r l s instability and how to circumvent it. Since it is the l a s t t a l k , I a l s o t h i n k that a b r i e f outlook to promising f u t u r e work is in order. F i r s t of all, we want to d i s t i n g u i s h the systems that we are working on and which we want to call Pseudo-one-dimensional Molecular C r y s t a l s , from the one-dimensional square planar complex systems like Krogmann's S a l t and the not so strictly one-dimensional molecular c r y s t a l s of the TTF-TCNQ type. Our systems are composed of e i t h e r mixed valence c a t i o n s of square planar ligand-bridged b i s - t r a n s i t i o n metal c h e l a t e s with acceptor anions such as TCNQ¯ as the counterion or of donor c a t i o n s with l i g a n d bridged b i f u n c t i o n a l t r a n s i t i o n metal complex counterions. In both cases we will d e a l with r a d i c a l i o n s a l t s of charge t r a n s f e r systems. The p a r t i c u l a r l y i n t e r e s t i n g aspect of these compounds lies i n the coexistence of i n t r a m o l e c u l a r as w e l l as i n t e r m o l e c u l a r exchange and e l e c t r o n t r a n s f e r i n t e r a c t i o n s between the t r a n s i t i o n metals i n v o l v e d . These systems are designed to maintain the a n i s o t r o p i c p r o p e r t i e s of one-dimensional molecular c r y s t a l s and have incorporated i n them the first possibility of a v o i d i n g the t r a n s i t i o n to an i n s u l a t i n g P e i e r l s s t a t e . E l i m i n a t i n g the m e t a l - i n s u l a t o r t r a n s i t i o n of h i g h l y conducting compounds is of course equivalent to c r e a t i n g organic and organometallic metals in the a c t u a l sense. However, the molecular design does not e l i m i n a t e the possibility that we may create a true i n s u l a t o r , although from all energetic considerations t h i s would not be very likely. One-dimensional Systems In order to b e t t e r e x p l a i n the reasons f o r our suggesting b i f u n c t i o n a l l i g a n d bridged systems f o r t h i s i n v e s t i g a t i o n , we l i k e to f i r s t take a look at the known i n o r g a n i c and organic one-dimensional molecular c r y s t a l s .

392

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28.

MUELLER-WESTERHOFF

AND

HEiNRicH

Molecular

Crystals

393

Inorganic L i n e a r Systems Several t a l k s i n t h i s symposium discussed p r o p e r t i e s of a one-dimensional i n o r g a n i c s a l t , which c u r r e n t l y i s very much en vogue: the s o - c a l l e d Krogmann's S a l t ("KCP", K Pt(CN),0.3Br3H 0) i n which during the transformation from the n e u t r a l P t ( I I ) complex by bromine o x i d a t i o n to the mixed valence compound there occurs a s u b s t a n t i a l decrease i n the i n t e r m e t a l l i c ( i n t h i s case equal to the i n t e r p l a n a r ) d i s t a n c e s from 3.2 to 2.88 A, by means of which the overlap between the platinum 5d atomic o r b i t a l s i s i n c r e a s e d . T h i s gives r i s e to m e t a l l i c behaviour i n one dimension along the platinum backbone of the e s s e n t i a l l y one-dimensional molecular c r y s t a l . For s i m i l a r systems l i k e the mixed valence oxalates and o t h e r s , one-dimensional m e t a l l i c behaviour and h i g h a n i s o t r o p r a t i o f optical d othe related p r o p e r t i e s have been e s t a b l i s h e d behaves l i k e a semiconducto (1) 2

2

2

Organic L i n e a r Systems The organic counterpart to KCP i s the c l a s s of TCNQ r a d i c a l anion s a l t s ( 2 ) , which i s c u r r e n t l y being s t u d i e d i n a number of l a b o r a t o r i e s . The unique f e a t u r e of TCNQ s a l t s i s t h a t , due i n p a r t to the p o l a r i z e d charge d i s t r i b u t i o n i n the r a d i c a l anion, Coulomb i n t e r a c t i o n s favor a l i n e a r arrangement w i t h i n the molecular c r y s t a l s . Exceptions are known, e.g. TMPD-TCNQ, i n which the s e p a r a t i o n of the p o s i t i v e charge centers i n the c a t i o n i s i d e n t i c a l to the s e p a r a t i o n of the negative charge centers i n TCNQ anion; t h i s could w e l l be the main reason f o r a D-A-D-A-D-A s t a c k i n g being e n e r g e t i c a l l y favored by coulomb a t t r a c t i o n . While i n charge t r a n s f e r systems the overlap of the π-systems w i l l g e n e r a l l y favor a l i n e a r arrangement, the symmetry of the i n t e r a c t i n g molecular o r b i t a l s i s r a r e l y as s u i t e d f o r the formation of segregated stacks as i n TCNQ r a d i c a l anion s a l t . Due to the r a t h e r d e l i c a t e balance between Coulomb and packing f o r c e s , the molecular symmetry i s d i r e c t l y r e l a t e d to the a n i s o t r o p y o f molecular c r y s t a l s . Two main c l a s s e s of TCNQ s a l t s have to be d i s t i n g u i s h e d : Class C: TCNQ r a d i c a l anion s a l t s of Closed s h e l l c a t i o n s ; w i t h i n t h i s c l a s s , there are such d i v e r s e systems as LiTCNQ, C s (TCNQK, NMP-TCNQ, NEtP-TCNQ, Q-TCNQ, Et NH-TCNQ e t c . C l a s s 0: TCNQ r a d i c a l anion s a l t s of Open s h e l l c a t i o n s , comprising the most I n t e r e s t i n g systems l i k e BFD-(TCNQ)2, TTF-TCNQ, TMPD-TCNQ and o t h e r s . The c a t i o n s i n c l a s s C compounds do not d i r e c t l y p a r t i c i p a t e i n the e l e c t r o n i c conduction process, although i n some of them the q u a l i t i e s of p o l a r i z a b i l i t y and permanent d i p o l e moments have b e a r i n g on the Coulomb p o t e n t i a l along the conductive TCNQ chains· 2

J

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

394

C l a s s 0 compounds are t r u e charge t r a n s f e r s a l t s of c l o s e d s h e l l donor and acceptor molecules. I t should be kept i n mind, however that a t y p i c a l c l a s s C compound can a l s o be a CT s a l t : L i TCNQ i s a true charge t r a n s f e r s a l t but o f an open s h e l l donor and a c l o s e d s h e l l acceptor, l e a d i n g to a c l o s e d s h e l l donor c a t i o n and an open s h e l l acceptor anion. In C l a s s 0 s a l t s , the coexistence of r a d i c a l c a t i o n s and anions i s the key to high c o n d u c t i v i t i e s , s i n c e i n a d d i t i o n to the p r o p e r t i e s a v a i l a b l e a l s o i n c l a s s C compounds there i s the p o s s i b i l i t y i n donor and acceptor ions f o r both e l e c t r o n and h o l e conduction. In the TCNQ complex of b i s f u l v a l e n e - d i i r o n BFD-(TCNQ) which represents the f i r s t mixed valence complex s a l t of TCNQ, the i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r w i t h i n the F e ( I I ) - F e ( I I I ) systems seems to p l a y an important r o l e . However, here the l i m i t of intermolecula i t i s the overlap betwee T h i s k i n d of l i m i t a t i o n has prompted us to consider mixed valence systems with d i r e c t i n t e r m e t a l l i c bonds as the most promising approach to s t a b l e h i g h l y a n i s o t r o p i c organometallic metals. A

2>

T r a n s i t i o n Metal Complexes and TCNQ S a l t s I t i s a tempting approach, to t r y to combine the two known l i n e a r systems: l i n e a r mixed valence square planar t r a n s i t i o n metal complexes ( f o r which i n the f u r t h e r d i s c u s s i o n we w i l l only consider platinum as the c l a s s i c example) and organic or organometallic acceptors ( f o r which we w i l l take TCNQ as the example of choice - although i n i t s acceptor p r o p e r t i e s i t i s no longer p a r t i c u l a r l y outstanding)· Consider the two approximate spacing parameters of the f o l l o w i n g i l l u s t r a t i o n d e s c r i b i n g a TCNQ stack and a l i n e a r Pt c h a i n : In TCNQ the i n t e r p l a n a r spacing i s about 3.2 A, the overlap i s roughly between the center of the molecule to the center of the m a l o n i t r i l e group of i t s neighbors - g i v i n g us a packing d i s t a n c e o f 4.2 A as a average v a l u e . In KCP the Pt-Pt d i s t a n c e i s 2.88 A. Since we are d e a l i n g i n t h i s case w i t h square planar u n i t s w i t h a t o t a l negative charge of 2.3 u n i t s , i t i s reasonable to expect t h a t due to the l e s s e r coulomb r e p u l s i o n i n a n e u t r a l complex a Pt-Pt d i s t a n c e o f 2.7 to 2.8 À can be obtained, with t h i s number being an important f a c t o r f o r determining the s t o i c h i o m e t r y _ o f square p l a n a r mixed valence systems c o n t a i n i n g stacks of TCNQ~ as the counterions. As shown i n the f o l l o w i n g diagram there i s only the one allowed r a t i o of 2.8/4.2 « 0.67 TCNQ s per Pt i n uniformly stacked s o l i d s . Although s e v e r a l attempts have been made i n other l a b o r a t o r i e s to produce conductive organometallic mixed valence systems of the above k i n d , such approaches cannot be s u c c e s s f u l s i n c e there i s a discrepancy between the Pt-Pt d i s t a n c e of 2.8 Â f o r optimal i n t e r a c t i o n s and the minimal d i s t a n c e (3.2 A) f o r 1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28.

MUELLER-WESTERHOFF

TCNQ stack

AND

HEiNRiCH

Molecular

Crystals

395

Pt chain

Pt/TCNQ = 4.2/2.8 = 1.5 ( i . e . : TCNQ/Pt = 0.67)

prganic l i g a n d molecules imposed by r e p u l s i o n f o r c e s . The only way by which i n such compounds s i g n i f i c a n t metal-metal i n t e r a c t i o n s can be obtained i s t o d i s t o r t the molecules from t h e i r planar ground s t a t e and t o create dimeric s p e c i e s . Such s t r u c t u r e s are reminiscent o f the P e i e r l s s t a t e o f one-dimensional systems. A thorough c r y s t a l l o g r a p h i c study (4) of the unsubstituted d i t h i e n e - p l a t i n u m complex showed such dimers. Schematically, the d i s t o r t i o n i s shown i n the f o l l o w i n g diagram.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

396

EXTENDED

INTERACTIONS

BETWEEN

Pt-

I I

3.2

I

*^pt' 2.8

METAL

IONS

r 3.2

Pt-

I I ι

3.2

Pt

I I ι

3.2

Pt

3.2

> t ^

^ P t V

L

I

I

uniform

(no s i g n i f i c a n t Pt-Pt I n t e r a c t i o n s )

D i s t o r t e d (strong p a i r w i s e Pt-Pt I n t e r a c t i o n s )

We consider t h i s d i s t o r t i o n t o be one o f the main reasons f o r the absence o f organometallic t r a n s i t i o n metal complexes o f more than semiconductor q u a l i t i e s (with the BFD system above being the only exception so f a r ) . I t i s reasonably safe t o p r e d i c t that there can be no m e t a l l i c systems based on square planar l i n e a r l y overlapping organometallic t r a n s i t i o n metal complexes. One a l t e r n a t i v e s t r u c t u r e that avoids the s t e r i c problems i s based on the use o f polymeric a s s o c i a t e s o f the (C^H^T1) and (Cgt^EiOg type. For CpTl, a l i n e a r a l t e r n a t i n g Cp and T l arrangement i n the s o l i d i s known and p a r t i a l o x i d a t i o n of such a stack by TCNQ could l e a d t o T 1 ( I ) - T 1 ( I I I ) mixed valence c a t i o n s segregated from stacks o f TCNQ anions. Attempts i n our l a b o r a t o r y to prepare such systems have been u n s u c c e s s f u l . Our approach t o avoid the l i g a n d r e p u l s i o n problems and t o s t i l l u t i l i z e the advantages o f d i r e c t metal-metal bonding through d o r b i t a l s as i n KCP l i e s i n the c o n s t r u c t i o n o f b i f u n c t i o n a l organometallic s p e c i e s , which o f f e r s e v e r a l s u r p r i s i n g advantages. X

2

Pseudo-One-Dimensional Systems The p r i n c i p l e o f our approach l i e s i n the s y n t h e s i s o f two c l a s s e s o f π-ligand bridged b i m e t a l l i c planar t r a n s i t i o n metal complexes. The s i g n i f i c a n t p o i n t i n pursuing the s y n t h e s i s o f these complexes l i e s i n t h e i r unique p o s s i b i l i t y o f avoiding

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28.

MUELLER-WESTERHOFF AND HEINRICH

Molecular

Crystals

397

the P e i e r l s State. As a matter of f a c t , these compounds have to undergo a d i s t o r t i o n i n order to become m e t a l l i c ! ! Herein l i e s t h e i r p a r t i c u l a r promise. The displacement out of the compound plane should have a n e g l i g b l e i n f l u e n c e on the i n t r a m o l e c u l a r interactions· We are c u r r e n t l y l o o k i n g at two main c l a s s e s of complexes. In the f i r s t c l a s s , t r a n s i t i o n metals are complexed by r i g i d b i f u n c t i o n a l l i g a n d systems. The s y n t h e s i s of such l i g a n d s poses severe d i f f i c u l t i e s , which are understandable t o anyone

f a m i l i a r with the p r e p a r a t i o n of m a c r o c y c l i c compounds. However, we have r e c e n t l y succeeded i n preparing such compounds and complex systems - although these experiments do not as yet allow the p r e p a r a t i o n of l i g a n d s i n u s e f u l q u a n t i t i e s . The second approach i s to use l i g a n d exchange r e a c t i o n s to c o n s t r u c t complexes that e x h i b i t strong Intramolecular interactions. In t h i s approach, we s a c r i f i c e molecular r i g i d i t y f o r s y n t h e t i c s i m p l i c i t y . U s e f u l b r i d g i n g l i g a n d s which are incorporated i n t h i s s y n t h e s i s are of the 2 , 2 - b i p y r i m i d y l type to give us complexes of the f o l l o w i n g general type. f

Both methods produce p l a n a r systems c o n t a i n i n g two metal atoms. For the f o l l o w i n g d i s c u s s i o n , we wish to represent these systems by the n o t a t i o n

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS BETWEEN

398 p

p

t

M E T A L IONS

t

which s c h e m a t i c a l l y equals a s i d e view o f these systems. The e n t i r e complex c a r r i e s e i t h e r a p o s i t i v e o r negative p a r t i a l charge, depending on the counterlon i n v o l v e d . The Two Stacking Arrangements There a r e two p r i n c i p a l ways i n which complexes o f t h i s type together l i k e a l i n e a r stack o f counterions can be arranged i n r e g u l a r stacks i n molecular c r y s t a l s and s t i l l f u l l y u t i l i z e the s t a b i l i z a t i o n o f f e r e d by the overlap o f d o r b i t a l s . These two arrangements, termed by us the " s t e p " and " l a d d e r " stackings r e q u i r e q u i t e d i f f e r e n t stoechiometries i n t h e i r mixed valence state. As a s i d e l i n e , i a l s o a few n e u t r a l mixe our l a b o r a t o r y . These, o f course, form a completely d i f f e r e n t c l a s s o f compounds and w i l l be described s e p a r a t e l y .

Pt

Pt Pt

Pt STEP Arrangement — P t

Pt Pt

Pt

Pt

Pt

Pt

Pt

Pt

Pt

Pt

Pt

Pt

Pt

LADDER Arrangement

The STEP Arrangement Considering spacing and stoechiometry f o r a molecular c r y s t a l made up from stacks o f TCNQ r a d i c a l anions and b i f u n c t i o n a l mixed valence c a t i o n s i n the step arrangement, we have only one f r e e parameter that determines the r e l a t i o n between stoechiometry

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28.

MUELLER-WESTERHOFF

AND

HEiNRicH

Molecular

Crystals

399

and i n t r a m o l e c u l a r Pt-Pt d i s t a n c e s . In both stacks the i n t e r p l a n a r spacing i s 3.2 Â, the TCNQ chain spacing i s 4.2 Â, which has to be matched as i l l u s t r a t e d i n the f o l l o w i n g diagram. For a 1:1 stoechiometry (one Pt per TCNQ), the i n t r a m o l e c u l a r Pt-Pt d i s t a n c e has to be 7.8 A. For n o n - i n t e g r a l stoechiometries, the values i n the f o l l o w i n g t a b l e apply.

Pt-Pt Distance (intramol., a) 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Stacking Distance (c) 5.12 5.92 6.80 7.70 8.62 9.55 10.50

Pt/TCNQ R a t i o 1.64 1.50 1.24 1.10 0.98 0.88 0.80

In order to achieve s i g n i f i c a n t i n t e r m o l e c u l a r metal-metal overlap, the molecules have to d i s t o r t i n the same way as i t was found f o r the d i t h i e n e - P t complex dimer. However, s i n c e the dimers i n t h i s case of b i f u n c t i o n a l systems are connected

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

400

INTERACTIONS BETWEEN M E T A L IONS

by a conductive π-electron l i g a n d b r i d g e , they a r e i n essence s t a b i l i z i n g the m e t a l l i c s t a t e through t h i s d i s t o r t i o n . As i s evident from the above sketch, we now have systems with i n t r a m o l e c u l a r i n t e r a c t i o n s as w e l l as d i r e c t i n t e r m o l e c u l a r metal-metal c o u p l i n g . The step arrangement appears t o be s t a b l e and not subject t o f u r t h e r d i s t o r t i o n s . The LADDER Arrangement The a l t e r n a t i v e s t a c k i n g arrangement depicted above o f f e r s s i g n i f i c a n t advantages over the step arrangement, s i n c e the r e l a t i o n between packing requirements and i n t r a m o l e c u l a r Pt-Pt d i s t a n c e has been removed. B r i d g i n g l i g a n d s o f any s i z e can be accomodated as long as they do not deviate from p l a n a r i t y . As i s evident from the f o l l o w i n

-Pt-

-Pt-

I

4.2

-Pt-

"Λ

3.2 -Pt-

-Pt-

.it

4.2

-Pt-

-Pt-

the c o n f i n i n g l i m i t a t i o n i n the spacing o f complex u n i t s r e l a t i v e to TCNQ u n i t s leads t o a f i x e d stoechiometry o f (Pt-Pt)/TCNQ = 1.31, which f o r simple TCNQ s a l t s corresponds t o a Pt o x i d a t i o n s t a t e o f 2.66. Again, as i n the step arrangement, a d i s t o r t i o n o f the b i f u n c t i o n a l molecule i s needed to maximize the metal-metal overlap and t o achieve the m e t a l l i c s t a t e :

Pt 3.2

2.8

Pt

I 2.8

3.2

Pt

3.2

Pt

I

2.8

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28.

MUELLER-WESTERHOFF

AND

HEiNRicH

Molecular

Crystals

401

Although the advantages of t h i s arrangement are obvious, there i s one p o s s i b i l i t y which might lead to a p a i r w i s e " i n phase" d i s t o r t i o n as i l l u s t r a t e d below, which would lead to a much l e s s conductive s t a t e . In t h i s case, the p r o p e r t i e s w i l l be s i m i l a r to the mixed valence BFD-(TCNQ>2 system mentioned e a r l i e r . A p r i o r i , there are no c r i t e r i a to s a f e l y p r e d i c t which of the two d i s t o r t i o n s w i l l occur - the s t a b i l i z e d m e t a l l i c s t a t e or the r e a l P e i e r l s s t a t e . Since extended i n t e r a c t i o n s w i l l s t a b i l i z e the t o t a l system, we would expect the "out of phase" m e t a l l i c s t a t e to be e n e r g e t i c a l l y favored. I t i s important

Pt

11 Pt

Pt I

Pt I

λ Pt !

k to r e a l i z e that the two s t a t e s should not i n t e r c o n v e r t , s i n c e to do so, we would have to break a metal-metal bond, move the metal through the molecular plane and reform a metal-metal bond on the other s i d e . Thus, even i f m e t a l l i c conduction i s not achieved, we are e l i m i n a t i n g the m e t a l - i n s u l a t o r t r a n s i t i o n and are beginning to understand which i n f l u e n c e the d e v i a t i o n s from s t r i c t one-dimensionality have on the p r e s e r v a t i o n of r e g u l a r l y spaced t r a n s i t i o n metal complex systems. Literature Cited 1. 2.

3.

4.

See proceeding papers by K. Krogmann, A. N. Bloch and H. R. Z e l l e r f o r d e t a i l s and r e f e r e n c e on KCP. Coleman, L. Β., Cohen, M. J . , Sandman, D. J . , Yamagishi, F. G., G a r i t o , A. F. and Heeger, A. J . , S o l i d State Commun. (1973) 12, 1125 and F e r r a r i s , J . , Cowan, D. O., Walatka, J . and P e r l s t e i n , J . Η., J . Amer. Chem. Soc. (1973) 95, 948 d e s c r i b e TTF-TCNQ and g i v e r e f e r e n c e to e a r l i e r TCNQ work. Mueller-Westerhoff, U. T. and E i l b r a c h t , P., J . Amer. Chem. Soc. (1972) 94, 9272. Other BFD s a l t s were a l s o prepared by Cowan, D. O. and LeVanda, C., i b i d . (1972) 94, 9271. Browall, K. W., Bursh, T., I n t e r r a n t e , L. V., and Kasper, J . S., Inorg. Chem. (1972) 11, 1800.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

402

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

INDEX

Charge densities, nuclear quadrupole resonance derived Charge transfer Chlorines, anionic Chloro-bridged copper (II) dimers Chromium (III) dimers Chromium doped V 0 Chromium (III) ions Cobalt

A Absorption solid state techniques, single crystal Absorptivity, molar Acetonitrile A c i d activity, Lewis Addition reactions, oxidative Adduct structures, solvent Amine-halide complexes Amine-TaSo complexes 2-Aminoantnracene Anion bridged compounds Anion bridges Anionic chlorines Anisotropy Ising-like magnetic Antiferromagnetic coupling Antiferromagnetic interactions Antiferromagnetic salts Arsenic

343 277 301 309 70 328 325 38 25 2 27 239, 248 236 196 199,346,375 202 210 59,202 241 185 14

2

Complex compounds, square planar .... 350 Complexes directed synthesis of linear chain metal 314 square planar d 301 RMX 154 of tantalum sulfide, organic intercalation 23 Compounds metallic 3 semiconducting 3 TCNQ 350 transitional 3 Computation of field-swept epr spectra 40 Conduction band 372 Conductivity, electrical 341 Cooperative magnetic order 34 Cooperative transitions 36 Copper (II) 94 complexes 109,124 superexchange interactions in 108 dimers 76,84 chloro-bridged 131 Correlation of structural and magnetic properties 118 Corundum 17 Counterion tetraphenylborate 76 Coupling antiferromagnetic 59, 202 exchange 143 ferromagnetic 167 hyperfine 55 spin orbit 224 Covalency effects 159 Covalent energy 366 Cr(urea) -Cr(urea) pairs 59 Crystal chemistry of V 0 , high temperature 16 polarized reflectance spectra, single .... 301 structure of V ( urea ) I 54 studies, mixed291 Crystalline K P t ( C N ) , electrochemical oxidation of 367 Crystals magnetically dilute molecular 34 8

3

Β Band, parabolic free electron Band-structure energy Biscyclopentadienyl titanium (III), linear chain complexes from Bonding contacts, hydrogenBridges, anion Bridging angle hydroxo group ligand interactions through a units, symmetry of the Bronzes, perovskite

372 359 142 85 236 101 121 76 66 81 2

C Cations, shielding between dimeric Cesium metal triiodides Cesium nickel trihalides Chain compounds mixed valence metal partially oxidized single and mixed valency single valence metal transition metal Chain metal complexes Chains, extended metal Chains, linear optical properties of Chalcogenides extended interactions i n transition metal

3

29 304 196 131 94 16 164 12,194

77 182 188 246 333 234 239 236 314 276 194 164 23 1 12

e

3+

e

2

3+

3

6

2

3

4

403

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

404

EXTENDED

Crystals (Continued) pure stoichiometric single rhodium with square complexes, electronic spectra of C s M I transition metal iodides Cubic tungsten bronzes [Cu(diamine)OH]., [Cu(pyrazine) ( N Ô ) ] „ Cyanides Cyanoplatinates

2 306 254 182 2 112 132 298 347

3

2+

3

2

INTERACTIONS

BETWEEN

METAL

IONS

Excited state, delocalized Exciton-exciton transitions Exciton+magnon transition Exciton model Extended interactions in transition metal oxides and chalcogenides Extended metal chains

293 171 169 310

F Fe(bipyridine)CL Fe(morphylyldtc) I Fe ( phenanthroline ) C I , Fermi level Γ. electron density enhancement at the Ferrocene polymers Ferromagnetic coupling Ferromagnetic interaction Ferromagnetism

205 38 205 390 27 72 167 205 218

F i e l d dependent susceptibility Field-swept epr spectra for systems with large interelectronic interactions .... Fields, perturbation formulation to predict transition Formic acid Free electron band, parabolic Free electron model Frequency-shift perturbation Frohlich transition, Peierls-

224

2

D Davydov components of transitions Decylamine-TaS complex Delocalized excited state Delocalized molecular orbitals Destabilization of Pt 2.3+ Dibenzylamine Di-n-butyl form amide Diethylamine Diformylhydrazine Diimine ligands, ferrous chloride polymers of Dilute molecular crystals, magnetically.... Dimeric cations, shielding between Dimers, ehloro-bridged copper (II) Dimers, chromium ( III ) Dioxalatoplatinates Directed synthesis of linear chain metal complexes Directly interacting metal ions 242, 2

246 24 293 66 364 2 2 24 29 205 34 77 131 94 350 314 250

40 41 351 372 365 45 373

G Gas, one dimensional electron Glycinato complex Guaninium complex

Ε

1 276

358 95 130

Eigenfield equation 41 properties of 42 H Electrical conductivity 341 Electrical resistivities of C s N i I 192 Halide-bridged outer-sphere dimers 89 Electrochemical measurements 72 Halide ions 346,350 Electrochemical oxidation of K P t ( CN ) 367 Halide lattices, multisite magnetic Electron interactions in 51 -accepting ligands 323 Halides, hydrated ternary transition metal 238 band, parabolic free 372 Halogen bonding, metal183 density enhancement at the Fermi level 27 Heat capacity 196 gas, one dimensional 358 Heisenberg interaction, isotropic 36 Electroneutrality 350 Hemin, imidazolate bridged polymer of Electronic an iron (III) 221 and magnetic properties 142 Hexaflurophosphate salts 67 spectra 189 Hexaurea-metal ( III ) halide lattices 51 of crystals with square complexes 254 H i g h temperature crystal chemistry of structure of Krogmann salts 356 V O . 16 structure of planar d complexes 302 Hydrated ternary transition metal halides 238 transition 318 Hydrazines, substituted 27 Energy, covalent 366 Hydrogen atoms, hydroxo-bridge 119 Entropy change 196 Hydrogen-bonding contacts 85 E P R spectra for coupled C r ( u r e a ) Hydroxo group, bridging 121 Cr(urea) pairs 59 Hyperfine coupling 55 E P R spectra for systems with large I interelectronic interactions 40 Equation Imidazolate bridged polymer of an properties of eigenfield 42 iron (III) hemin 221 Van Vleck 95, 111 Increased interlayer spacings 31 E S R transitions, singlet-triplet 87 Infinite R M X linear chain complexes .... 142 Exchange coupling 143 Inorganic linear systems 393 Instability, Peierls 392 Exchange paths 36 a

2

4

2

H

8

0

0

3 +

3+

: {

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

405

INDEX

Insulator transition, metal16 Interacting metal ions 242 Interaction, ferromagnetic 205 Interaction, isotropic Heisenberg 36 Interactions antiferromagnetic 241 through a bridging ligand 66 in copper (II) complexes, superexchange 108 in C r ( u r e a ) X 54 in hexaurea-metal ( III ) halide lattices, multisite magnetic 51 interatomic 381 interdimer 128 intermolecular 321 metal-metal 66 in molecular crystals, intermolecular magnetic exchange 34 Ru-Ru 69 systems with large interelectroni in transition metal oxides and chalcogenides, extended 1 weak metal-metal 68 Interatomic interactions 381 Intercalation complexes of tantalum sulfide, organic 23 Intercalation of N-methylform amide 29 Interdimer interactions 128 Interelectronic interactions 40 Interligand repulsion 350 Intermolecular interactions 321 Intermolecular magnetic exchange interactions i n molecular crystals 34 Intermolecular separation 390 Intervalence transfer bands 73 Iodides, C s M I transition metal 182 Ionic states, transitions to 266 Ions, d 323 Ions, directly interacting metal 242, 250 Ions, halide 350 Iridium complexes, platinum and 381 Iron (III) bis(dithiocarbamates) 34 Iron (III) hemin, imidazolate bridged polymer of an 221 Ising-like anisotropy 202 Ising model, one-dimensional 199 Isomer shifts, Môssbauer 215 Isomorphism 294 Isotropic Heisenberg interaction 36 e

3

3

8

Κ K PdCl4 289 K PtBr 257 K PtCl 257,289 Kramers doublet 37 Kramers-Kronig analysis 280 Krogmann salts, electronic structure of .... 356 2

2

4

2

4

L Ladder arrangement Lattices, hexaurea-metal (III) halide .... Lewis acid activity Ligand bridged compounds, polymeric -bridged ruthenium complexes interaction through a bridging

400 51 328 71 68 66

Ligand (Continued) mixing, metal parameters, variation of Ligands, electron accepting Ligands, organic Linear chain complexes from biscyclopentadienyl titanium (III) Linear chain metal complexes, directed synthesis of Linear chain series R M X Linear chains, optical properties of Linear systems, inorganic Linear systems, organic Localized and collective states Lowest-energy allowed electronic transition 3

388 319 323 205 142 314 194 164 393 393 235 276

M

in hexaurea-metal (III) halide lattices, multisite 51 in molecular crystals, intermolecular 34 moments 207 order 37 cooperative 34 properties of chromium ( III ) dimers .... 94 properties, correlation of structural and 118 properties of C s M I transition metal iodides 182 susceptibility 152, 185 and thermal properties of [ (C H ) NH] M X · 2H 0 194 Magnetically dilute molecular crystals .... 34 Magnetization studies 207 Magnon assisted transitions 173 Magnus green salt 261, 285, 319 solid solutions 331 Magnus salts 276 Manganese ( II ) dimers 90 Metal chain compounds, mixed valence 246 chain compounds, single valence 239 chain compounds, transition 236 chains, extended 276 complexes, transition 394 compounds, polymeric, mixed-valence transition 66 -halogen bonding 183 -insulator transition 16 iodides, C s M I transition 182 ions, directly interacting 242, 250 -ligand mixing 388 -metal interactions 66 weak 68 oxides and chalcogenides, extended interactions i n transition 1 plane, tetra coordinated 350 -sulfur bonds 12 systems, triatomic 154 Metallic compounds 3 Metallic Pt salts, 1-D 346 Metalloporphyrins 221 Metals, 1-D 331 N-Methylformamide, intercalation of ... 29 3

3

3

3

2

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

406

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

N-Methylformanilide 29 1-Methyl-l-phenylhydrazine 31 Microspecular reflectance attachment ... 302 Mixed-crystal studies 291 M i x e d metal T i ( I I I ) compounds 143 Mixed valence metal chain compounds 234, 246, 331 Mixed valence solids 324 M n ( I I ) in tetrahydrofuran 150 Model, molecular orbital 120 Molar absoiptivity 309 Molar susceptibility 185 Molecular crystals 392 intermolecular magnetic exchange interactions i n 34 magnetically dilute 34 Molecular orbital model 120, 312 Molecular orbitals, delocalized 66 Molecular structure studies 213 Mossbauer isomer shifts 21 Môssbauer spectroscopy 31 Multisite magnetic interactions in hexaurea-metal(III) halide lattices 51

Ν Nickel compounds dimers dimethylglyoxime dimethylglyoximate trihalides, cesium Ν M R spectroscopy Nuclear quadrupole resonance derived charge densities

14 173 77 282 244 188 318

Paramagnetic species Partially oxidized chain compounds Peierls-Frohlich transition Peierls transition i n one-dimensional solids Perovskite bronzes Perovskites Perturbation formulation to predict transition fields Perturbation, frequency-shift Phenanthroline complexes π-exchange Piperidine Planar complex compounds, square Planar d complexes, electronic structure of Planar d systems Platinum chains, mixed-valence 8

Ι,Γ-Polyferrocene compounds Polymer of an iron (III) hemin, imidazolate bridged Polymeric, ligand-bridged compounds Polymeric, mixed-valence transition metal compounds Polymers of diimine ligands Polymers, ferrocene Polymers, ruthenium Polymorphism Properties of eigenfield equation Pseudo-one-dimensional systems Pt(en)Br Pt(en)Cl, Pt(CN) " Pt salts, 1-D metallic Pure stoichiometric single crystals Pyridine-TaS complex 4

Ο One-dimensional behavior electron gas Ising model solids systems Optical properties of linear chains Orbital overlap Order, magnetic Organic intercalation complexes of tantalum sulfide Organic ligands Organic linear systems Outer-sphere dimers, halide-bridged Overlap, orbital Oxalato complex Oxidation of crystalline K P t ( C N ) , electrochemical Oxidative addition reactions Oxyfluorides Oxidized chain compounds, partially 2

23 205 393 89 368 99

4

367 325 7 333

Ρ Packing Palladium (II) Parabolic free electron band Paramagnet

2

2

164 358 199 372 392 164 368 37

37,77 254 372 196

372 2 236 41 45 97 84 24 350

8

2

29

63 333 373

302 277 254 331 71 221 71 66 205 72 72 38 42 396 266 266 388 346 2 24

Q Quadrupole resonance derived charge densities, nuclear

29

R RbMnF Reactions, oxidative addition Reflectance attachment, microspecular... Reflectance spectra, single crystal polarized Reflectance spectrometer Reflection spectroscopy, specular Refluxing, formic acid Repulsion, interligand Resistivity Resonance derived charge densities, nuclear quadrupole R h ( C O ) acac Rhodium crystals Rhodium derivatives R M X systems 154, R u - R u interactions 3

2

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

171 325 302 301 379 278 351 350 9 29 301 306 328 164 69

407

INDEX

Ruthenium complexes, ligand-bridged ... Ruthenium polymers

68 72

Salt, Magnus green 285 solid solutions 331 Salts antiferromagnetic 185 1-D metallic Pt 346 electronic structure of Krogmann 356 hexaflurophosphate 67 platinum 336 TCNQ 394 S C F - X a - S W scattered wave method .... 382 Selenocyanate dimers 81 Semiconducting compounds 3 Shielding between oimeric cation Shift perturbation, frequency4 σ-exchange 84 Single crystal absorption techniques 301 Single crystals, pure stoichiometric 2 Single and mixed valency chain compounds 234 Single phase solid solutions 337 Single valence metal chain compounds .... 239 Singlet-triplet E S R transitions 87 Singlet-triplet splitting 101 Solid-state absorption 277 Solid state galvanic cell 367 Solid state interactions in P t ( C N ) " complexes 381 Solids, mixed valence 324 Solids, Peierls transition i n onedimensional 372 Solvent adduct structures 38 Spacings, increased interlayer 31 Spectra for coupled C r ( urea ) - C r ( urea ) pairs, E P R 59 of crystals with square complexes, electronic 254 electronic 189 isolated-molecule 277 Pt Br 264 single crystal polarized reflectance .... 301 Spectrometer, reflectance 379 Spectroscopic and magnetic properties of C s M I transition metal iodides 182 Spectroscopic methods 318 Spectroscopy, specular reflection 278 Specular reflection spectroscopy 278 Spin density 38 Spin Hamiltonian parameters 160 Spin orbit coupling 224 Spin wave 167 Splitting, singlet-triplet 101 Square complexes, spectra of crystals with 254 Square planar complex compounds . 3 0 1 , 3 5 0 Stacking arrangement 398 Step arrangement 398 Structural and magnetic properties, correlation of 118 Substituted hydrazines 27 4

6

2

G

2

3

3 +

n

6

3+

Sulfur bonds, metal12 Superconductivity 24 Superexchange 101,164 interactions i n copper (II) complexes 108 Susceptibility field dependent 224 and magnetization studies 207 molar 185 temperature variation of the inverse .... 126 Symmetry of bridging units 81 Synthesis of linear chain metal complexes, 314

Tantalum sulfide, organic intercalation complexes of

23

T C N Q compounds 350 T C N Q salts 394 Temperature crystal chemistry of V 0 , high 16 Temperature, transition 38 Temperature variation of the inverse susceptibility 126 Ternary transition metal halides, hydrated 238 Tetrachloroplatinate(II) 288 Tetrahydrofuran, M n ( I I ) in 150 Tetrakis(phenylisonitrile) cobalt (II) 328 Tetraphenylborate counterion 76 Thermal properties of [ ( C H ) N H ] MX · 2H 0 194 Thiocyanate dimers 81 Tight-binding model 359 Titanium (III) 142 Titanium-zinc complexes 145 Ti(urea) 63 Transfer bands, intervalence 73 Transition electronic 318 exciton+magnon 169 fields, perturbation formulation to predict 41 lowest-energy allowed electronic 276 metal chalcogenides 12 metal complexes 394 metal compounds 66, 236 metal-insulator 16 metal oxides and chalcogenides 1 in one-dimensional solids, Peierls 372 temperature 38 Transitions cooperative 36 exciton-exciton 171 to ionic states 266 magnon assisted 173 singlet-triplet E S R 87 Triatomic metal systems 154 Triplet E S R transitions, singlet87 Triplet splitting, singlet101 Tungsten bronzes, cubic 2 Tungsten (VI) oxide 2 Two-phase complexes 29 2

3

3

3

3

2

e

3 +

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

408

EXTENDED

2

BETWEEN

METAL

IONS

W

V Vanadium (IV) oxide Van der Waals forces Van Vleck equation Variation of ligand parameters V 0 , high temperature crystal chemistry of V ( u r e a ) I , crystal structure of

INTERACTIONS

5 23 95, 111 319

Wave, spin Weak metal-metal interactions

167 68

Ζ

: i

6

3

16 54

Zinc complex Zinc complexes, titanium-

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

143 145