Reactivity of Metal-Metal Bonds 9780841206243, 9780841208063, 0-8412-0624-4

Table of contents :
Title Page......Page 1
Copyright......Page 2
ACS Symposium Series......Page 3
FOREWORD......Page 4
PdftkEmptyString......Page 0
PREFACE......Page 5
1 Metal-Metal Multiple Bonds and Metal Clusters New Dimensions and New Opportunities in Transition Metal Chemistry......Page 6
Survey of M-M Bonds and Clusters......Page 7
Evidence for Multiple Bonds......Page 9
Strengths of M-M Bonds......Page 14
Comparison of Clusters and Multiple Bonds......Page 15
Relevance and Uses......Page 16
Literature Cited......Page 19
Coordination Numbers and Geometries......Page 22
Ligand Substitution Reactions (17)......Page 27
Stereochemical Lability......Page 28
Organometallic Reactions......Page 29
Direct Attack on the M-M Multiple Bond......Page 38
Conclusions......Page 39
Abstract......Page 40
Literature Cited......Page 41
Experimental......Page 45
Rectangular Clusters......Page 46
New Ternary Molybdenum Oxides......Page 53
Abstract......Page 63
Literature Cited......Page 64
4 Rhodium Carbonyl Cluster Chemistry Under High Pressures of Carbon Monoxide and Hydrogen Polynuclear Rhodium Carbonyl Complexes and Their Relationships to Mononuclear and Binuclear Rhodium Carbonyl Complexes1......Page 65
RESULTS AND DISCUSSION......Page 67
ACKNOWLEDGEMENT......Page 85
LITERATURE CITED......Page 86
5 Photochemistry of Metal-Metal-Bonded Transition Element Complexes......Page 88
Photochemistry of Dinuclear Complexes......Page 89
Photochemistry of Trinuclear Complexes......Page 105
Photochemistry of Tetranuclear Complexes......Page 107
Literature Cited......Page 111
6 Thermal and Photochemical Reactivity of H2FeRu3(CO)13 and Related Mixed-Metal Clusters......Page 114
Results and Discussion......Page 115
Summary and Outlook......Page 134
Literature Cited......Page 135
7 Kinetic Studies of Thermal Reactivities of Metal-Metal-Bonded Carbonyls......Page 138
Reactivities and Reaction Mechanisms of M2(CO)10 (M2 = Mn2, Tc2, MnRe, and Re2) and some Substituted Derivatives......Page 140
Reactivities and Reaction Mechanisms of Co2(CO)8 and Some Substituted Derivatives......Page 149
Reactivities and Reaction Mechanisms of Some Metal-Carbonyl Clusters......Page 150
Reactivities and the Strengths of Metal-Metal Bonds in Organometallic Compounds......Page 157
Recent Kinetic Studies of Some Co4 Clusters......Page 161
Summary......Page 164
Literature Cited......Page 166
8 Metal-Metal Bond Making and Breaking in Binuclear Complexes with Phosphine Bridging Ligands......Page 170
Metal-Metal Bond Formation and Rupture in Diphosphine Bridged Dinuclear Metal Complexes......Page 176
2-Diphenylphosphinopyridine, a Ligand for the Stepwise Construction of Binuclear Complexes......Page 179
Literature Cited......Page 187
Synthesis and Characterisation of Electron-Deficient Hydrido and Methyl Diplatinum Complexes......Page 189
Binuclear Reductive Elimination Reactions......Page 193
Catalysis of the Water Gas Shift Reaction......Page 196
Acknowledgments......Page 197
Literature Cited......Page 198
Diatomic Molecules of Transition Metal Elements.......Page 199
Binary metal carbonyl clusters......Page 202
Compounds containing multiple metal-to-metal bonds......Page 203
Literature cited......Page 206
11 Breaking Metal-Metal Multiple Bonds Their Use as Synthetic Starting Materials......Page 208
Cleavage Reactions by σ-Donors.......Page 211
Cleavage Reactions by π-Acceptor Ligands.......Page 213
Conclusions.......Page 218
Literature Cited......Page 219
12 The Novel Reactivity of the Molybdenum-Molybdenum Triple Bond in Cp2Mo2(CO)4......Page 222
General Reactivity Considerations......Page 223
Synthesis and Structure of Cp2M2(CO)4......Page 225
Nucleophilic Addition to ΜΞΜ Bonds......Page 228
Electrophilic Additions to ΜΞΜ Bonds......Page 244
Summary and Prognosis......Page 252
Acknowledgements......Page 254
Literature Cited......Page 255
DI-IRON AND DIRUTHENIUM METALLOCYCLES.......Page 259
DIMOLYBDENUM AND DITUNGSTEN METALLOCYCLES.......Page 267
LITERATURE CITED.......Page 270
14 The Coordination Chemistry of Metal Surfaces......Page 272
Experimental......Page 274
Discussion......Page 275
Acknowledgment......Page 294
Literature Cited......Page 295
15 New Approaches to the Chemistry of Di- and Trimetal Complexes Synthesis, Chemical Reactivity, and Structural Studies......Page 297
COMPOUNDS WITH BRIDGING ALKYLIDENE AND ALKYLIDYNE LIGANDS.......Page 298
REACTIONS OF LOW VALENT METAL COMPOUNDS WITH COMPLEXES CONTAINING METAL—METAL MULTIPLE BONDS.......Page 307
LITERATURE CITED.......Page 310
B......Page 312
C......Page 313
F......Page 316
I......Page 317
M......Page 318
P......Page 321
R......Page 322
V......Page 323
Z......Page 324

Citation preview

Reactivity of Metal-Metal Bonds Malcolm H. Chisholm, EDITOR Indiana

University

Based on a symposium sponsored by the Division of Inorganic Chemistry at the Second Chemical Congress of the North American Continent (180th A C S National Meeting), Las Vegas, Nevada, August 25-26, 1980.

ACS SYMPOSIUM SERIES 155

AMERICAN

CHEMICAL

SOCIETY

WASHINGTON, D. C. 1981

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Library of Congress CIP Data Reactivity of metal-metal bonds. (ACS symposium series; 155 ISSN 0097-6156) Includes bibliographies and index. 1. Metal-metal bonds—Congresses. 2. Reactivity (Chemistry) —Congresses. I. Chisholm, Malcolm. II. American Chemical Society. Division of Inorganic Chemistry. III. Series: American Chemical Society. A C S symposium series; 155. QD461.R38 ISBN 0-8412-0624-4

546'.3 AACR1

81-361 A S C M C 8 155 1-327 1981

Copyright © 1981 American Chemical Society A l l Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating new collective work, for resale, or for information storage and retrieval systems. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto.

PRINTED IN THE UNITED STATES OF AMERICA

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

ACS Symposium Series M. Joan Comstock, Series Editor

Advisory Board David L. Allara

James P.

Kenneth B . Bischoff

Marvin

Donald D .

Leon Petrakis

Dollberg

Lodge Margoshes

Robert E . Feeney

Theodore

Jack H a l p e r n

F . Sherwood

Brian

Dennis

W.

M.

Jeffrey

Harney Howe

James D . Idol, J r .

Provder Rowland

Schuetzle

Davis L . Temple, J r . Gunter

Zweig

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

FOREWORD T h e A C S S Y M P O S I U M SERIES was founded in 1 9 7 4 to provide

a medium for publishing symposia quickly in book form. T h e format of the Series parallels that of the continuing A D V A N C E S I N 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. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted.

Both reviews

and reports of research are acceptable since symposia may embrace both types of presentation.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

PREFACE

T

he chemistry of compounds containing metal-metal bonds is one of the most rapidly developing areas of modern coordination chemistry.

A t this time, virtually all the transition elements are known to form homoor heterodinuclear compounds with metal-metal

bonds that may be of

integral (1, 2, 3, 4) or fractional order (•£, 1|, 2i, 3|).

There are also large

classes of cluster compounds ranging from polynuclear metal carbonyls and other organometallics to polynuclear metal halides, oxides, and chalconides that contain delocalized metal-metal bonds.

M u c h of the initial

interest in these compound structures, bonding, and electronic properties.

However, there is now a

growing recognition that the reactivity patterns

associated

with these

compounds will provide a rich and fruitful field of research. This volume is based on the first ACS-sponsored symposium devoted to this topic. The authors, through their research interests and findings, present a survey of the types of reactivity that presently have been established.

These

include metal-metal bond rupture and formation, photolysis, substitution, oxidative-addition, reactions.

reductive-elimination, oligomerization,

and

template

N o doubt this group of reactions will be further elaborated

upon in the future and new modes of reactivity will be discovered.

Truly,

compounds containing metal-metal bonds offer new dimensions and opportunities for reactivity. I would like to thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, the A C S Division of Inorganic Chemistry, and the Union Carbide Corporation for support used to organize this symposium.

MALCOLM H. CHISHOLM

Department of Chemistry Indiana University Bloomington, Indiana 47405 November 21,

1980.

vii

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

financial

1 M e t a l - M e t a l Multiple Bonds and M e t a l Clusters New Dimensions and New Opportunities in Transition Metal Chemistry F. A. COTTON Department of Chemistry, Texas A&M University, College Station, TX 77843

The chemistry of th based on a sound foundation of broad, basic principles around the turn of the century as a result of the efforts of Alfred Werner, who was the first chemist to make sense out of an enormous body of experimental facts that had been painstakingly accumulated by himself, by the Danish chemist S. M. Jørgensen, and by a number of others. Working without the aid of any direct structural data, Werner was able to combine intuition, geometric reasoning and experiment to develop the central principle of his coordination theory, namely, that an ionized transition metal atom would always be surrounded by a set of neutral molecules (e.g., H O, NH ) and/or anions (e.g., CN , Cl , OH ) arranged in a geometrically well-defined pattern (square, tetrahedral, octahedral). This general conception dominated transition metal chemistry for more than six decades and even today is applicable to a vast amount of chemistry, albeit with the benefit of some further knowledge and insight, such as: (1) recognition that coordination numbers other than those emphasized by Werner (e.g., five-, seven- and eight-coordination) are important; (2) recognition that coordination geometry, especially for 5-, 7- and 8-coordination, is not always rigid; (3) much more detailed knowledge of equilibria, kinetics and spectra; (4) a quantum-mechanically sound understanding of metal-ligand bonding. -

2

-

-

3

There are, however, s e v e r a l aspects of contemporary t r a n s i t i o n metal chemistry whose e x i s t e n c e could not have been extrapolated from the Wernerian p r i n c i p l e s . Among these one could mention considerable areas of metal carbonyl type chemist r y , much of the current field of organometallic chemistry and, most unambiguously, the chemistry of compounds c o n t a i n i n g metal-metal bonds. Although Werner d e a l t e x t e n s i v e l y with p o l y n u c l e a r complexes, these were conceived simply as two or more mononuclear complexes u n i t e d only by the ligands they shared. 0097-615 6 / 81 / 0 1 5 5-0001 $ 0 5 . 0 0 / 0 © 1981 American Chemical Society

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

REACTIVITY

OF

M E T A L - M E T A L

BONDS

The n o t i o n of d i r e c t metal-metal bonds i s , to the best of my knowledge, t o t a l l y absent from the work of Werner, h i s coworkers or h i s d i r e c t f o l l o w e r s . I t i s a modern, "non-Wernerian" concept, and i t i s the subject of t h i s symposium. Even while Werner was s t i l l a l i v e , the f i r s t non-Wernerian compounds were discovered. In 1907 the compound " T a C ^ ^ l ^ O " was reported and i n 1913 reformulated, c o r r e c t l y , as TagCli^·7Η2θ (1) · During the t h i r d decade of t h i s century polynuclear h a l i d e compounds of molybdenum were discovered (2). The true s t r u c t u r e s of these compounds could not be i n f e r r e d by the a v a i l a b l e experimental methods and d e s p i t e t h e i r f a i l u r e to f o l l o w the general (Wernerian) patterns of behavior they a t t r a c t ­ ed l i t t l e a t t e n t i o n . Even when, i n 1935, C. Brosset (3) showed that t h e t u n g s t e n atoms i n [ ^ C l g ] ^ were very c l o s e together (ca. 2.5A) and again a decade l a t e r (4) showed that the a f o r e ­ mentioned lower h a l i d e octahedral M 0 5 groups wit interatomic d i s t a n c e i n m e t a l l i c molybdenum, no new research appeared i n the l i t e r a t u r e . The presence of Tag octahedra i n the lower tantalum c h l o r i d e was discovered i n 1950 and i t was e x p l i c i t l y concluded that metal-metal bonds were present i n t h i s and r e l a t e d compounds (5), but s t i l l , no one apparently was i n t e r e s t e d i n l o o k i n g f u r t h e r at such " c u r i o s i t i e s . " It was i n 1963, with yet another a c c i d e n t a l d i s c o v e r y , that of the [ R e C l ] i o n (6, 7) that the f i e l d of "metal atom c l u s t e r " chemistry r e a l l y had i t s b i r t h , since t h i s discovery provoked the f i r s t general d i s c u s s i o n of the existence and probable importance of the e n t i r e c l a s s of "metal atom c l u s t e r " compounds (6). I t was i n the Re^ c l u s t e r s a l s o that the f i r s t m u l t i p l e M-M bonds were e x p l i c i t l y and unequivocally recognized (6) and t h i s was soon followed by the d i s c o v e r y of the f i r s t quadruple bond (8) and then the f i r s t t r i p l e bond (9). Since these seminal d i s c o v e r i e s , the f i e l d of metal atom c l u s t e r s and M*-M m u l t i p l e bonds has a r i s e n , grown and f l o u r i s h e d . 3

o

3

3

1 2

Survey of M-M

Bonds and C l u s t e r s

The compounds under c o n s i d e r a t i o n here are r e s t r i c t e d to those i n which there are d i r e c t M

2

elec­

2.

Dinuclear

CHISHOLM

(ΜΞΜ) ä

ing

Transition

Compounds (3). M^

central

6 +

unit

molybdenum and tungsten the

formation

bonds tion of

of

a

(d-d , xz xz

d -d ) yz yz

rise

atoms to

Ilia,

ligands. atoms

to about

the

giving

ligands

below)

been

In

atoms

z 2

)

ΜΞΜ bond a r i s e s

atom

lie

which

in

are

a plane,

either

giving

staggered, the

metal

atom

lie

in

the M-M bond depends rise

to

staggered

a

plane,

and

the

on the requirements of

IVa,

eclipsed

IVb

(shown

situations.

Illb

1/

\

Μ = =

l

Μ

/

1/

Μ ΞΞΞΞΞ Μ

/\\

/\ IVa

IVb

coordination

the

structures

and

VI

below, in

the c o o r d i n a -

commonly 3 or 4, but examples

Ilia

formed

from

and a degenerate p a i r of π

these compounds,

is

for

fo

or intermediate

For

synthesized

depending upon the requirements of

each

conformation

(4).

-d

structures

Illb

Similarly,

bonded

z 2

each metal

state

or e c l i p s e d ,

recently

known. For c o o r d i n a t i o n number 3, the three

bonded to

ground

19

A large number of compounds c o n t a i n ­ have

σ bond ( d

5 and 6 are also

Complexes

in which a c e n t r a l

number of the metal

ligand

Metal

a

of

numbers

5 and 6,

W (0 CNEt ) Me

reveal

2

2

2

that

pentagonal

4

2

which

less

and W ( 0 C N M e ) , 2

f i v e equivalent

plane

are

(using

metal

2

2

6

common,

shown

in V

bonds can r e a d i l y be s,

ρ ,

ρ ,

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

d

and

20

REACTIVITY

(M=M) at

Compounds

present,

(6).

not common.

i n both M o ( 0 P r )

with

respect t o each metal

2

based pyramidal

VII 0 = OPr

number 5 i s , however,

(7) and M o ( 0 B u ) ( C 0 )

( 8 ) , which have,

t

8

2

6

atom, t r i g o n a l

bipyramidal

and square

s t r u c t u r e s , r e s p e c t i v e l y . See VII and VIII below.

VIII 1

BONDS

Compounds containing M=M bonds are,

The c o o r d i n a t i o n

seen

1

O FM E T A L - M E T A L

0 = OBu

1

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

Dinuclear

CHISHOLM

10+

(M-M) and

Compounds.

tungsten

nature

of

Transition

Single

bridging

ligands

21

Complexes

M-M bonds

the +5 o x i d a t i o n

in

the

Metal

state (9L

formed by molybdenum are dependent

For example,

on the

MO^CI-JQ

is

paramagnetic

(JO) and does not show s t r u c t u r a l evidence (Y\) f °

a

Oxygen

M-M bond.

bond of

ligands,

however,

Mo^X^iOPr )^

below

(]3)

compounds

with

number 6 i s tions

of

metal

which

Mo-to-Mo

seen

metal

inbetween

i n IX. d

orbitals

the s t r u c t u r e shown i n IX

of 2 . 7 3 Ä . The c o o r d i n a t i o n

which

have

from the i n t e r a c -

their

1

the sake

here to M-M bonds cases

examples

Cp Mo (C0) 2

atoms

brevity,

I

I

in

dimers),

6

d

of

(M-M)

n

d

electrons

the l a t t e r

state

X = Br or C I

have

r e s t r i c t e d my a t t e n t i o n

electrons

n

are used t o form M-M

(2.448(1)

which

M-M bonds

i n which not

to M-M bonding.

both

contain

+1 (formally

Well

(ΜΞΜ) (J_5) and

are Cp^Mo^iCO)^

they

molybdenum are d^-d^

of the ΕΑΝ r u l e and the observed

and 3 . 2 3 5 ( 1 )

t o have M-M t r i p l e and s i n g l e

having

compounds

contribute

number

but by c o n s i d e r a t i o n s

distances

bond.

order and have considered only the

( J 6 ^ compounds

oxidation

M-M s i n g l e

^X

( J 4 ) and d i n u c l e a r

order

the a v a i l a b l e

2

of

L'-0^

of i n t e g r a l

1

There a r e , however, d i n u c l e a r compounds

fractional

known

directed

OR

R

where a l l the a v a i l a b l e

bonds.

lobes

the bridgin

i s thus well suited f o r t h i s type of d - d

For

the M-M

characterizations

The M-M bond a r i s e s

X*^

M-M

have

distances

OR

all

to favor

(]2) as i s seen i n the recent s t r u c t u r a l 1

of

appear

r

8)

are

commonly considered

bonds, r e s p e c t i v e l y .

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

22

REACTIVITY

Ligand S u b s t i t u t i o n Mononuclear cally

labile

Cr(3+),

Co(2+) are ty

labile.

substitution

"inert" Group ples

are

classified

important

the

size of

as

kineti-

and undergo

in

of

determining

the

the

e.g.

and high

spin

differences in kinetic l a b i l i ­

ligands,

valence

satisfy

BONDS

ligand s u b s t i t u t i o n r e a c t i o n s :

are i n e r t , while Cr(2+)

number

which

the

electrons

ΕΑΝ

rates

rule

Other

of

formal

ligand positive

on the metal.

are

Many

substitutionally

substitutio

6 transition of the

be broadly

These dramatic

are

and the

complexes

can

r a t i o n a l i z e d by ligand f i e l d c o n s i d e r a t i o n s .

which

charge

M E T A L - M E T A L

(17)

i n e r t toward and Pt(4+)

are e a s i l y

factors

compounds

or

Co(3+)

Reactions

O F

meta

latter

carbony

compound

provid

goo

phenomenon. On the other hand, the c o o r d i n a -

t i v e l y unsaturated square planar complexes of the group 8 t r a n s i ­ tion

elements

labilizing which is

allow

All

of

these

of

the

planar

control

in

considerations

position

(S 2).

and

to the group phenomenon,

octahedral

isolation

The

N

trans-effect

the

2

4

+

with

4

excess

of

carry

and tungsten.

(M=M)^

Kinetically,

addition

square

molybdenum

Thus, Mo R (PMe^) PMe^

for

kinetic

around the

(6).

in the trans

reactions

of

complexes isomers

of

and square planar compounds.

chemistry

ed

association

substitution,

documented

can

ligand

a group

undergoing

octahedral

tions

by

e f f e c t of

is

well

and

react

and

they

(ΜΞΜ)^

are

compounds PMe^

to

over Ligand

than

(ΜΞΜ) w i l l a

the d i n u c l e a r

substitution

reac­

moieties are well document­

+

slower

give

into

the

nmr

time-scale.

not exchange coordinated

single

a d i f f e r e n t phosphine w i l l

PMe^

resonance,

lead to r a p i d

but

scrambling

on the s y n t h e t i c t i m e - s c a l e . In

the

reagents

LiR

r e t e n t i o n of formed

which

reaction (2

between

equiv),

configuration then

anti-W Cl (NEt ) 2

substitution 08).

isomerizes

to

2

2

of

Kinetically a mixture

4

and

alkyllithium

Cl-by-R

occurs

with

anti-W R (NEt ) 2

of

anti

2

and

rotamers, with the l a t t e r being the favored rotamer. This t u t i o n r e a c t i o n can be viewed as an example of an S 2 f

2

4

gauche substi­

process

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

is

in

2.

Dinuclear

CHISHOLM

Transition

which the new bond i s square the

plane

(^9).

cogging

rotation Ligand

formed as

of

the

isomerization.

ligand

substitution

(CH SiMe ) 2

3

readily

and

2

(CH SiMe ) 2

3

HNMe

of

2

give

which

2

1,1

whereas with C 0

formed

and

2

while

the

these

3

to

(20).

anti=—^

control seen

in

in the

(ΜΞΜ) r e a c t

4

1,2-Mo (NMe ) 2

do

not

2

l,T-Mo (NMe )(0 CNMe )(CH SiMe ) , 2

2

(1

2

the

react.

)

2

2

isomerize

1,2-Mo (0Bu ) (CH SiMe ) , 25°C),

not

is

and

atmos,

2

mol than

2

1,1-

once

barrier

kinetic

center

because

Additio

yields

4

Kcal

1 ,2-Mo Br (CH SiMe )

to

2

24

faster

example

of

arises

an energy

= ca.

t

dimolybdenum

respectively,

4 >

(21_).

tively,

a

produces

ft

broken w i t h i n a

rotamer

kinetically

Another

at

Hexane s o l u t i o n s

LiNMe

(E

thus

23

the o l d bond i s

groups

9

bond

is

gauche

with

NR

M=M

substitution

following.

Complexes

Formation of the anti

effect

about

Metal

2

2

2

Clearly,

dinuclear

a

3

rich

compounds

4

substitution and

3

to

yields

1,2-isomer

chemistry

remains

respec

4

1,1-isomer

be

does

surrounds

explored

and

exploited.

Stereochemical Three

types

(1)

rotations

at

each

change

Lability of

stereochemical

about M-M bonds,

metal

center

and

(2) c i s ^ .

(3)

have been observed:

>trans

isomerizations

bridge^^terminal

ligand

ex­

processes.

Rotations time-scale) bond.

In

have

the

pounds

ΜΞΜ bond

the

compounds c o n t a i n i n g σ ^ by

the and

rotation

of

is

than

ca.

steric

neutral

rotation

properties 2

15 Kcal

9 Kcal

2

the 3

2

and

4

to

be

ligands. show

E

which only

The com­ for

A c t

M-M

2

conformers

3

have

4

not been frozen

(22).

compounds M o ( 0 R ) L , nitrogen donor

2

moiety,

m o l " ^ . For M o M e ( C H S i M e ) , the b a r r i e r

mol'^

2

2

+

appears

of

(on the nmr

would rupture the δ (ΜΞΜ)^

1,2-Mo (NMe ) (CH SiMe )

out on the nmr t i m e - s c a l e The

are not observed

the c e n t r a l

configuration,

2

1,1-

less

about

and are not expected since t h i s

restricted

a

lability

6

2

where R = a l k y l

ligand,

or SiMe

contain three oxygen

3

and L =

atoms

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

and

24

R E A C T I V I T Y

one

nitrogen

25).

Thus,

which

is

the OR

atom coordinated

M E T A L - M E T A L

each molybdenum

atom

B O N D S

(23,24,-

there are a p a i r of mutually trans OR ligands and one

trans

to the N atom. At low temperatures in toluene-dg,

nmr spectra are c o n s i s t e n t with the expected 2:1 r a t i o of ligands

the

OR

described

ligands

Mo^OPr )g(NCNMe ), an asymmetric

ca.

there i s +35°C,

ratio

is

exchange thus,

the

the

temperature,

all

time-scale.

For

nmr are

different

Mo^t p-NCNMe^) moiety (26). reveals

four

types

because

Here the low

of

OR ligands

r a t i o 1:1:2:2. Upon r a i s i n g the tempera-

a collaps

consistent

three OPr with

the

resonances

view

that

in

cis

the

integral

^ ^ trans

OPr

1

r a p i d at one molybdenum, but not at the other. These

processes

parallel

nate

atoms

and 220 MHz,

3:2:1,

exchange

molybdenum

spectrum

in the expected i n t e g r a l ture,

on

central

limiting

Upon r a i s i n g

equivalent

the

2

temperature

above.

become

1

of

to

O F

have

been

shown

the common l a b i l i t y

mononuclear

complexes

(27).

to

be

intramolecular

associated For

the

and

with f i v e c o o r d i -

dinuclear

compounds,

however, the f i f t h c o o r d i n a t i o n s i t e i s the other metal atom. The (M=M)

compounds (8),

Mo^OPr ^

which

contain

b r i d g e ^ = ^ = terminal larly,

the

change

between

group

compounds

(7)

and

bridging

OR

ligands,

2

bridging (j>).

Organometal1ic

Reactions

Mo^OBu^U-CO) show

rapid

exchange on the nmr t i m e - s c a l e .

W Me (0 CNEt )

nmr t i m e - s c a l e

Tolman (28)

(M=M)

1

2

and

2

2

4

terminal

and W ( 0 C N M e ) 2

2

2

carbamato

6

Simi-

show

ligands

has suggested that a l l commonly o c c u r r i n g

on

exthe

organ-

ometal l i e r e a c t i o n s , i n c l u d i n g those that are important in c a t a l ysis,

can

be c l a s s i f i e d

microscopic tion,

(2)

reverse:

Lewis

(1)

by

f i v e named r e a c t i o n s ,

Lewis

acid a s s o c i a t i o n

base

and d i s s o c i a t i o n ,

and d e i n s e r t i o n (ligand migration r e a c t i o n s ) , tion

and

reductive

reductive

elimination,

decoupling.

With

the

each with

association

and

(5)

(4)

of

dissocia-

(3)

insertion

oxidative addi-

oxidative

exception

its

and

the

coupling simple

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

and Lewis

2.

Dinuclear

C H I S H O L M

acid

Transition

Metal

association-dissociation

Complexes

reactions,

25

we have

studied

exam-

p l e s of a l l of the aforementioned r e a c t i o n s .

Lewis

Base

Association

and

Dissociation.

alkoxides

M^iOR)^ r e v e r s i b l y add donor

compounds

(23,24):

the

bulkiness

of

the R

addition reactions, c.f.

(23,24)

position and

L.

of In

these

reversible

Mo = 2.242(1) Ä i n

Mo (OSiMe ) (HNMe ) ,

do

18

not

attain

However,

for

an

compounds

atoms

have

(CO)^

compounds

metal-ligand order,

three

one,

reversible

the

Π6)

the metal

atoms

i n which the metal

of

Mo-to-Mo

found

Reactions.

M (0R)^

compounds

2

compounds

(29).

An

catalysis

by t r a c e s

and

C0

the

Cp M ~ 2

2

two new

for

distances Cp Mo (C0) 2

2

of and

4

of

example

of

a

facile

is

in

the

reac­

seen

which

2

These r e a c t i o n s

by a d i r e c t i n s e r t i o n mechanism ing

bonds

with Mo-to-

6

of e l e c t r o n s , e.g.

insertion-deinsertion reaction

between 2

since

the formation

c.f.

3.235(1 ) Ä

Insertion-Deinsertion

2

3

respectively.

2

(0 C0R)

base

occur only with a reduction in M-M bond

to

and

2

2

ΜΞΜ

shell

(M = Mo and W),

Π5)

Cp Mo (C0)^,

tions

2

containing

bonds w i l l

from

2.448(1) Ä 2

6

a completed valence

Lewis

are e s s e n t i a l l y unchanged, 2

3

M^iOR)^

dependent on

= 2.222(1) Ä i n Mo (0CH CMe ) 2

dinuclear

to give

equilibrium is

the M-M distances

Mo-to-Mo

ligands

The

were

give

shown

M (0R) ~ 2

4

to proceed

( i . e . not by a mechanism i n v o l v ­

alcohols)

with energies of

activation

of not greater than 20 Kcal m o l " . 1

Oxidative-Addition studying of

oxidative

ΡΓ 00ΡΓ ί

the

leads

ί

halogens

Mo (0Pr ) X n

2

6

additions, two

to

4

additions to

Cl , 2

(M-M)

Br

is

Mo^OPr ^ and

I

change

achieved.

Kirkpatrick

to M o ^ O P r ) ^ (M=M)

1

2

Chuck 1

(7).

proceeds

2

where X = CI,

a stepwise

one,

Reactions.

in

Br M-M

A large

or

has

(ΜΞΜ) (30).

been

Addition

A d d i t i o n of each of

to

give

I.

In

these

bond order, number of

the

compounds oxidative-

from three

related

to

additions

have been noted, but await d e t a i l e d s t r u c t u r a l c h a r a c t e r i z a t i o n s .

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

26

R E A C T I V I T Y

The

formation

of

W^tNMe^)^ and P^OH addition: view

of

W^(p - H ) ( 0 P r ) ^

(excess)

W^OPr )^ the

the

reaction

i[W^(μ -Η) (OPr )^] skeleton

molecule

is

with

and long

(3.407(1) ft) W-to-W distances

is

Curtis

and coworkers

of

between

alternating

short

their

The

(2.446(1) Ä)

c o n s i s t e n t with the pres-

have also

in

below.

respectively.

documented a number of

studies

of the r e a c t i v i t y

Cp Mo (C0) . 2

2

4

Reductive ries

(32^)

reactions

2

shown

ence of W-to-W double and non-bonding d i s t a n c e s ,

oxidative-addition

B O N D S

(31_). An ORTEP

1

W^y-H^O-^

centrosymmetric

M E T A L - M E T A L

can also be viewed as an o x i d a t i v e -

PrVrl -

central

in

n

2

1

O F

of

El imi n a t i o n s .

1,2-M R (NMe ) 2

2

2

between 1 , 2 - M C l ( N M e ) 2

equiv),

2

2

compounds

4

pathways

affords of

particularly

alkyl

the

4

(33,34)

i - P r , n-Bu,

opportunity of

groups

interesting

clear σ-alkyl

complexes

coordinated comparisons

of

the

Figure 1

structure (34).

This

of

an extensive

from

the

the view

to can

2

emphasizes

the

dimetal be

made

(2 and

3

decomposition centers. with

elements,

Some

mononu­

which have

(35).

Mo Et (NMe ) 2

LiR

2

studying

se-

reaction

sec-Bu, t - B u , CH CMe

transition

been the subject of much i n v e s t i g a t i o n The

of

(ΜΞΜ) and a l k y l l i t h i u m reagents,

where R = CH^, E t ,

Ch^SiMe^,

The synthesis

2

4

the

molecule

is

virtual

C

?

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

shown

in

axis

of

2.

Dinuclear

C H I S H O L M

Transition

symmetry which

exists

the

were

hydrogens

Metal

f o r gauche located

of

the

C-H

bonds.

Two

27

1^-M^X^tNMe^J^

and

stereoview of the molecule i s

Complexes

refined,

and

is are

limits shown

which, C-C

is

the

important

points

can

bond,

from

indicate (36^).

along group

relatively

is

short

e-H-to-Mo

atoms

M-M

axis

are

be

keeping

on the d i s t a l

"what of

the

about Mo-N, to

2.4 Ä,

the

ethyl

seen:

groups

(1) the

is,

atom

Mo-C as

distances

directly

bonded.

provide

β and γ

Thus,

the

are

as

atoms ethyl

3.25(5) Ä,

conformation about the

which

this as

methyl

Mo^N^C^

is

in

and

hydrogens

are each

introduces

groups,

atoms

but

and

only

a

the

little

proximal methyl hydrogens and

rigid,

CH---Mo

groups the

distances

readily

answered:

but

allowing

rotations

atom of

in

is

distances of

between the methyl

molybdenum

to

This

molybdenum

question

unit

The methyl

from

shown i n Figure 3. The

natural

Figure 4, the

planar,

atom

the ΜΟΞΜΟ bond. This, leads one

closest This

are

plane.

and C-C bonds produces C H — M o

shown

ligand

which

hydrogen

the

across

type?"

central

to

one

involving

are the

this

within

two 3 -hydrogen

units,

within

between

are the distances

molecules

and

distances

other molybdenum atoms wonder,

a

th

CH—Mo

hydrogen

to

The

contained

distances

longer

molybdenum

(2) The MoNC

the

shortest

the

the

together with the staggered

interaction

methyl

Newman p r o j e c t i o n below,

σ-bonded.

taken

aligned

Figure 2

of experimental e r r o r , p e r f e c t l y staggered. in

equidistant

ligand

in

All

shown which shows the o r i e n t a t i o n s

conformation about the C-C bonds of the ethyl the

compounds.

the

to

which

ethyl

2.3

protons it

is

ligands

two molybdenum atoms

Η — M o distances across the ΜΟΞΜΟ bond are the s h o r t e s t .

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

of not

thus

and the

REACTIVITY

O F

M E T A L - M E T A L

BONDS

Figure 1. An ORTEP view of the Mo Et (NMe ) molecule emphasizing the vir­ tual C axis of symmetry. Pertinent structural parameters are Mo Mo (M=M) = 2.206(1) A, Mo—N = 1.96 A (av), Mo—C = 2.18 A (av), Mo—Mo—N angle = 103° (av) and Mo—Mo—C angle = 101° (av). 2

2

2 h

2

Figure 2. Stereoview of the Μο Εί (ΝΜβ )ι, molecule looking down the bond. This stick view of the molecule emphasizes the positions of all the atoms. 2

2

2

Mo—Mo hydrogen

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

CHISHOLM

Dinuclear

Transition

Metal

Complexes

H(^0)

Mo(l)

Mo(2)

Figure 3. A line drawing of one Mo NC> fragment showing the Mo HC distances that arise to the two hydrogen atoms that are confined in the plane of the Mo—NC, unit. (Mo(2) H(44) 2.77 A; Mo(2) H(40) 3.18 A; Mo(l) H(40)2.96 A) 2

Figure 4. A line drawing of one Mo,-ethyl fragment showing the short CH Mo distance that arises from rotations about Mo—C and C—C bonds (Mo(l) Η 2.36 A;Mo(2) Η 2.97 A)

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

30

REACTIVITY

Although

the

below +100°C, reductive and

Bu.

(37)

2

elimination

is

type

fairly

addition

of

compounds

C0

-

2

containing

2

2

products.

formed

and

cross-over

3

2

x

give

the

and Mo (PhN Ph)

4

2

3

2

has

been

shown

by

of two

Pr

alkyl e.g.

butane.

The

bonded

as the molybdenum

CH =CD

2

and CD CH D

the

appropriate

2

use

cause

quadruply

(38)

only

3

will

chemistry,

M-M

4

stable

when R = E t ,

1-butene +

When R = CH CD ,

it

substrates

alkenes,

+

BONDS

thermally

dialkylmononuclear

PhNNNHPh

Mo (0 CNMe ) 2

and

[(PPh ) Pt]

and

2

are

a number of

alkanes

common i n

2

compounds

4

M E T A L - M E T A L

reductive disproportionation

n

3

2

of

of

(PPh ) Pt(Bu )

are

2

the a d d i t i o n of

This

ligands

Mo R (NMe )

O F

of

3

2

experiments

lecular. The r e a c t i o n between M o ( C H C H ) ( N M e ) 2

dg

has

been

build-up be

of

seen.

followed an

The

at

intermediate, spectrum

molecule

has

the

mid-point

of

the

2

4

2

2

and C0

4

nmr

in t o l u e n e -

2

spectroscopy.

namely M o E t ( N M e ) ( 0 C N M e ) 2

C

Mo-Mo

axis

2

2

to

of

bond,

2

this

2

2

2

is

found

following

esting

reaction

possibilities.

involved

in

the

The

slow

2

2

2

2

"Mo (NMe ) (0 CNMe ) 2

2

2

2

2

C0 -

Alternatively,

into

the Mo-NMe

2

2

Mo Et (0 CNMe ) , 2

pound

2

2

2

4

2

2

2

2

step

2

the

in

c.f.

(5)

2

4

reactive

slow of

2

the of

through for

poses two i n t e r ­

ethane

could

and

be

ethylene

which would then generate a species

2

highly

n

bonds

W Me (0 CNEt ) 2

2

reductive-elimination

from M o E t ( N M e ) ( 0 C N M e ) 2

2

that

(29)

2

2

is

the forma­

t i o n of the intermediate M o E t ( N M e ) ( 0 C N M e ) 2

can

2

with the view

as

pathway

The

intermediate

symmetry passing

i.e.

The r e a c t i o n

2

2

e n t i r e l y consistent

a virtual

Μο (0Βυ^) (0 00Βυ^) ·

3

by

corresponding

shown i n F i g u r e 5 and i s the

2

-30°C

step

to

f u r t h e r r e a c t i o n with

could

involve

Mo Et (NMe ) (0 CNMe ) 2

2

the

which

2

2

2

2

structurally

then

rapidly

C0 2

2

to

insertion give

say,

c h a r a c t e r i z e d com­

eliminates

ethane

and

ethylene.

Mo (NMe ) 2

2

6

+ ArNNNHAr

(excess)

-

Mo (NMe ) (ArN Ar) 2

2

4

3

2

(ΜΞΜ) + Me NH 2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

(1)

2.

Dinuclear

CHISHOLM

Mo R (NMe ) 2

2

2

Transition

+ ArNNNHAr

4

Metal

(excess)

-

Mo R (NMe ) (ArN Ar) 2

Mo R (NMe ) 2

2

2

+ ArNNNHAr

4

Mo (ArN Ar) 2

We

have

pathways,

based

triazenes (3)

3

has

relating

a

to

inclined

distinguish

toward

with

the

between

favoring

reactions

the

2

occurs.

When

structure

R =

of

Et

the

is

and

former

observed

sonable

to

the M o R ( N M e ) ( A r N A r )

suppose 2

that

2

2

elimination

of

alkane

unsaturated

molecules

reaction 2

3

C

2

2

generating

Mo (NMe ) (ArN Ar) 2

2

3

2

time, nothing

intramolecular,

2

axis

2

of

It

which

2

3

the

4

2

symmmetry

i s not unrea­ initial

forma­ undergo

coordinatively react

rapidly

d e f i n i t i v e can be said concerning the

quantitative

it

is

and i r r e v e r s i b l e - no scrambling

of

i s observed.

occurs

by

a

rate

determining

c y c l o m e t a l l a t i o n r e a c t i o n across by the

close

bond i s

9

CH—Mo

distance

formed, e l i m i n a t i o n of the

be

a

W R (NMe L ?

2

(M=M).

A p l a u s i b l e i n t e r p r e t a t i o n of these r e s u l t s

ingly,

3

Mo Me (NMe ) ~

intimate mechanism of e l i m i n a t i o n beyond noting the f a c t s :

then

(2)

reaction

compounds, which then

with the excess t r i a z i n e to give M o ( A r N A r )

tion

with

shown in Figure 6. The mole­

3 proceeds v i a

and alkene 2

Bu ,

compound

imposed

tion

labels

these

2 and 3 above. The d i f f e r e n c e between

crystallographically

At t h i s

(3)

2

the two 4-coordinate molybdenum atoms.

of

(2)

2

-

where Ar = p - t o l y l ,

2

(ΜΞΜ) + Me NH

2

+ alkane + 1-alkene + HNMe

analogy

The molecular

(ArN Ar) ,

3

unable

are

1,

2

solel

reaction

occurs.

(M=M)

4

been

we

in

2

(excess)

on the

rests

3

cule

yet

shown

CH CMe , 2

as

though

pathway

and

3

2

31

Complexes

9

elimination slower

of

and e i t h e r

C0

shown i n Figure 4.

or

is

akin

to

a

suggested

Once the Mo-H

alkane i s r a p i d . Rather i n t e r e s t ­

step: 9

which

the M-M bond. This i s

alkene

final

step

i s that e l i m i n a ­

from the d i n u c l e a r center may analogous

reactions

triazines yield

involving

1 mole of

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

alkane

32

R E A C T I V I T Y

O F

M E T A L - M E T A L

B O N D S

Figure 5. H NMR spectrum recorded at —30°C, 100 MHz, during the reaction between Mo2Et (NMe ) and C0 , showing the formation of the intermediate Mo Et2(NMe )2(O CNMe ) , along with the products C H C H , and Mo (0 CNMe ) . The solvent is toluene-d and the protio impurities are indicated by (*). The signal noted by (**) corresponds to Mo^O^CNMe^^ which, because of its very low solubility, is mostly precipitated as it is formed. l

2

J

2

z k

li

2 h

2

g 2

2

h

2

6

2

2

8

Figure 6. An ORTEP view of the Mo>Me (NMe ) (C H N C H ) molecule (C,H = p-tolyl) viewed down the Mo=Mo bond emphasizing the C axis of molecular symmetry which relates the two 4-coordinate molybdenum atoms. The Mo Mo distance is 2.175(1) A and Mo—C = 2.193(4) A. 2

S

2 2

7

8

3

7

H 2

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2.

Dinuclear

CHISHOLM

and as

yet

Transition

Metal

Complexes

uncharacterized tungsten

33

compounds

which

r e t a i n the

elements of the alkene. It tive

is

clear

from t h i s

brief

elimination

sequences

have

from the

presence of

summary that d i n u c l e a r reduc­ pathways

the dimetal

center,

a c c e s s i b l e to mononuclear complexes

Oxi dati ve-Coupli ng oxidative-coupling dimetal

center

and

and

are

which in

addition

Reducti ve-Decoupli ng.

in

uniquely to

those

(39).

reductive-decoupling

seen

arise

the work

of

of

Stone,

Examples

ligands

of

about

a

Knox

and t h e i r

or

functional

coworkers (40) and are

D i r e c t Attack on the M-M M u l t i p l e Bond A

number

groups

have

Cp Mo (C0) 2

2

enes In

small

been to

4

(41), all

of

found

give

allenes

of

4-electron

to

add

molecules

across

the

M-M

triple

bond

in

adducts C p M o ( C 0 ) ( u n ) , where un = a c e t y l ­ 2

(42),

these donors

unsaturated

4

cyanamides

adducts, to

2

the

(43)

and thioketones

unsaturated

the dimetal

center,

fragments

(44).

act

as

thus reducing the M-M

bond order from three to one. The react

same group of

with

Mo (0Pr )g, n

2

characterized

by

a

observed

(45)

for

however,

with

a

unsaturated molecules have been found to but

full

as yet none of the adducts X-ray

study.

Mo^OPr ) (NCNMe ) 1

6

mode

of

2

addition

The are

spectroscopic

entirely

directly

has been data

consistent,

analogous

to

that

observed above. Carbon monoxide has been shown the

ΜΞΜ bond of

VIII

before.

carbene-like class

of

Similar

We

2

have

addition

reactions

additions

reported

Mo (0Bu ) t

of

6

to

(8) to r e v e r s i b l y add across

give M o ^ O B u * ) ^ -CO)

previously

speculated

to

a M=M

bond

could

that

are

common

CO across

to

be

(46) one

dinuclear

Pd-Pd and Pt-Pt

(M=M). See that of

this

general

compounds.

bonds have been

(47).

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

34

REACTIVITY

O F

M E T A L - M E T A L

BONDS

A d d i t i o n of n i t r i c oxide to M^iOR)^ compounds gives [M(0R) ~ 3

N0]

compounds. The l a t t e r do not c o n t a i n M-M bonds:

2

t u r e of

[Mo(0Pr ) N0] 3

shown i n VII, a formal

M-M bond i n the

as

3

compounds

2

evidence

bonds

above)

by the

that

(2

equiv)

that

ligands

the such

WiOBu^)^-

also

and

2

have found that compounds c o n t a i n

cleaved

Mo (0Bu ) 2

6

the

addition

isocyanides.

(M=M)

t

by

reacts

are

eliminated azides to

2

from

to

give

both

2

the

certain

with each of molecular

and

t r i p l e bond i s

of

Very r e c e n t l y we have

a r y l azides ( >4 equiv) t Mo(NAr) (0Bu ) , r e s p e c t i v e l y . In

metal-metal

Mo(0) ?

reactions,

cleaved and the elements of 2Bu^0

metal

center.

Though

the

addition

of

low valent mononuclear t r a n s i t i o n metal complexes

is

known to g i v e

of

the ΜΞΜ bond seen in these r e a c t i o n s , once again,

the

In

addition

fact

has been s t r u c t u r a l l y confirmed f o r

are

(51)

the

aryl

a l s o supported ( i n

are r e a d i l y cleaved by donor

unsaturated molecules such as

oxygen t (OBu )

1

(49).

M=M

found

is

cited

Walton and coworkers (50) ing

Mo^OPr )^

(ΜΞΝ->0). The absence of any d i r e c t

t r i p l e bonds

This

struc-

has been replaced by

1

[M(0R) N0]

pyridine.

r e l a t e d to that of

the

a Mo-to-Mo d i s t a n c e of 3.325(1) Ä (48).

bridged dimers

(NOMpy)

closely

the ΜΞΜ bond i n M o ^ O P r ) ^

structural

alkoxy

2

but has

sense,

two m e t a l - l i g a n d

to

is

1

a r y l i m i d o compounds

(52h

the ease of

cleavage

emphasizes

l a b i l i t y of the d i n u c l e a r center toward a d d i t i o n r e a c t i o n s .

Conclusions The

time

reactivity tial

for

catalytic

of

is

ripe

for

dinuclear

cyclic

has

of

by M u e t t e r t i e s ,

tively

hydrogenated to alkenes

rate

acetylene

et

determining adducts

metal

reactions,

step

al.

(53),

compounds. as

is

?

that

involves d

9

9

CO

the

The poten­

It

for

has been

can be s e l e c ­ by C p ^ o ^ C O ) ^ :

dissociation

(2) We

in

required

(1)

alkynes

(cj_s 2H-addition)

Cp Mo (C0) (R C ). ?

developments

already been r e a l i z e d .

shown,

the

exciting

transition

sequences

reactions,

truly

have

from

found

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

the that

2.

Dinuclear

CHISHOLM

μ-Η) (0Ρτ )

will

Ί

2

1-butene

to

speculate metal of

1 4

about

the it

intimate

centers

will

f o r example

surely

mechanisms more

impact fair of

35

Complexes

olefins:

ultimate is

prove

t h e i r analogues

Metal

isomerize

cis-2-butene,

chemistry,

the

Transition

it

selectively

(54). While of

reactions

challenging

one can but

dinuclear

to say that

takes

transition

the e l u c i d a t i o n

at

dinuclear

and f a s c i n a t i n g

metal

than

did

at mononuclear c e n t e r s .

Acknowledgements I

thank

administered Naval

Research

Camille awards

and the National

support

particularly and which

of

grateful Henry have

various

Science

aspects

Foundation

of t h i s

to the A l f r e d P. Sloan Dreyfus

allowed

promoting

my research

long

collaborations

time

the Petroleum Research

Fund

by the America

Research

financial

Corporation,

Teacher-Scholar me a d d i t i o n a l

in this

area.

for

work.

I

their

am also

Foundation and the Grant

degrees

Finally,

I

Program of

freedom i n

acknowledge my

with F. A l b e r t Cotton and coworkers

Texas

A&M U n i v e r s i t y ,

as well

as my own group

tions

are c i t e d i n the r e f e r e n c e s .

for

whose

at

contribu­

Abstract All of the reaction chemistry surrounding mononuclear tran­ sition metal compounds could just as easily be carried out at a bimetallic center. reactions

Indeed, there should be additional types of

uniquely associated

with the M-M bond. This general

premise is discussed in light of the recent reactivity found for dimolybdenum and ditungsten compounds. For a given M X+ center, 2

the preferred coordination geometries,

ligand substitution beha­

viors and dynamical properties (fluxional behavior) are noted. The ability of these compounds to undergo Lewis base association and

dissociation

reactions,

migrations, oxidative

reversible

insertions

or ligand

addition and reductive elimination reac­

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

36

tions,

REACTIVITY OF METAL-METAL BONDS

as well

as direct the

additions across the M-M bond are

discussed.

Finally,

possible mechanisms for one of these

reactions,

reductive elimination of alkane and alkene from a

dimolybdenum center, is considered in detail.

Literature Cited 1. Cotton, F.A.; Wilkinson, G. "Advanced Inorganic Chemistry", 4th Edition, 1980, Wiley-Interscience. 2. For recent reviews, see (a) Cotton, F.A., Chem. Soc. Rev., 1975, 4, 27; (b) Cotton, F.A. Acc. Chem., Res., 1978, 11, 225; (c) Templeton 3. For recent reviews, (a) , ; , Acc. Chem. Res., 1978, 11, 356 and (b) Chisholm, M.H. Faraday Society Symposium, 1980, 14, xxx. 4. For detailed calculations, see (a) ref. 2b; (b) Cotton, F.A.; Stanley, G.G.; Kalbacher, B.; Green, J.C.; Seddon, E.; Chisholm, M. H. Proc. Natl. Acad. Sci., U.S.A., 1977, 74, 3109; (c) Hall, M.B. J. Am. Chem. Soc., 1980, 102, 2104; (d) Bursten, B.E.; Cotton, F.A.; Green, J.C.; Seddon, E.A.; Stanley, G.G. J. Am. Chem. Soc., 1980, 102, 4579. 5. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Stults, B.R. Inorg. Chem., 1977, 16, 603. 6. Chisholm, M.H. Transition Metal Chemistry, 1978, 3, 321. 7. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Reichert, W.W. Inorg. Chem., 1978, 17, 2944. 8. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Kelly, R.L. J. Am. Chem. Soc., 1979, 101, 7645. 9. Cotton, F.A. Acc. Chem. Res., 1969, 2, 240. 10. Klemm, W.; Steinberg, H. Z. Anorg. Allgem. Chem., 1936, 227, 193. 11. Sands, D.E.; Zalkin, A. Acta Cryst., 1956, 12, 723. 12. See references to other studies cited in ref. 9. 13. Chisholm, M.H.; Kirkpatrick, C.C.; Huffman, J.C. J. Am. Chem. Soc., submitted.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2. CHISHOLM Dinuclear Transition Metal Complexes 37

14. 15. 16. 17.

18.

19.

20. 21. 22. 23. 24. 25.

Cotton, F.A.; Frenz, B.A.; Webb, T.R. J. Am. Chem. Soc., 1973, 95, 4431. Klinger, R.J.; Butler, W.; Curtis, M.D. J. Am. Chem. Soc., 1975, 97, 3535; idem, ibid, 1978, 100, 5034. Adams, R.D.; Collins, D.M.; Cotton, F.A. Inorg. Chem., 1974, 13, 1086. For general discussion, see (a) Basolo, F.; Pearson, R.G. "Inorganic Reaction Mechanisms", 2nd Ed., 1968, John Wiley Publishers; (b) Tobe, M.L. "Inorganic Reaction Mechanisms", 1972, T. Nelson Publishers; (c) Wilkins, R.G. "The Study of Kinetics and Mechanis Complexes", Allyn and Bacon Publishers, 1974. Chisholm, M.H.; Extine, M.W. J. Am. Chem. Soc., 1976, 98, 6393; Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Millar, M.; Stults, B.R. Inorg. Chem., 1977, 16, 320. See, Basolo, F.; Pearson, R.G. in "The Mechanisms of Inorganic Reactions", 2nd Ed., 1968, John Wiley and Sons Publishers, page 126. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Millar, M.; Stults, B.R. Inorg. Chem., 1976, 15, 2244. Chisholm, M.H.; Rothwell, I.P. J.C.S. Chem. Commun., 1980, xxx. Chisholm, M.H.; Rothwell, I.P. J. Am. Chem. Soc., 1980, 102, 5950. Chisholm, M.H.; Cotton, F.A.; Murillo, C.A.; Reichert, W.W. Inorg. Chem., 1977, 16, 1801. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Reichert, W.W. J. Am. Chem. Soc., 1978, 100, 153. See also the solid state structure and dynamical solution behavior of W (OPr ) (py) . Akiyama, M.; Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Haitko, D.A.; Little, D.; Fanwick, P.E. Inorg. Chem., 1979, 18, 2266. Chisholm, M.H.; Kelly, R.L. Inorg. Chem., 1979, 18, 2321. Muetterties, E.L. Acc. Chem. Res., 1970, 3, 266. i

2

26. 27.

6

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

REACTIVITY OF METAL-METAL BONDS

38

28. 29. 30. 31.

32. 33. 34. 35.

36.

37. 38. 39.

40.

41. 42.

Tolman, C.A. Chem. Soc. Rev., 1972, 1, 337. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Reichert, W.W. J. Am. Chem. Soc., 1978, 100, 1727. Chisholm, M.H.; Kirkpatrick, C.C.; Huffman, J.C. Inorg. Chem., in press. Akiyama, M.; Little, D.; Chisholm, M.H.; Haitko, D.A.; Cotton, F.A.; Extine, M.W. J. Am. Chem. Soc., 1979, 101, 2504. See Chapter 12 in this volume. Chisholm, M.H.; Haitko, D.A. J. Am. Chem. Soc., 1979, 101, 6784. Chisholm, M.H.; Haitko, D.A.; Huffman, J.C. J. Am. Chem. Soc., submitted. Kochi, J.K. "Organometallic Mechanisms and Catalysis", Academic Press Publishers, 1978, Ch. 12 and references therein. This contrasts with other significant and short CH---Mo interactions: Cotton, F.A.; Day, V.W. J.C.S. Chem. Commun., 1974, 415; Cotton, F.A.; LaCour, T.; Stanislowski, A.G. J. Am. Chem. Soc., 1974, 96, 754. Whitesides, G.M.; Gaasch, J.F.; Stedronsky, E.R. J. Am. Chem. Soc., 1972, 94, 5258. Cotton, F.A.; Rice, G.W.; Sekutowski, J.C. Inorg. Chem., 1979, 18, 1143. Reductive elimination: L M(H)R -> L M + R-H is generally much faster than by C-C bond formation. See, Norton, J.R. Acc. Chem. Res., 1979, 12, 139. Knox, S.A.R.; Stansfield, R.F.D.; Stone, F.G.A.; Winter, M.J.; Woodward, P. J.C.S. Chem. Commun., 1978, 221. See also, Chapter 13 in this volume. Bailey, W.I.; Chisholm, M.H.; Cotton, F.A.; Rankel, L.A. J. Am. Chem. Soc., 1978, 100, 5764. Bailey, W.I.; Chisholm, M.H.; Cotton, F.A.; Murillo, C.A.; Rankel, L.A. J. Am. Chem. Soc., 1978, 100, 802. n

n

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2. CHISHOLM Dinuclear Transition Metal Complexes

43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54.

39

Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Rankel, L.A. J. Am. Chem. Soc., 1978, 100, 807. Alper, H.; Silavwe, N.D.; Birnbaum, G.I.; Ahmed, F.R. J. Am. Chem. Soc., 1979, 101, 6582. Chisholm, M.H.; Kelly, R.L. Inorg. Chem., 1979, 18, 2321. Chisholm, M.H. Advances in Chemistry Series, 1979, 173, 396. See Chapters 8 and 9 in this volume and references therein. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Kelly, R.L. J. Am. Chem. Soc., 1978, 100, 3354. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Kelly, R.L. Inorg. Chem., 1979 Walton, R.A. Chapter 11 this volume. Chisholm, M.H.; Kirkpatrick, C.C.; Raterman, A, results to be published. Hillhouse, G.L. Ph.D. Thesis, Indiana University, 1980. Muetterties, E.L.; Slater, S. Inorg. Chem., in press. Akiyama, M.; Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Haitko, D.A.; Leonelli, J . ; Little, D. J. Am. Chem. Soc., in press.

RECEIVED November 21,

1980.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3 Structure and Reactivity of Some Unusual Molybdenum and Tungsten Cluster Systems R. E. McCARLEY, T. R. RYAN, and C. C. TORARDI Ames Laboratory and Department of Chemistry, Iowa State University,Ames,IA50011 One aspect of the r e a c t i v i t y of metal-metal bonds concerns the approaches to rational synthesi f metal cluste specie f increasing nuclearity. A the addition of a metal atom, metal ligand fragment or metal cluster fragment to an existing metal cluster species such that units of increasing size and complexity can be b u i l t up i n a controlled stepwise manner. This approach has been quite suc­ cessful when applied to organometallic clusters (1), but not nearly so effective i n the synthesis of cluster species to which only c l a s s i c a l ligands are attached. In the l a t t e r category the elements providing the most numerous examples of metal-metal bonded structures are those found early i n the 4d and 5d t r a n s i ­ tion series (2). Molybdenum i s p a r t i c u l a r l y outstanding i n the number, diversity and robustness of i t s cluster compounds (2). Thus i t i s natural to examine the r e a c t i v i t y of molybdenum clusters selected because of their a v a i l a b i l i t y , well-charac­ terized structural features, and particular metal-metal bond type or electronic arrangement. The work reported here concerns the addition of quadruply bonded dimers of the type Mo Cl L according to eqn. 1, 2

4

4

2 Mo.Cl.L. = M o , C l L . 4- 4L 2 4 4 4 8 4 0

(1)

whereby new t e t r a n u c l e a r c l u s t e r s o f r e c t a n g u l a r geometry a r e generated. F i n a l l y , a g l i m p s e o f e x c i t i n g c h e m i s t r y y e t t o come f r o m t e r n a r y o x i d e compounds i s p r o v i d e d b y t h e new m e t a l - m e t a l bonded s t r u c t u r e s NaMo.O. a n d Ba., Μο 0 . 4 6 1.13 8 16 1 0

η

1 Λ

Experimental D e t a i l s o f t h e e x p e r i m e n t a l work and s t r u c t u r e d e t e r m i ­ n a t i o n s w i l l be g i v e n i n s e p a r a t e p a p e r s t o be p u b l i s h e d i n r e g a r d t o t h e s e p a r a t e a s p e c t s o f work summarized h e r e .

0097-6156/81 /0155-0041 $05.00/0 © 1981 American Chemical Society

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

42

R E A C T I V I T Y

O F

M E T A L - M E T A L

B O N D S

Rectangular C l u s t e r s Molybdenum C l u s t e r s o f t h e Type M o ^ C l ^ L ^ . The c h e m i s t r y d e s c r i b e d h e r e was i n i t i a t e d w i t h t h e n o t i o n t h a t i t s h o u l d be p o s s i b l e t o o b s e r v e a d d i t i o n o f m o l e c u l a r s p e c i e s X-Y a c r o s s m u l t i p l e m e t a l - m e t a l b o n d s , f o r example as i n t h e p r o c e s s L M= n

ML

n

+

L M= ni

X-Y

ML ι n

Such a p r o c e s s was c o n s i d e r e d l i k e l y b e c a u s e i n t h e q u a d r u p l y bonded d i m e r s t h e m e t a l atoms have v a c a n t c o o r d i n a t i o n s i t e s t o a c c e p t t h e l i g a n d atoms o f X and Y, and t h e X-Y bond e n e r g y p l u s t h e weak 6-bond o f t h e d i m e r a r e compensated by f o r m a t i o n o f M-X and M-Y bonds. On t h e crowding of l i g a n d s abou a d d i t i o n c h e m i s t r y p r e v i o u s l y o b s e r v e d f o r t h e s e d i m e r s we s o u g h t t o p r e p a r e compounds h a v i n g w e a k l y bound l i g a n d s . D i s s o c i a t i o n i n s o l u t i o n as i n eqn. 2 Mo-Cl.L. = M o C l L + L 2 4 4 2 4 3 0

/

(2)

Q

w o u l d t h e n p r o v i d e s p e c i e s more r e a c t i v e t o w a r d s a d d i t i o n . To t h i s end we a t t e m p t e d t h e p r e p a r a t i o n o f Mo^Cl^(Ρφ^),, w h i c h b e c a u s e o f t h e l o w e r d o n o r s t r e n g t h and l a r g e s i z e or t r i p h e n y l p h o s p h i n e was e x p e c t e d t o meet t h e d e s i r e d r e q u i r e m e n t s . A l t h o u g h many d e r i v a t i v e s o f Mo C I ( P R ) , > where R = a l k y l , had been r e p o r t e d ( 3 ) , Mo C I (Ρφ ) had n o t . A l l a t t e m p t s t o p r e p a r e Μο^ΰΙ^ίΡφ^)^ t h u s f a r have f a i l e d , b u t i n t h e c o u r s e o f t h i s work a m o l e c u l e h a v i n g t h e d e s i r e d r e a c t i v i t y was o b t a i n e d v i a eqn. 3 q

(NH

) Mo

CI

· Η 0 + 2 φ Ρ + 2Ch OH 2

3

Μο α (Ρφ ) (ΟΙ ΟΗ) 2

4

3

2

3

3

2

— 3 excess

• φ^Ρ

+ 5NH C1 + H 0 4

2

(3)

The compound i s c r y s t a l l i z e d f r o m m e t h a n o l as t h e s o l v a t e Mo 01^(Ρφ ) (CH 0H) -nCH OH ( n ^ 2 . 2 ) . A s t r u c t u r e d e t e r m i n a t i o n (4; r e v e a l e d t h e m o l e c u l a r c o n f i g u r a t i o n shown i n F i g . 1. A n o t e w o r t h y f e a t u r e o f t h e s t r u c t u r e i s t h e l o n g Mo-0 b o n d , 2.211(5)Ä, t o t h e c o o r d i n a t e d m e t h a n o l l i g a n d s ; o t h e r f e a t u r e s of the s t r u c t u r e are not unusual. Based on t h e bond r a d i u s f o r Mo d e r i v e d f r o m t h e a v e r a g e M o - C l d i s t a n c e , 2.404(2)Ä, t h e Mo-0 bonds a r e ca.. 0.14Ä l o n g e r t h a n t h e sum o f c o v a l e n t r a d i i . This f e a t u r e of the s t r u c t u r e suggests easy l o s s of the methanol l i g a n d s i n accordance w i t h the r e a c t i o n subsequently e x p l o r e d . Some r e a c t i o n s o f M o C l , ( Ρ φ ) ( C H 0 H ) a r e g i v e n i n Scheme I . Upon d i s s o l v i n g t n i s compound i n b e n z e n e a t 25° a 3

3

3

2

3

2

3

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

McCARLEY E T

AL.

Figure 1.

Molybdenum

Molecular

and

Tungsten

structure of

Cluster

Systems

Mo Cl,,(P ) (CH OH) 2

3 2

3

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

44

R E A C T I V I T Y

O F

M E T A L - M E T A L

Scheme I Reactions Μο 01 (Ρφ ) (ΟΗ 0 H ) 2

4

3

2

25°

2

CH OH/C H 3

1 0

1 8

HC1 [MoCl (CH_0H)]

[Μο01 (Ρφ.)] Ζ J η

Ζ

ο

o

9

η

y e l l o w powder

brown p p t .

RCN

Mo Cl (PR ) 4

8

3

[MoCl (NCR)] ζ η

4

brownish yellow

0

crystals

y e l l o w powder

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

B O N D S

3.

M C C A R L E Y

E T A L .

Molybdenum

and Tungsten

Cluster

Systems

45

r e a c t i o n l e a d i n g t o t h e p r e c i p i t a t i o n o f a brown s o l i d i s c o m p l e t e w i t h i n £a. 30 m i n . T h i s compound i s d i f f i c u l t t o o b t a i n i n p u r e form by t h i s r e a c t i o n , because o f secondary r e a c t i o n w i t h metha­ nol. A n a l y t i c a l d a t a and i . r . s p e c t r a i n d i c a t e d however t h a t a l l m e t h a n o l l i g a n d s h a d been l o s t f r o m t h e s t a r t i n g m a t e r i a l and t h e brown p r o d u c t c o n s i s t e d o f [MoCl (Ρφ ) ] . I n s o l u b i l i t y o f t h i s p r o d u c t p r e v e n t e d d e f i n i t i v e c h a r a c t e r i z a t i o n , h e n c e e f f o r t s were made t o s e c u r e more t r a c t a b l e d e r i v a t i v e s . R e a c t i o n o f t h e brown [MoCl^(Ρφβ)1 w i t h t r i b u t y l p h o s p h i n e a t 25° i n b e n z e n e l e d t o d i s p l a c e m e n t o f Ρφ^ and f o r m a t i o n o f a b r o w n i s h y e l l o w s o l u t i o n , f r o m w h i c h b r o w n - y e l l o w c r y s t a l s were o b t a i n e d . A structure d e t e r m i n a t i o n r e v e a l e d a tetramer o f t h e c o m p o s i t i o n Mo^Cl (PBu^)^, b u t s a t i s f a c t o r y refinement o f t h e s t r u c t u r e coula n o t be a c h i e v e d . Refinement o f t h e s t r u c t u r e o f t h e t r i e t h y l p h o s phine d e r i v a t i v e Mo^ClgCPE o f t h e m o l e c u l e i s show We n o t i c e i m m e d i a t e l y t h a t t h e m o l e c u l e c o n t a i n s f o u r Mo atoms i n a r e c t a n g u l a r a r r a y , b r i d g e d on e a c h l o n g edge b y two CI atoms. W i t h i n t h e r e c t a n g l e t h e Mo-Mo bond d i s t a n c e s a r e 2.211(3)Ä on t h e s h o r t edges and 2.901(2)Ä on t h e l o n g e d g e s . The l o n g d i s t a n c e f a l l s w e l l w i t h i n t h e r a n g e o f c a . 2.5 t o 3.2Ä known f o r Mo-Mo s i n g l e bonds ( 5 ) . The s h o r t bond d i s t a n c e s a r e i n e x c e l l e n t agreement w i t h t h o s e known f o r compounds w i t h Mo-Mo t r i p l e bonds (6), e.g. Mo (NMe ) 2.214(3)Ä; Mo (OCH CMe ) , , 2.222(2)Ä; Mo (OSiMe ) ( H M e 2.242 (1)Ä. Thus t h e s t r u c t u r a l d a t a c l e a r l y i n d i c a t e t h e r e c t a n g l e c o n s i s t s o f s i n g l e bonds on t h e l o n g edges and t r i p l e bonds on t h e s h o r t e d g e s . Remembering t h a t t h e r e c t a n g l e i s formed f r o m two q u a d r u p l y bonded d i m e r s we may s c h e m a t i c a l l y r e p r e s e n t t h e r e a c t i o n a s a 2 + 2 a d d i t i o n process 2

Mol

Mol

F Mo

I f t h i s v i e w i s v a l i d we m i g h t e x p e c t t h a t t h e ό-bonding o r b i t a l s i n t h e d i m e r s a r e r e c a s t i n t h e t e t r a m e r t o f o r m t h e l o n g σ-bonds. A f t e r c h a r a c t e r i z a t i o n o f t h e Mo C l g C P R ^ ) ^ d e r i v a t i v e s i t n a t u r a l l y was o f i n t e r e s t t o e s t a b l i s h more g e n e r a l s y n t h e t i c routes t o the r e c t a n g u l a r c l u s t e r s o f t h i s type. Poor o v e r a l l y i e l d s o f c a . 20 p e r c e n t b a s e d on c o n v e r s i o n o f Mo ΰΙ^ίΡφ^)^ (CH 0 H ) t o Mo C I (PR ) a s d i s c u s s e d above p r e s e n t e d an i m p e d i ­ ment t o f u r t h e r c n e m i s t r y o f t h e t e t r a m e r s . A l s o we w i s h e d t o know i f t h e d i m e r M o ^ Ι ^ ί Ρ φ ^ ) ^ (CH^OH)^ was u n i q u e i n l e a d i n g t o f o r m a t i o n o f t h e r e c t a n g u l a r u n i t s , a r e s u l t w h i c h seemed r a t h e r unlikely. I n d e e d , s u b s e q u e n t work p r o v e d t h a t much more e f f i ­ c i e n t s y n t h e s e s c o u l d be o b t a i n e d f r o m more c o n v e n i e n t s t a r t i n g 2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

46

R E A C T I V I T Y

O F

M E T A L - M E T A L

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

B O N D S

Molybdenum

McCARLEY E T A L .

3.

and Tungsten

Cluster

47

Systems

m a t e r i a l s . The governing f a c t o r proved to be simply s t o i c h i o ­ metric c o n t r o l of the added ligand/metal r a t i o . Some s u c c e s s f u l s y n t h e t i c r e a c t i o n s are enumerated i n Scheme I I . Reactions 5 and 6 are p a r t i c u l a r l y convenient s i n c e the most common s t a r t i n g m a t e r i a l f o r s y n t h e s i s of quadruply bonded dimers of molybdenum, v i z . Mo^CO^CCH )^, i s converted d i r e c t l y i n t o the rectangular c l u s t e r s i n high y i e l d . The products of r e a c t i o n s 7, 8 and 9 are formulated as tetramers and assumed to c o n s i s t of rectangular c l u s t e r u n i t s because t h e i r yellow c o l o r and e l e c ­ t r o n i c absorption maxima are q u i t e s i m i l a r to Mo^Clg(PBu^),, and each i s e a s i l y converted i n t o Mo^Clg(PEt^)^ or Ho^Cl^(PEfLj^ upon r e a c t i o n a t 25° with the s t o i c h i o m e t r i c amount of PEt^. l presence of excess PEt^ a l l of these tetramers are converted back i n t o dimers: nt

n

e

Mo.Cl.L. + 8PEt 4 8 4 3 The c o n t r a s t i n c o l o r between the dimers Mo C l ^ L ^ , gener­ a l l y red to blue, and the tetramers, brown to yellow, i s q u i t e striking. The intense band assigned as the δ -> δ* t r a n s i t i o n i n the dimers (7), a t ca. 500-600nm, disappears upon formation of the tetramers and i s replaced by new bands a t 375-440nm(strong) and 600-700nm(weak). Absorption maxima f o r these bands obtained from r e f l e c t a n c e s p e c t r a of s e v e r a l of the d e r i v a t i v e s are given i n Table I, The assignment of these t r a n s i t i o n s must await a thorough study of the s p e c t r a . However one p o s s i b i l i t y f o r t h e i r -1 Table I.

-3

Band Maxima (cm

χ 10

) f o r Mo^ClgL^

L

PBu^

Ρφ^

EtCN

MeOH

ν

14.3

14.5

15.6

16.7

weak

22.7

23.1

24.4

26.7

strong

v

2

1

assignment i s based on the Mo s which can be derived from the s e t of four d - o r b i t a l s a p p r o p r i a t e f o r δ-bonding i n the short d i r e c ­ t i o n , and σ-bonding i n the long d i r e c t i o n of the r e c t a n g u l a r cluster. In the i d e a l i z e d symmetry of the c l u s t e r found i n the s t r u c t u r e of Mo4Cl8(PEt3>4 these MO s c o n s i s t of the bonding set a (σ-δ), b ( o - ö * ) , and the antibonding s e t a ( o * - ö ) , bg(o*-ö*), with r e l a t i v e energies a 2 and Mo at 1100° s e v e r a l c r y s t a l l i n e phases were observed. A s i n g l e c r y s t a l of one phase e x h i b i t i n g columnar growth h a b i t was secured and the s t r u c t u r e determined. The composition of t h i s compound, Ba Mo 0 , was e s t a b l i s h e d from the s t r u c t u r e refinement (R = 0.033, R = 0.042), which a l s o showed the c r y s t a l was t r i c l i n i c with l a t t i c e constants a = 7.311(1), b = 7.453(1), c = 5.726(1)Ä, α = 101.49(2)°, Β = 99.60(2)°, γ = 89.31(2)°, Ζ = 1, PI. ft

The S t r u c t u r e of B a i s extremely i n t e r e s t i n ture again i n v o l v e s i n f i n i t e chains of metal c l u s t e r u n i t s boun^ together i n such a way that tunnels are constructed f o r the Ba ions. T h i s f e a t u r e of the s t r u c t u r e i s remarkably l i k e that of NaMo^O^ as shown i n F i g . 7, which i s a view down the c - a x i s , a l s o the tunnel a x i s . The repeat d i s t a n c e along the c - a x i s i s almost e x a c t l y twice that i n NaMo^O^, a r e s u l t of the f a c t that the Mo-Mo separations are a l t e r n a t e l y long and short, though the 0-0 spacings are roughly equal at 2.86A. Secondly, the i n f i n i t e chains are of two types: one which c o n s i s t s of n e a r l y r e g u l a r Μ

0

0

a

S

s

h

o

w

n

i

n

F i g

8

a

n

d

t

h

e

o

t

h

e

r

c l u s t e r s Mo^Og = ° 4 ° 2 8 / 2 6 7 3 * ' having c l u s t e r s of the same b a s i c s t r u c t u r e but w i t h two elongated Mo-Mo bont^s_ as shown i n F i g . 9. F i n a l l y , the p o s i t i o n s occupied by the Ba ions w i t h i n the tunnels are of three k i n d s , each w i t h only f r a c t i o n a l occupation number, v i z . B a l , 0.86; B a l l , 0.13; B a l l l , 0.13. The arrangement i s such that s i t e s I and I I , or I I and I I I cannot be occupied simultaneously because of t h e i r c l o s e proximity. A view perpendicular to one of the chains i s shown i n F i g . 10. I t i s seen that the metal atoms f i l l a l l o c t a h e d r a l s i t e s between two oxide l a y e r s , each three atoms wide by i n f i n i t e length. The d i s c r e t e c l u s t e r u n i t s are formed by displacement of the metal atoms along a l i n e which i s c o l i n e a r with the chain, such that the Mo-Mo spacing i s a l t e r n a t e l y long and s h o r t , e.g. 3.16 and 2.57A. A l l 0-atoms capping t r i a n g u l a r faces above and between c l u s t e r u n i t s along the chain are t r i g o n a l pyramidal, and those bound on the edges are t r i g o n a l planar. Metal-metal bonding w i t h i n the c l u s t e r s of r e g u l a r geometry may be understood as r e s u l t i n g from 10 e l e c t r o n s i n bonding σo r b i t a l s d i r e c t e d along the 5 bonded edges. T h i s i n d i c a t e s the net o x i d a t i o n s t a t e s of 3+ f o r the Mo atoms on the shared edge of the two t r i a n g l e s , and 4+ f o r the two atoms on the outer apiceg^ Each r e g u l a r c l u s t e r u n i t then must assume a n i o n i c charge Mo,0 . I t i s d i f f i c u l t to reason why the d i s t o r t i o n i n the o

0

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

56

REACTIVITY

OF

M E T A L - M E T A L

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

BONDS

3.

McCARLEY E T A L .

Molybdenum

and

Tungsten

Cluster

Systems

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

57

58

REACTIVITY

OF

M E T A L - M E T A L

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

BONDS

McCARLEY E T A L .

3.

Molybdenum

and Tungsten

Cluster

59

Systems

" i r r e g u l a r " c l u s t e r u n i t occurs e x a c t l y as i t does. From the Mo-Mo d i s t a n c e s i n these u n i t s i t appears the three short bonds are of order 1 . 0 and the two elongated bonds are approximately of order 0 . 5 . Since the atoms on the shared edge are involved i n ca. 2 . 5 bonds each, t h e i r net o x i d a t i o n s t a t e i s approximately 3 . 5 + , whereas the atoms on the outer apices are involved i n only 1.5 bonds and have approximate s t a t e 4 . 5 + . In both cases the shortest Mo-0 d i s t a n c e s are those about the outer a p i c a l atoms, which lends support to t h e i r assessment of a higher net o x i d a tion state. Based on these c o n s i d e r a t i o n s the^compound perhaps i s b e t t e r formulated as Ba ( M o Og )(Mo Og ). The astute reader w i l l " r e c o g n i z e that t h i s compound i s a metal-metal bonded adaptation of the well-known h o l l a n d i t e s t r u c ture ( 1 9 ) . F u r t h e r , the metal c l u s t e r c o n f i g u r a t i o n i s l i k e that i n CsNb^Cl ( 2 0 ) , though the l a t t e r compound possesses a double l a y e r s t r u c t u r e rather tha F i n a l l y these new s t r u c t u r e p o s s i b l e , and an excitement about new compounds of unusual s t r u c ture and p r o p e r t i e s yet to come from s t u d i e s of h i g h l y reduced ternary and quaternary r e f r a c t o r y metal oxide systems. 13

Abstract Synthesis, structure and properties of the new tetranuclear clusters Mo Cl L (L = PBu , PEt , EtCN and MeOH) and W Cl (PBu )4 are described. These compounds result from condensation of quadruply bonded dimers in a 2 + 2 cycloaddition-ligand elimination process and exhibit rectangular geometry of the M cluster. Metal-metal bond distances of 2.211(3) and 2.901(2) for Mo Cl8 (PEt ) , and 2.309(2), 2.840(1)Åfor W Cl (PBu ) were found for the short and long edges of the rectangular cluster units. The short distances agree well with known M-M triple bond lengths, and the long distances fall within the range known for M-M single bonds. Also described are the synthesis and structure of two new ternary molybdenum oxides with metal-metal bonded cluster units, Ba Mo O and NaMo4O . In each case the structure features Infinite chains of metal clusters crosslinked by M-O-M bonds in such a way that unidirectional tunnels are formed for accommodation of the Na or Ba ions. In Ba Mo O the clusters consist of Mo O units with the connectivity relation Mo O O O and the coplanar metal atoms at the vertices of two triangles sharing a common edge. The overall structural features (both Mo-Mo ang Mo-O bonding) suggest the formulation Ba l.13 (Mo O 8) (Mo O 8). The extremely interesting structure of NaMo O consists of infinite chains of octahedral clusters Mo6 fused on opposite (parallel) edges and bridged on the outwardly exposed edges by O-atoms. Within the chains the connectivity is Mo Mo / O O ; if the crosslinking between chains is included the connectivity becomes Na [(Mo Mo O O )O ]. The Compound is tetragonal and exhibits metallic conductivity. 4

8

4

3

3

4

8

3

4

4

3

4

4

1.13

8

16

4

2

8/2

2-

4

4

4 2

4

2+

1.13

8

8/2

2/2

16

16

6/3

2+

2

3

6

+

4

8

0.26-

4

6

2

8/2

+

-

2

4/2

2/2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

60

REACTIVITY OF METAL-METAL BONDS

Literature Cited

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

See for example Vahrenkamp, Η., Structure and Bonding, 1977, 32, 1-56. Cotton, F. Α.; Wilkinson, G., Advanced Inorganic Chemistry, 4th Ed., John Wiley and Sons, New York, 1980, p. 1094. San Filippo, Jr., J., Inorg. Chem., 1972, 11, 3140. McGinnis, R. N.; Ryan, T. R.; McCarley, R. E., J. Am. Chem. Soc., 1978, 100, 7900. Cotton, F. Α., J. Less-Common Met., 1977, 54, 3. Chisholm, Μ. H.; Cotton, F. Α., Acc. Chem. Res., 1978, 11, 356. Trogler, W. C.; Gray, Η. Β., Acc. Chem. Res., 1978, 11, 232. Sharp, P. R.; Schrock, R. R., J. Am. Chem. Soc., 1980, 102, 1430. Cotton, F. Α.; Felthouse Soc., 1980, 102, McCarroll, W. H.; Katz, L.; Ward, R., J. Am. Chem. Soc., 1957, 79, 5410. Ansell, G. B.; Katz, L., Acta Cryst., 1966, 21, 482. Cotton, F. Α., Inorg. Chem., 1964, 3, 1217. Torardi, C. C.; McCarley, R. E . , J. Am. Chem. Soc., 1979, 101, 3963. Pauling, L., The Nature of the Chemical Bond, 3rd Ed., Cor­ nell University Press, 1960, p. 400. Lokken, D. Α.; Corbett, J. D., Inorg. Chem., 1973, 12, 556. Simon, Α.; Holzer, N.; Mattausch, Ηj., Z. Anorg. Allg. Chem., 1979, 456, 207. Poeppelmeier, K. R.; Corbett, J. D., J. Am. Chem. Soc., 1978, 100, 5039. Mattausch, Ηj.; Hendricks, J. B.; Eger, R.; Corbett, J. D.; Simon, Α., Inorg. Chem., 1980, 19, 2128. Wells, A. F., Structural Inorganic Chemistry, 4th Ed., Clarendon Press, Oxford, 1975, p. 459. Broil, Α.; Simon, Α.; von Schnering, H. G.; Schafer, H., Ζ. Anorg. Allg. Chem., 1969, 367, 1.

(The Ames Laboratory i s operated f o r the U.S. Department of Energy by Iowa State U n i v e r s i t y under Contract No. W-7405-Eng-82. This research was supported by the A s s i s t a n t Secretary f o r Energy Research, O f f i c e of Basic Energy Sciences, WPAS-KC-02-03.)

RECEIVED

November 21,

1980.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4 Rhodium Carbonyl Cluster Chemistry Under H i g h Pressures of Carbon Monoxide and Hydrogen Polynuclear Rhodium Carbonyl Complexes and Their Relationships to Mononuclear and Binuclear Rhodium Carbonyl Complexes 1

JOSÉL. VIDAL, R. C. SCHOENING, and W. E. WALKER Union Carbide Corporation, P.O Bo 8361 South Charleston WV 25303

The complexity of the chemistry of rhodium carbonyl clusters has been responsible for the slow development of this field of chemistry. (1) We have been involved for some time in the study of these species under high pressures of carbon monoxide and hydrogen. (2) The study of these systems by high pressure infrared spectroscopy showed the relatively low fingerprint ability of this technique(3) and the possibility that some species could remain undetected as a consequence of an overlapping of spectral features. The importance of determining the presence in those systems of rhodium carbonyl complexes of nuclearity lower even than that of such simple clusters as [Rh (CO)15] is rooted in the relatively recent proposals that mononuclear transition metal complexes of Co(4) and Ru(5) are able to hydrogenate carbon monoxide and of mechanisms for this behavior.(6) In contrast, polynuclear complexes have been mentioned as probably enhancing this reactivity.(7) Thus, it was of interest to determine the presence of mono- and binuclear-rhodium complexes in a system showing the catalytic formation of polyalcohols from synthesis gas. The participation of mono- and binuclear anionic, cationic and neutral complexes in the aggregation and fragmentation reactions of rhodium carbonyl clusters under ambient or low pressure conditions has been described before. For instance, [Rh(CO) ] has been shown(1) to participate in condensation reactions that increase -

5

-

4

1

This is the fourth part of a series. 0097-6156/81/0155-0061$05.75/0 © 1981 American Chemical Society

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

62

REACTIVITY OF METAL-METAL BONDS

stepwise the s i z e and degree of reduction of the c l u s t e r s (equations 1-3). It was a l s o reported t h a t the formation of t h i s anion could o r i g i n a t e in the fragmentation of the c l u s t e r s by carbon monoxide facilitated by a basic medium. Rh4(C0)i2

CRh(C0)4]" - — ^ [Rh«5 (CO)i

+

[Rh (C0)i5]-

+

5

CRh(C0) ]- — [ R h 4

[Rh6(C0)i ] "" 2

5

+ [Rh(C0) ]"

(I)

(C0>1 ^ ] ' + 4C0

—[Rh (CO)! ] ~ 3

4

(2)

2

6

7

6

+ 3C0

(3)

Another simple rhodium carbonyl complex a l s o known t o be involved in the fragmentation and aggregation r e a c t i o n s of c l u s t e r s i s Rh2(C0)8« T h i s s p e c i e s has been shown t o p a r t i c i p a t e In the r e a c t i o n s of neutral rhodium carbonyl species In e i t h e r m a t r i x e s ^ * o has not y e t been implicate clusters. 2Rh6(C0)6

+

24C0

3Rh427] " +

2

3

3

+ 2CH3CN

[Rhi (C0)27] ~ + 2Br" — ^ 3

5

(8)

[ R h ! ( C 0 ) 5 ] " + [Rh(CO) Br ]" 4

4

2

2

(

9

)

2

The p o t e n t i a l c a t a l y t i c a b i l i t y of polynuclear complexes for the conversion of C0:H2 into chemicals*!.* is in c o n t r a s t with recent r e s u l t s i n v o l v i n g mononuclear speci es* .»^.* and the proposal of a mechanism f o r such c o n v e r s i o n . ^ Our i n t e r e s t in understanding the behavior of rhodium carbonyl c l u s t e r s in systems which c a t a l y t i c a l l y convert CO:H2 into alcohols* .* prompted us t o t e s t the p o t e n t i a l presence of mononuclear and b i n u c l e a r rhodium carbonyl complexes in these systems. A positive characterizatio these circumstances woul rhodium carbonyl c l u s t e r s under high pressure of carbon monoxide. It could a l s o show the existence of a p a r a l l e l behavior between the chemistry of these species under ambient and high pressures of carbon monoxide, and i t may shed some l i g h t on the c a t a l y t i c r e a c t i o n s o c c u r r i n g in those systems.* .* Because of our previous success in applying F o u r i e r transform i n f r a r e d spectroscopy t o the study of the rhodium carbonyl c l u s t e r s under high pressures of carbon monoxide and hydrogen* .*.!*, we have a p p l i e d the same technique and equipment in t h i s work.* .* The temperature has been kept In a l l these experiments below 2 0 0 ° with maximum pressures of 832.0 atm t o maximize the trend towards fragmentation of c l u s t e r s . The absence of bases, e . g . , s a l t s or amines, in the systems under e v a l u a t i o n in t h i s work was d e s i r a b l e t o e l i m i n a t e the ambiguity t h a t would r e s u l t from the enhancement of the fragmentation of c l u s t e r s by carbon monoxide in a b a s i c medium. *L* 4

3

3

2

3

RESULTS AND DISCUSSION Rhodium carbonyl c l u s t e r s with encapsulated heteroatoms, e . g . , [ R h ( C 0 ) C ] - , [RhgPCCO^l ] " and [ R h i 7 S 2 * C 0 ) ] ~ , have been observed t o be more r e s i s t a n t t o fragmentation under 500-1000 atm of C0:H2 than homometallic rhodium c l u s t e r s . Polynuclear species account f o r 95-98 and 80-85 percent, r e s p e c t i v e l y , of the t o t a l rhodium content of those systems, while the greatest c a t a l y s t a c t i v i t y has been noted with the l a t t e r complexes. CRh(C0>4D" is the only fragmentation product detected in both instances.* .* The reasons above d i r e c t e d our a t t e n t i o n t o the r o l e of [Rh(C0>4]" in t h e fragmentation-aggregation r e a c t i o n s of c l u s t e r s . T h i s anion is generated during the fragmentation of [ R h 7 ( C 0 ) i 6 Ϊ " under 600 atm of C0:H2 (Figure 1, equation 1 0 ) . [ R h ( C 0 ) 4 ] " is a l s o formed under s i m i l a r c o n d i t i o n s from other c l u s t e r s in t h e presence of bases, e . g . , [Rh^ 2*CO>3o!] ~ (equation 2

6

2

3

1 5

3 2

3

5

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

64

REACTIVITY

OF

M E T A L - M E T A L

BONDS

Figure 1. 1R spectra under 537 atm of CO:H (1:1) and 125°C of: (a) a solution of [(Ph P) N][Rh(CO)J in 18-crown-6; (b) a solution of [(Ph P) N] [Rh (CO) ] in 18-crown-6; (c) a solution of [(Ph P) N] [Rh (CO) ] in 18-crown-6; (d) solution (c) at atmospheric conditions. Assignments of the spectra shown in the figures. 2

s

2

s

3

2

3

7

2

2

16

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

12

30

4.

viDAL E T

Rhodium

AL.

Carbonyl

Cluster

65

Chemistry

11). The same behavior has been observed under ambient c o n d i t i o n s . *J_* [ R h 7 ( C 0 ) | 6 ] ~ + 7C0 — ^

[Rh (C0)15]- + 2[Rh(C0) ]-

3

5

(10)

4

[ R h i 2 ( C O ) 3 o ] ~ + R3N + CO —=»» 2[Rh5(CO) 1 5 Ι Γ + 2 [ R h ( C 0 ) ] -

(11)

2

4

The p a r a l l e l between the aggregation r e a c t i o n s of c l u s t e r s with L~Rh(C0) ]~ under ambient and high pressure c o n d i t i o n s is more c l e a r l y shown by the comparison of the a b i l i t y of these r e a c t i o n s t o b r i n g about the growth of c l u s t e r s one atom and one negative charge at a time under both c o n d i t i o n s . T h i s behavior (equations 2 and 3) has been a l s o noted under 537-840 atm (equation I I, F i g u r e 2). liL> 4

(

CRhgP(C0) |] " + [Rh(C0) 2

2

The fragmentation r e a c t i o n s of QRhi 3(C0)2 H3D "" and CRh-j 3 ( C 0 ) 2 H 2 H " have not been p r e v i o u s l y s t u d i e d , although is has been reported that these c l u s t e r s are formed together with [ R h i ( C 0 ) 2 5 ] ~ and [Rhi ( C 0 ) 2 7 ] " " during the r e a c t i o n of [ R h i 2 ( C 0 ) 3 Q l ~ with hydrogen at 8 0 ° and 1 atm [equation ]3)*±1>1Σ> (equ 2

4

3

4

4

3

4

5

2

[Rh

(C0)3 ] -

+ H

2

1 2

0

-[Rh 3(C0) 1

2

2 4

H ] x

( 5

-

n )

-

+

[Rh

1 4

(C0)

+ [Rh (C0)27] " 3

] 4

2 5

(

1

3

)

1 5

We a l s o examined the fragmentation and aggregation r e a c t i o n s of C R h i 3 ( C 0 ) 2 H ] ~ " under high pressures of CO and Η ·It was of i n t e r e s t in t h i s respect t o determine which rhodium carbonyl complexes r e s u l t from the former r e a c t i o n and whether the p a r a l l e l formation of any organic products derived from the hydrogenation of carbon monoxide is a l s o o c c u r r i n g . The l a t t e r p o s s i b i l i t y was considered because of the presence of hydrides In the c l u s t e r and the Involvement of hydrldo carbonyl complexes in the hydrogenation of carbon monoxide.*—* Unfortunately, i t was not p o s s i b l e t o detect any o r g a n i c products formed from t h i s r e a c t i o n even a f t e r the c y c l i c r e p e t i t i o n of the transformations below (equation 14). ( 5

4

2

CRh 3(C0) H3] " 2

1

24

CO

H

[Rh5(C0) ]" 15

n )

x

2

BASE 10 ATM CO

»

{

CRh (C0)24H ] " 3

13

2

CO

H

[Rh (C0) ]5

1 5

(14)

2

+

[Rh(C0) ]4

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

REACTIVITY

1

cm" (A)

(B)

OF

M E T A L - M E T A L

BONDS

1

cm" (C)

Figure 2. IR spectra under 537 atm of CO:H (1:1) and 180°C: (a) a solution of [Cs(18-crown-6) ] [Rh P(CO) ] in 18-crown-6; (b) a solution of [Cs(18-crown6) ]s[Rh P(CO) ] in 18-crown-6; (c) same solution as in (b) before high-pressure treatment. 2

n 2

n

10

y

21

22

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

4.

viDAL E T

Rhodium

AL.

Carbonyl

Cluster

67

Chemistry

The products detected and the r e l a t i v e c o n d i t i o n s r e q u i r e d f o r t h e i r formation (equation 14) are c o n s i s t e n t with the trend towards increased fragmentation by carbon monoxide upon i n c r e a s i n g the negative charge: metal atom r a t i o , e . g . , Bh6i4*- R h ( C 0 ) Rh (CO)j .*i> presence of LRn(C0)4_r in the baste system above could have been a s c r i b e d t o p a r t i a l fragmentation of [Rhi3(C0)24H2ZP~ but a q u a n t i t a t i v e i n f r a r e d study of t h a t s o l u t i o n showed a molar r a t i o of [ R h 5 ( C 0 ) ] " t o [ R h ( C 0 ) ] " of c a . , 2:3. T h i s , together with our i n a b i l i t y t o detect any hydrogenation product of carbon monoxide and with the high a c i d i t y of HRh(C0)4, was i n d i c a t i v e of the p o t e n t i a l formation of t h i s mononuclear hydride during the fragmentation of [Rhi 3 ( 0 0 ) 2 ^ 3 ] . ~ A comparison of the i n f r a r e d spectrum of [Rh5(C0)i5J~ and t h a t r e c e n t l y reported*. * f o r HRh(C0)4 (Figure 1 and T a b l e I) shows the o v e r l a p p i n g of the main f e a t u r e s of both s p e c t r a of [ R h 5 ( C 0 ) i 5 ] " from th a f t e r r e a c t i o n of [Rhi3(C0)24H3D with carbon monoxide leaves the p a t t e r n expected f o r HRh(C0)4 (Figure 3 ) . We have concluded t h a t the fragmentation and aggregation r e a c t i o n s of [ R h ( C 0 ) 2 4 - H ] " " " ( x = x,3) could be w r i t t e n as below (equations 15 and I6) 2

6

T

1 5

6

1 5

h

e

6

4

2

9

( 5

1 3

x )

x

CRhi (C0)24H ] -

+ 18C0 — ^ 2[Rh«5(C0)i 5 ] " + 3HRh(C0)

[Rhi3(C0)24H2D "

+ 18C0 + 2R N — 2 [ R h 5 ( C O ) 1 5 ] -

2

3

3

3

4

( 15)

3

+ 3CRh(C0)4]- + 2 [ R N H ]

(16)

+

3

The c h a r a c t e r i z a t i o n of HRh(C0)4 was e s s e n t i a l t o our s t u d i e s . T h i s species was expected t o be analogous t o HM(C0)4(M=Co, Ir) p r e v i o u s l y reported by Whyman, ϋ but i t was not p o s s i b l e t o detect i t s formation upon r e a c t i n g Rh4(C0)i2 with C0:H2«*JJL* In a d d i t i o n , i t s presence could not be confirmed during the protonation of [Rh(C0)4]~. Instead, Rh6(C0)i5 was formed when t h a t r e a c t i o n was c a r r i e d out at ambient conditions.*!^ We looked at the r e a c t i o n of Rh44

HRh(C0)4

HCo(C0)4

Compound

TABLE I.

viDAL E T

Rhodium

AL.

Carbonyl

Cluster

Chemistry

1

cm"

Figure 3. IR spectra of J8-crown-6 solutions of: (a) [Cs(18-crown-6) ] [Rh (CO)> H ] under 10 atm of CO; (b) solution in (a) after addition of N-methylmorpholine; (c) solution in (a) under 140 atm of CO and 80°C; (d) solution in (b) under 140 atm of CO and 40°C; (e) solution in (c) after Fourier-subtraction of the pattern of [Rh,(CO) ]-; (f) HRh(CO) obtained from Rh (CO) and CO:H>. u ii

fl

:s

15

h

k

12

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

13

70

REACTIVITY

O F

M E T A L - M E T A L

BONDS

Another pathway f o r the preparation of HRh(C0>4 was found t o be the a c i d i f i c a t i o n of s o l u t i o n s of the conjugate base [Rh(C0)4]~ under high pressure of C0:H2* The formation of Rh6(C0)i6 caused by o x i d a t i o n of the anion by the a c i d i s precluded by b r i n g i n g the a c i d in contact with the anion under 1000 atm. of C0:H2. In t h i s way, the c o o r d i n a t i o n to carbon monoxide present in rhodium tetracarbonyI is r e t a i n e d and the tendency t o aggregation i s precluded. A comparison with the b a s i c i t y of the other two tetracarbonyI metal late anions (Co and Ir) as i n d i c a t e d by the s t o i c h i o m e t r i e s for t h e i r protonation t o HM(C0)4 under s i m i l a r c o n d i t i o n s (Table II) shows the f o l l o w i n g t r e n d of b a s i c i t y : [I r ( C 0 ) ] " " > C C o ( C 0 ) ] " > C R h ( C 0 ) 4 ] - . In f a c t , we have been able t o e s t a b l i s h the order of a c i d i t i e s of the conjugate acids by preforming them in dodecane s o l u t i o n s by means of the r e a c t i o n of C0:H2 with C02(C0>8, Rh4(C0)i2 and lr4(C0>i2> followed by of d i f f e r e n t b a s i c i t i e s Hlr(C0)4 does not r e a c t with an excess of N-methyImorpholine (pka (25°) = 7.4), HCo(C0)4 and HRh(C0)4 reacted q u a n t i t a t i v e l y . On the other hand, HCo(C0)4 did not r e a c t with N,N-d imethyIa n i l i n e (pKa (25°) = 4.8) but i t deprotonated HRh(C0) . A l l the r e s u l t s above i n d i c a t e a s c a l e of a c i d i t i e s in which the second row t r a n s i t i o n metal complex is the most a c i d i c . We do not have an explanation f o r t h i s at hand although i t appears t h a t a s i m i l a r order has been found f o r some s i m i l a r complexes in the iron t r i a d . 1Z> 4

4

4

(

Order of A c i d i t y Cobalt T r i a d

HRh(C0)4

Iron T r i a d

Ru

> HCo(C0)4 >

Fe

>Hlr(C0)4 >

0s

The other neutral species we have studied with respect t o i t s p o t e n t i a l p a r t i c i p a t i o n in the fragmentation-aggregation r e a c t i o n s of rhodium carbonyl c l u s t e r Is Rh2(C0)8* This species has not been p r e v i o u s l y implicated in these r e a c t i o n s in the case of a n i o n i c rhodium carbonyl c l u s t e r s , although i t s involvement in such r e a c t i o n s f o r neutral c l u s t e r s (equation 4) has already been shown. An i n d i c a t i o n of the presence of t h i s species In these types of r e a c t i o n s could be the formation of Rh6(C0)i6 in the r e a c t i o n of [Rh5(C0)i5~)~ with carbon monoxide, as observed by C h i n i , e t a l . , \ L § £ * and by u s ^ ^ b ) under ambient c o n d i t i o n s 3[Rhi2 σ or -»• σ Lowest E x c i t e d States. I t i s apparent from the d e s c r i p t i o n of the e l e c t r o n i c s t r u c t u r e of Mn2(CO)io, (8., 9) , Scheme I, that complexes having a s i n g l e M-M bond could have low-lying e x c i t e d s t a t e s that should be mor respect to e i t h e r M-ligan In p a r t i c u l a r , M n 2 ( C O ) i and i t s t h i r d row analogue, Re2(CO)io, exhibi£ low-lying e x c i t e d s t a t e s a r i s i n g from π-d σ* and O •> σ t r a n s i t i o n s (8, 9^, K), 11) . These high symmetry , 0

b

(OC) MC2)

(OC) M

5

5

M(CO)

(E^M(CO)

(

5

d 2 2. χ-y

d 2 2 χ-y

d 2 ζ

d 2 ζ

TT-d

- f f - *+h -H-

Scheme I .

One-electron l e v e l diagram f o r Mn (CO) , Re (CO) , W (CO) 2

1Q

2

1 Q

2

2 1Q

^, etc.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Jir-d

5.

WRIGHTON

7

E T

A L .

Metal-Metal-Bonded

87

Complexes

7,T

" d - d , homodinuclear dimers were the f i r s t to be subjected to a d e t a i l e d photochemical i n v e s t i g a t i o n (10, 11). Irradiation r e s u l t i n g i n π - d -** σ or *• σ t r a n s i t i o n s r e s u l t s i n c l e a n , quantum e f f i c i e n t , homolytic s c i s s i o n of the M-M bond to y i e l d r e a c t i v e , 17-valence e l e c t r o n r a d i c a l s , equation (1) (10, 11). V ^ I O

aitone

2 M ( C O )

'

( 1 )

5

The generation of 17-valence e l e c t r o n r a d i c a l s i s c o n s i s t e n t w i t h f l a s h i r r a d i a t i o n of the h e t e r o d i n u c l e a r dimer MnRe(CO)io that y i e l d s r a d i c a l c o u p l i n g products according to equation (2)

2*** but the p r i n c i p a l primary photoproduct suggests a mechanism other than d i s s o c i a t i v e l o s s o f CO to give M ( C 0 ) . For example, 2

0

5

2

2

0

0

3

2

9

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

88

REACTIVITY

OF

M E T A L - M E T A L

i r r a d i a t i o n of M n ( C O ) i n the presence of PPh M n ( C O ) ( P P h ) rather than the expected Mn (CO) T h i s l e d to the p o s t u l a t e d mechanism represented ( 4 ) - ( 6 ) . The s u b s t i t u t i o n l a b i l i t y of 17-valence hv Mn (CO) 2 -Mn(CO) 2

2

8

3

1Q

3

2

2

9

BONDS

leads to mainly PPh (11). by equations electron radicals 3

)

2

1Q

(4)

5

2 *Mn(CO) + 2PPh 5

2 -Mn(CO) PPh 4

•

3

•

3

2 «Mn(CO) (PPh ) 4

Mn (CO) (PPh ) 2

8

3

(5)

3

(6)

2

has been e l e g a n t l y elaborated by T, L. Brown and h i s co-workers (16, 17, 18, 19). The population of the σ o r b i t a l i n M ( C O ) obviously gives r i s e to considerabl labilit f th M - M bond Th d s t a t e of these molecule f o r photoreaction mean y y produced y i e l d s r a d i c a l s . The e x c i t e d s t a t e cleavage rates can be concluded to be >10 s" , s i n c e no emission has ever been detected from these species and the r e a c t i o n s cannot be quenched by energy t r a n s f e r . The e x c i t e d s t a t e cleavage rate of >10 s"" i s many orders of magnitude l a r g e r than from the ground s t a t e and r e f l e c t s the p o s s i b l e consequence of a one-electron e x c i t a t i o n . The question of whether both depopulation of c j ^ and population of σ are necessary i s seemingly answered by n o t i n g that i r r a d i a t i o n at the low energy t a i l of the π-d σ absorption gives a quantum y i e l d nearly the same as that f o r + σ e x c i t a t i o n . However, the r e l a t i v e quantum y i e l d s do not r e f l e c t the r e l a t i v e e x c i t e d s t a t e r a t e constants f o r r e a c t i o n . I t i s only s a f e to conclude that e i t h e r π-d + σ or o σ r e s u l t s i n dramatic l a b i l i z a t i o n compared to ground s t a t e r e a c t i v i t y and that the r a t e constant i s l a r g e enough i n each case to compete with i n t e r n a l conversion to an unreactive s t a t e . Another ambiguity i s that the s t a t e achieved by a given wavelength may not be the s t a t e from which r e a c t i o n occurs. The r e a c t i v e s t a t e s , f o r example, could be t r i p l e t s t a t e s that only give r i s e to weak absorptions from the ground s t a t e . E l e c t r o c h e m i s t r y and redox chemistry provide p o s s i b l e ways of a s s e s s i n g the consequence of one-electron depopulation or population of o r b i t a l s . The M ^ C O ^ Q species s u f f e r r a p i d M - M cleavage upon reduction (population of σ ) or o x i d a t i o n (depopulation of σ^) (20, 21). But c y c l i c voltammetry only r e v e a l s that the cleavage r a t e from the r a d i c a l anion, M ( C 0 ) io > r a d i c a l c a t i o n , M ( C O ) i t , i s >~10 s" . This i s c e r t a i n l y c o n s i s t e n t but d i r e c t measurements of the r a t e remain to be done. Comparison of the photochemistry and the thermal r e a c t i v i t y of M ( C O ) i o i s of i n t e r e s t . Considerable e f f o r t has been expended to show that thermolysis can lead to M - M bond cleavage. The a c t i v a t i o n energy from M - M d i s s o c i a t i o n has been examined and c o r r e l a t e d with the p o s i t i o n of the -> σ absorption band from Λ

2

10

1 0

1

!

b

7

2

o

r

3

2

1

0

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

0

1

5.

WRIGHTON

Metal-Metal-Bonded

E TA L .

Complexes

89

a number of d e r i v a t i v e s of Mn2(CO)io . The question i s : what i s the r e l a t i v e importance of M-ligand cleavage vs. M-M cleavage i n the ground state? A recent report bears on t h i s question. Thermal r e a c t i o n of MnRe(CO)i with PPh was examined (22). The i n t e r e s t i n g f i n d i n g i s that the primary products do not include M n ( C O ) ( P P h ) which would be expected i f Mn(C0) were produced i n the rate l i m i t i n g step (see equations ( A ) - ( 6 ) ) . Thus, the ground state pathway f o r t h i s complex appears to be mainly CO l o s s , not Mn-Re bond cleavage. The p h o t o r e a c t i v i t y appears to be dominated by Mn-Re bond cleavage (11). A d i f f e r e n c e i n M-ligand vs. M-M cleavage i s seemingly e s t a b l i s h e d . The e x c i t e d s t a t e r e s u l t s i n a r e l a t i v e l y low b a r r i e r to M-M cleavage compared to M-ligand cleavage. Since the e x c i t e d s t a t e r e a c t i o n i s so clean and quantum e f f i c i e n t (vide i n f r a ) i t i s tempting to g e n e r a l i z e the s p e c i f i c i t y found f o r e x c i t e d s t a t e M-M cleavage i n MnRe(CO) . But t h i s g e n e r a l i z a t i o s t a t e have a lower a c t i v a t i o f o r M-M cleavage. Such i s apparently not the case f o r c e r t a i n species such as ( n - C H ) C r ( C O ) 6 that l i k e l y e x i s t i n solution i n e q u i l i b r i u m with the ( n - C H ) C r ( C O ) r a d i c a l (23). The species [ ( r | - a l l y l ) F e ( C O ) ] a l s o e x i s t s i n the monomeric form i n s o l u t i o n (24). Weak M-M s i n g l e bonds do e x i s t and the ground s t a t e M-M cleavage can c l e a r l y dominate the chemistry. L i g h t may serve only to a c c e l e r a t e the observed r a t e . 0

2

8

3

3

2

5

f

10

5

5

2

2

5

5

3

3

3

2

There are now known a large number of other p h o t o s e n s i t i v e d i n u c l e a r complexes that can be formulated as having a 2-electron sigma bond (25-29). Designation of the e l e c t r o n i c c o n f i g u r a t i o n by the d c o n f i g u r a t i o n of the 17-valence e l e c t r o n fragment that would be obtained by homolytic cleavage allows a convenient cataloguing of the species s t u d i e d so f a r . The t r a n s i t i o n metal-metal bonded complexes i n the summary, Table I , have a l l

transition. G e n e r a l l y , a ττ-d ~> 0 * feature i s a l s o observable at lower energy than the a -+ σ*, and i r r a d i a t i o n at t h i s energy r e s u l t s i n homolysis but with a somewhat lower quantum y i e l d . None of the complexes l i s t e d are complicated by b r i d g i n g ligands (see succeeding section) and none give s i g n i f i c a n t y i e l d s f o r CO l o s s as the primary photoprocess. A l l of the M-M s i n g l e bonded complexes can be formulated w i t h i n the framework suggested by Scheme I I . The 17-valence e l e c t r o n r a d i c a l s studied a l l have about the same group electro­ n e g a t i v i t y based on the a b i l i t y to p r e d i c t the p o s i t i o n o f the b * absorption of h e t e r o d i n u c l e a r dimers, M-M from the a -+ σ energy i n M-M and M -M (28). S i g n i f i c a n t i o n i c c o n t r i b u t i o n to the M-M bond w i l l l i k e l y r e s u l t i n a s i t u a t i o n where M-M bond cleavage i s h e t e r o l y t i c or where some other r e a c t i o n , M-ligand cleavage f o r example, dominates the e x c i t e d b

G

σ

f

f

f

b

1

?

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

90

REACTIVITY

Scheme I I .

O F

M E T A L - M E T A L

BONDS

General one-electro s i n g l e bonded complexes.

s t a t e processes. I r r a d i a t i o n of the (n-C H )M(C0) X (M = Mo, W) (30), ( n - C H ) F e ( C 0 ) X (31), or Re(C0) X (32) (X = h a l i d e or pseudohalide) y i e l d s CO e x t r u s i o n ; i n these cases the -*· σ t r a n s i t i o n i s very high i n energy. Complexes that e x h i b i t a formal two-electron sigma bond between a t r a n s i t i o n metal and main group metal are of i n t e r e s t but have r e c e i v e d r e l a t i v e l y l i t t l e study. Species such as (CH ) SnMn(C0) are thermally q u i t e rugged but are photo­ s e n s i t i v e (33). I r r a d i a t i o n of (n-C H )Mo(CO) SnMe i n the presence of P(OPh) y i e l d s CO s u b s t i t u t i o n (34). The question of course i s whether the primary photoreaction i s l o s s of CO or homo l y s i s of the Sn-M bond. The complexes R SnCo(C0) Li*_ represent an i n t e r e s t i n g s e t of Sn-M bonded complexes, because the 17-valence e l e c t r o n Co-fragment £s of d^ c o n f i g u r a t i o n and the complex has only one l o w - l y i n g σ o r b i t a l . For example, Scheme I I I shows a simple, but very adequate, one-electron l e v e l diagram f o r Ph SnCo(C0) L. I r r a d i a t i o n of t h i s complex does not l e a d to e f f i c i e n t formation of C o ( C 0 ) L that would be an expected c r o s s - c o u p l i n g product from photogenerated r a d i c a l s . Rather, CO s u b s t i t u t i o n occurs with a quantum y i e l d of -0.2 a t ^ 366 nm (35) . The lowest e x c i t e d s t a t e i s very l i k e l y a π-d + ö e x c i t a t i o n and does not appear to r e s u l t i n s u f f i c i e n t Sn-M l a b i l i t y t o compete with Co-CO cleavage w i t h i n the e x c i t e d s t a t e l i f e t i m e . The r e l a t e d system R SiCo(C0) l i k e w i s e only gives CO e x t r u s i o n upon p h o t o e x c i t a t i o n (36). In both of these systems the -»· σ i s l i k e l y much higher i n energy than the π-d ->· σ t r a n s i t i o n s . Even i f the -** σ * e x c i t a t i o n i s achieved i t i s not c l e a r the Sn-M or Si-M bond cleavage can compete with M-CO cleavage rates i n the e x c i t e d s t a t e . I t appears that bonds between group IV elements and t r a n s i t i o n metal carbonyls are not e f f i c i e n t l y cleaved compared to the e f f i c i e n c y f o r M-CO cleavage. 5

5

3

3

5

2

5

3

5

5

5

5

3

3

3

3

3

n

3

2

3

6

2

tt

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

n

5.

WRIGHTON

E T

Metal-Metal-Bonded

AL.

L(OC) Co

L(OC)

3

Complexes

Co

SnPh O ILL 1 1

3

91

C£^

SnPh,^3

σ d 2 ζ

-4-1

b d

d

2

xy, x -y d

d

x , yz Z

2

"H"

"H"

-ff^tt-

"H"

Scheme I I I .

One-electro symmetry d^, 17-valence coupled to Ph~Sn.

electron radical

b. Photochemistry of Bridged, M-M S i n g l e Bonded Complexes. The unsupported M-M s i n g l e bonds, Table I, give c l e a n rupture of the M-M bond upon p h o t o e x c i t a t i o n . Bridged M-M bonds represent a s i t u a t i o n that could d i f f e r s i g n i f i c a n t l y s i n c e the b r i d g i n g group could prevent the d i s s o c i a t i o n of the 17-valence e l e c t r o n fragments. The doubly CO-bridged C o ( C O ) , and (n-C H ) 2 M 2 (CO) i* (M = Fe, Ru) s p e c i e s have been examined. Of these, the most i n t e r e s t i n g i s (n-CsHs) 2 R U 2 (CO) ι» which e x i s t s as a non-bridged or bridged s p e c i e s , equation ( 7 ) , i n s o l u t i o n at 298 K. The 2

8

5

5

r a t i o of the two forms depends on the solvent with alkane s o l v e n t s g i v i n g approximately a 1/1 r a t i o , w h i l e p o l a r s o l v e n t s such as CH CN give n e a r l y 100% of the bridged form (37, 38)„ The 366 nm quantum y i e l d f o r r e a c t i o n according to equation (8) i s 3

(n-c H ) Ru (co) 5

5

2

2

^ • 2 (n-c H )Ru(:co) ci — 4 solvent h

4

0

>

1

c

c

l

5

5

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

(8)

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

6

2

7

5

5

7

3

3

9

9

2

5

7

(d -d )

7

N

(L=P(n-Bu) ,Ρ(OPh) (ά -d )

5

(M = Mo,

2

W)

3

3

5

5

5

5

T

3

T

5

(M=Mo,W; M =Mn,Re)

5

(n-C H )M(C0) M (C0)

5

7

9

5

7

(d -d )

(n-C H )Fe(C0) Co(C0) (P(0Ph) ) ( d - d )

5

J ,5 ( n - c H ) F e ( c o ) M ( c o ) ( n - c H ) (d'-d-O

5

7

(d -d )

7

(n-C H )Fe(CO) Mn(CO)

2

1()

5

(M=Mo, W ) ( d - d )

11

d )

(M=Mn, R e ) ( d - d )

2

MnRe(CO)

1()

5

Co (CO) L

2

M (CO)

5

6

-

(n-C H ) M (CO)

n

Photochemistry of Dinuclear, M-M

Complex ( d

Table I-

M-M Bond Cleavage (26) (from c r o s s - c o u p l i n g and halogen atom r e a c t i o n s )

1

Fe-Co Bond Cleavage (29) (from c r o s s - c o u p l i n g r e a c t i o n )

Fe-M Bond Cleavage (27,28,40) (from c r o s s - c o u p l i n g and halogen atom a b s t r a c t i o n reactions)

S i g n i f i c a n t l y lower quantum^yield f o r ^ π - d -> σ vs. a -*· σ . b

Heterodinuclear but same d c o n f i g u r a t i o n at each center.

Fe-Mn Bond Cleavage (40) (from c r o s s - c o u p l i n g r e a c t i o n s )

n

Halogen atom a b s t r a c ­ t i o n products not s t a b l e at 298 K.

Electronic structure w e l l - e s t a b l i s h e d (8,9),

M-M or Re-Mn Cleavage (11) (from c r o s s - c o u p l i n g and trapping r e a c t i o n s ) Co-Co Bond Cleavage (29) (from c r o s s - c o u p l i n g r e a c t i o n s )

P o p u l a t i o n of ->• σ gives higher quantum y i e l d than π - d σ .

Comment

M-M Bond Cleavage (25) (from c r o s s - c o u p l i n g and trapping reactions)

(Ref.)

Species Having a Two-Electron Sigma Bond.

Primary Photoreaction

or M-M*

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

n

n

(continued)

5

3

4

5

3

3

9

-main group)

5

(d -d )

3

(d -main group)

(d -main group)

3

Co(CO) (P(0Ph) )SnPh

5

Mn(C0) SnMe

5

(n-C H )Mo(CO) SnMe (d

5

(n-c H )M(co) co(co)

Complex ( d - d )

Table I,

Loss o f CO (35) (from p h o t o s u b s t i t u t i o n )

Not w e l l - e s t a b l i s h e d (33)

Loss o f CO (34) (from p h o t o s u b s t i t u t i o n )

M-Co Bond Cleavage (27,28,40) (from c r o s s - c o u p l i n g and halogen atom r e a c t i o n s )

Primary P h o t o r e a c t i o n (Ref.)

Loss o f CO or Mn-Sn cleavage would rationalize results.

Comment

94

REACTIVITY

OF

M E T A L - M E T A L

BONDS

e s s e n t i a l l y the same i n CH3CN and i n isooctane where the f r a c t i o n of i n c i d e n t l i g h t absorbed by the non-bridged form changes d r a m a t i c a l l y (38). The l a c k of a change i n the quantum y i e l d suggests that the bridged species i s cleaved as e f f i c i e n t l y as the non-bridged species by e x c i t a t i o n corresponding to t r a n s i t i o n s that lead to population of the O o r b i t a l . The (n-C5H5)2Fe2(C0)i* i s f u l l y bridged i n any s o l v e n t . I r r a d i a t i o n at 298 K i n the presence of CCli* a c c e l e r a t e s the thermal r a t e of formation of (n-C H )Fe(CO) C1 (38, 39). Lightinduced c r o s s - c o u p l i n g of (r)-C H ) F e (CO) with the r a d i c a l precursors M ( C O ) , (Μ = Mn, Re) ( n - C H ) M ( C 0 ) CM = Mo, W) i s observed (28, 40), c o n s i s t e n t with c l e a n photogeneration of the (n-C H )Fe(C0) radical. I r r a d i a t i o n of ( n - C H ) F e ( C O ) at low temperature i n s o l u t i o n s c o n t a i n i n g P ( 0 R ) r e s u l t s i n an i n t e r ­ mediate hypothesized to be a CO-bridged species that has a ruptured Fe-Fe bond (41) f o r the r e a c t i o n s of photoexcite invoking the prompt photogeneration of f r e e r a d i c a l s . However, the r a d i c a l c r o s s - c o u p l i n g i s l i k e l y best explained by the u l t i m a t e generation of the f r e e r a d i c a l s but t h i s may not be required i f the l i f e t i m e of the CO-bridged species not c o n t a i n i n g the Fe-Fe bond i s s u f f i c i e n t l y long. In any event, population of s t a t e s where the σ o r b i t a l i s populated leads to a s i g n i f i c a n t l y weakened M-M i n t e r a c t i o n and net cleavage does occur. Prompt CO e j e c t i o n does not seem to be an important process f o r e i t h e r the Fe or Ru s p e c i e s . 5

5

2

5

5

2

5

2

2

h

1 0

5

5

5

2

2

2

6

5

5

2

2

k

3

The C o ( C 0 ) a l s o e x i s t s as bridged and non-bridged forms i n s o l u t i o n (42). I r r a d i a t i o n of C o ( C 0 ) and ( n - C H ) M ( C O ) (M = Mo, W) leads to the expected r a d i c a l coupling product (n-C H )M(CO) Co(C0K (28). But the quantum e f f i c i e n c y i s not known. I r r a d i a t i o n of C o ( C 0 ) at low temperature has been shown to lead to l o s s of CO, equation ( 9 ) , and the r e a c t i o n i s 2

8

2

5

5

8

5

5

2

2

6

3

2

8

low

hv temperature matrix

8

Co (C0) 2

?

+

(9)

CO

wavelength dependent but quantum y i e l d s have not been reported (43). Reaction according to equation (9) leads to the suggestion that d i s s o c i a t i v e l o s s of CO can become the dominant photoreaction when the cage e f f e c t becomes severe enough to preclude separation of the 17-valence e l e c t r o n r a d i c a l s . The importance of the b r i d g i n g CO s i n 298 K s o l u t i o n s i s d i f f i c u l t to assess at t h i s time. Some of the usual chemical probes are u n s a t i s f a c t o r y here because the C o ( C O ) i s l a b i l e thermally i n the presence of p o t e n t i a l e n t e r i n g l i g a n d s . The CoiCO)^ r a d i c a l does not seem to react r a p i d l y enough with CCl^ to provide a reasonable measure of the e f f i c i e n c y of the photogeneration of Co(C0) (27^ ^ 8 ) . The s i t u a t i o n i s f u r t h e r complicated by the f a c t that the C (CO),,X (X = C l , Br, I) species are thermally l a b i l e , even i f formed. 1

2

8

u

0

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5.

WRIGHTON

Metal-Metal-Bonded

E T A L .

Complexes

95

I n t u i t i v e l y , i t would seem that b r i d g i n g ligands would r e s u l t i n s i g n i f i c a n t l y lower quantum y i e l d s f o r M-M cleavage. Such i s not found experimentally. I f a r e t a r d i n g e f f e c t i s to be q u a n t i t a t e d i t w i l l l i k e l y come from a d i r e c t measurement o f the e x c i t e d s t a t e rate constants. Table I I summarizes the key r e s u l t s t o date. c. Photochemistry of Dinuclear Complexes Having M u l t i p l e M-M Bonds. The cornerstone example of strong, m u l t i p l e M-M bonds i s the Re = Re quadruple bonded R e C l ~ (44). T h i s substance was found to s u f f e r Re = Re bond cleavage from an upper e x c i t e d state produced by o p t i c a l e x c i t a t i o n i n CH CN s o l u t i o n , equation (10) (45). The lowest e x c i t e d s t a t e , a s s o c i a t e d with the 2

2

8

3

R e

2

C 1

2

8 "

CH^CN ' 2 R e C l ( C H C N ) 4

3

(10)

2

δ δ t r a n s i t i o n (46) d i s r u p t the bond enough to y i e l d cleavage. Even the upper e x c i t e d s t a t e s u f f e r s cleavage v i a a t t a c k of the CH CN as determined from f l a s h p h o t o l y s i s s t u d i e s (47), The photoinduced cleavage places an upper l i m i t on the Re = Re bond energy, but the energetics are obscured by the f a c t that r e a c t i o n i s not d i s s o c i ­ a t i v e homolytic cleavage. The ^ e C l " and R e B r ~ were found to be luminescent from the δ-* δ e x c i t e d s t a t e (48, 49). In the f i r s t report (48) d e s c r i b i n g the low temperature (1.3 K) emission of R e X " (X = CI, Br) and M o C l ~ the emission of -100 ns l i f e t i m e was a t t r i b u t e d to the t r i p l e t s t a t e , but subsequently (49) i t was reported that the complexes R e X " and M o C l ( P R ) i are emissive i n f l u i d s o l u t i o n at 298 Κ with l i f e t i m e s i n the 50-140 ns regime. The d e t a i l e d study (49) l e d to the conclusion that the emission originates from a d i s t o r t e d singlet s t a t e . Related Mo=Mo quadruple bonded complexes apparently do emit from a t r i p l e t state at low temperature (4§ with a l i f e t i m e of ~2 ms for Mo^O^CF^ at 1.3 K. The Mo CliiI^ ) species have been found to undergo photooxida^ion in chlorocarbon solutLcn (4,5p, but like the photochemistry for Re2X " &5, h]), the react ion occurs from an upper e x c i t e d s t a t e . The Mo=Mo quadruple bonded species Κι+Μο (S0i») i» has been i r r a d i a t e d i n aqueous s u l f u r i c a c i d s o l u t i o n (50). The photo­ chemistry proceeds according to equation (11). The disappearance 3

2

2

2

2

2

8

2

8

l+

8

2

8

2

2

2

1+

3

t

2

8

8

3

4

2

2

H

(aq)

+

M o

2

( S 0

)

A

4 4 "

2

hE

+

3

Mo (S0 ) " 2

4

4

(11)

quantum y i e l d at 254 nm was found to be 0.17, but the r e a c t i o n can also be e f f e c t e d with v i s i b l e l i g h t . This system exemplifies the notion that i f the l i f e t i m e of the e x c i t e d s t a t e i s not c o n t r o l l e d by d i s s o c i a t i v e M-M bond cleavage, then bimolecular processes may be p o s s i b l e . The M o ( S C \ ) i s another example of a m u l t i p l e M-M bond that cannot be e f f i c i e n t l y cleaved i n a d i s s o c i a t i v e manner. S i m i l a r l y , p h o t o e x c i t a t i o n of Μο Χ **" (X = C l , Br) i n aqueous acid r e s u l t s i n photooxidation. Clean formation of Mo Cl H can be observed. Again, rupture of the Μ Ξ Μ quadruple bond i s not found (50). 2

2

0

3

2

8

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5

2

2

5

5

2

Co (CO)

n

8

2

2

( -C H ) Ru (CO)

5

4

4 5

2

2

hv

5

2

2

5

2

2

3

2

2

( C O )

5

+ 3

(28) A

8

2

5

2

2

2

4

+

5

5

5

2

C o

5

2

2

( C O

n

5

5

6

>7

+

C

0

5

5

2

(

3

—

}

4

Comment

(27,28)

Low temperature form i s f u l l y bridged; room temperature form i s 1/1 i n hydrocarbon solution.

Quantum y i e l d independent of whether 1/1 bridged/nonbridged or 100% bridged,

CO-bridged, Fe-Fe cleaved intermediate observed at low temperature (40).

Implies formation of free r a d i c a l s .

08,39) Quantum y i e l d reasonable but thermal r e a c t i o n i s detectable suggesting r e l a t i v e l y small degree of l a b i l i z a t i o n upon photoexcitation.

2(n-C H )Ru(CO) Cl (38)

(38,AO)

hv

2

6

2

2( -C H )M(CO) Co(CO)

2

2

3

3

P(OR)

5

(n-c H ) M (co)

8 low temp/ matrix

3

5

hv hydrocarbon

co (co)

C o

5

5

(n-c H ) M (co)

( -C H ) Ru (CO) - ^ - ^

n

5

+

5

2(n-C H )Fe(CO) Cl

2 (n-C H )Fe(CO) -

4

4

(n-C H ) Fe (CO) P(OR)

5

(ri-C H ) Fe (CO)

5

Mo(n-C H )(CO)

hydrocarbon

5

(n-c H ) Fe (co)

5

hV (n-C H ) Fe (CO) —

P h o t o r e a c t i o n (Ref.)

Photochemistry of Bridged D i n u c l e a r Metal-Metal Bonded Complexes.

(n-C H ) Fe (CO)

Complex

Table I I ,

NO

Η > r α Ο ζ σ

m

Η > r ι

m

O τι

H «


as

WRIGHTON

5.

E T

Metal-Metal-Bonded

AL.

Recent s t u d i e s of (η-C R ) C r (CO)^ (R = that are formulated as show that the dominant to equation (12) (53). 5

5

2

2

5

5

2

97

the hydrocarbon s o l u b l e complexes H, Me) and ( -C ) (ri-C Me ) C r (CO),, , having a Cr Ξ Cr t r i p l e bond, (51_, 52) photoreaction i s l o s s of CO according The t r i c a r b o n y l species accounts f o r the n

( -C R ) Cr (CO) n

Complexes

2

4

a

l

^

n

e

5

5

•

5

2

( -C R ) Cr (CO) CO n

5

5

2

2

3 +

(12)

i n c o r p o r a t i o n of CO upon i r r a d i a t i o n of (rj-C R5 ) C r (CO)^ under a C 0 atmosphere. I r r a d i a t i o n of (ri-C H ) (n-C R5 ) C r (C0) produces no d e t e c t a b l e amount of ( - C H ) C r (C0) or (r]-C Me ) C r (C0) and no measurable ( -C H )(n-C Me )Cr (CO) i s formed when a 1/1 mixture of the symmetrical s p e c i e s i s i r r a d i a t e d . The negative r e s u l t s from the attempted c r o s s - c o u p l i n g experiments r u l e out an important r o l e f o r the prompt generatio p h o t o e x c i t a t i o n of the quantum y i e l d f o r l o s s of CO, equation (12), i s wavelength dependent; i r r a d i a t i o n at the lowest absorption (-600 nm) r e s u l t s i n no r e a c t i o n , but near-uv i r r a d i a t i o n gives a quantum y i e l d of >10" (53). The wavelength dependence i s reminiscent of Co (C0)e i n low temperature matrices where the p o t e n t i a l fragments are tethered by the b r i d g i n g groups and the cage e f f e c t s o f the matrix (43). I r r a d i a t i o n of the ( n - C H ) V ( C O ) , V=V double bonded species r e s u l t s i n l o s s of CO, not V=V bond cleavage upon photoe x c i t a t i o n (54). The c r u c i a l r e s u l t comes from an experiment where the (n-CsHs) V (CO)5 i s i r r a d i a t e d at -50°C i n the presence of P E t P h i n THF s o l u t i o n . The only product observed i s (n-C H )2V (C0) PEt Ph. From the s t u d i e s of the quadruple, t r i p l e , and double bonded complexes examined thus f a r , i t w i l l prove d i f f i c u l t to photochemically cleave m u l t i p l e metal-metal bonds i n a d i s s o c i a t i v e f a s h i o n . Photochemical cleavage of m u l t i p l e bonds i s not taboo, since the l i g h t induced cleavage of O 2 and C O 2 i s w e l l known. But i n metal complexes i t appears that other processes such as solvent attack on the e x c i t e d s t a t e , e l e c t r o n t r a n s f e r , and l i g a n d d i s s o c i a t i o n can lead to e x c i t e d s t a t e d e a c t i v a t i o n before bond rupture can occur. As seen i n the summary i n Table I I I , d i s s o c i ­ a t i v e cleavage of a m u l t i p l e metal-metal bond remains to be accomplished. A l s o , upper e x c i t e d s t a t e r e a c t i o n i s the r u l e i n the m u l t i p l e bonded systems. 1 3

5

2

2

13

5

n

5

5

n

5

5

5

2

2

2

5

u

u

5

5

5

2

5

2

2

k

l|

2

2

5

2

5

2

2

5

2

2

5

5

2

t t

2

d. Photochemistry of D i n u c l e a r M-M Bonded Complexes Having Charge T r a n s f e r Lowest E x c i t e d S t a t e s . Complexes such as Re(CO) Re(C0) (l,10-phenanthroline) (55) and Ph SnRe(C0) 3(1,10-phenanthroline) (7, 56) and related canplexes can be formulated as having a M-M or M -M s i n g l e bond. Table IV summarizes the known photoprocesses f o r such complexes. The lowest e x c i t e d s t a t e i n t h ^ complexes has been i d e n t i f i e d as a r i s i n g from a (Μ-Μ)σ^ •> π phen charge t r a n s f e r t r a n s i t i o n (2, 55, 56). Importantly, the o r b i t a l of termination of the 5

3

3

f

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8

5

4-

4

3

2

2

5

5

2

2

(r,-C H ) V (CO)

(R = Me, H)

5

5

(n-C R ) Cr (CO)

2

Mo X

2

4

4

Mo Cl (P(n-Bu) )

2

Re Clg

2-

Complex

4

4

4

M-M Bond Order

Q

3

2

Loss of CO (54)

D i s s o c i a t i v e l o s s o f CO from upper e x c i t e d s t a t e s (53)

2

Emits a t 1.3 K; photooxidized i n H 0 t o give M o X H - (48^ 50)

Emits at 298 K o r lower (49); Photooxidized i n chlorocarbons (4, 50)

Solvent a t t a c k o f excited state leads to Re^Re Cleavage (45, 47)

Primary P h o t o r e a c t i o n (Ref.)

Table I I I . Photochemistry o f D i n u c l e a r M-M Multiple-Bonded Complexes.

ζ α

O

r

H >

m

k

H >

m

O •n

H ·
o

00

5.

WRIGHTON

Table IV.

E T

Metal-Metal-Bonded

A L .

Complexes

99

Primary E x c i t e d State Processes of ~MRe(C0) (l,10-phenanthroline) (2, 3

Complex

55, 56)

Photoprocess

Re(CO) Re(CO) (l:,10-phenanthroline) 5

3

Emission at 77 K Re-Re Bond D i s s o c i a t i o n a t 298 K

Ph ERe(C0) (l,10-phenanthroline) 3

3

(E = Sn, Ge)

Emission at 298 or 77 K Ε-Re Bond D i s s o c i a t i o n E l e c t r o n i c Energy T r a n s f e r Electron Transfer

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

100

REACTIVITY

OF

M E T A L - M E T A L

BONDS

1

t r a n s i t i o n i s not the σ* o r b i t a l associated with the M-M or M-M sigma bond. The o r b i t a l of termination i s l o c a l i z e d on the charge acceptor l i g a n d and population of i t i s not expected to s e r i o u s l y d i s r u p t the M-M or M-M bonding. Consistent with t h i s a s s e r t i o n i s the f a c t that the complexes are r e v e r s i b l y r e d u c i b l e on the c y c l i c voltammetry time s c a l e (56). However, the o x i d a t i o n of the complexes i s not r e v e r s i b l e and c y c l i c voltammetry shows that M-M or M-M* cleavage occurs f o r the r a d i c a l monocation at a r a t e of >10 s' (56). The lowest energy e l e c t r o n i c t r a n s i t i o n of the R e ( C 0 ) (1,10-phenanthroline) and r e l a t e d complexes i s expected to l a b i l i z e the M-M bond i n the sense that the M -core i s " o x i d i z e d " by the intramolecular s h i f t i n e l e c t r o n density. E x c i t a t i o n has been shown to y i e l d M-M bond cleavage; r e a c t i o n according to equation (13) i s r e p r e s e n t a t i v e (55). The quantum y i e l d of -0.2 f

3

1

2

8

2

Re (CO) (l,10-phenanthroline 0

Q

£

o

CCI

4 (1,10-phenanthroline)

(13)

was found to be independent of the e x c i t a t i o n wavelength from 550 nm to 313 nm. This wavelength independent quantum y i e l d £s consistent with r e a c t i o n that o r i g i n a t e s from the (Μ-Μ)σ^ •> π phen CT s t a t e that i s found to y i e l d emission (but no photoreaction) at low temperature. This r e s u l t shows that depopulation of an M-M core bond l e v e l l a b i l i z e s the M-M bond s u f f i c i e n t l y to allow cleavage w i t h i n the l i f e t i m e of the e x c i t e d s t a t e . The 77 K emission l i f e t i m e was found to be e x t r a o r d i n a r i l y long (~95 Psec), but emission was not detectable at a l l at 298 K where photoreaction occurs with a quantum y i e l d of -0.20. I t i s p o s s i b l e that emission i s not observable at 298 K because M-M bond cleavage occurs too f a s t ; t h i s would i n d i c a t e that the M-M d i s s o c i a t i o n rate i n the e x c i t e d s t a t e s i g n i f i c a n t l y exceeds 10 s " . Given that the ground s t a t e d i s s o c i a t i o n i s slow (

M\T

(15)

3

+ [Ph SnRe(C0) L J 3

(16)

3

,

TMPDEN,N,N\N -tetramethyl^-pheny3enedLani\e; ^^l^Mimed^^,4-b^3ridnium s i g n i f i c a n t l y downhill and the rate constants k and k a r e those expected f o r d i f f u s i o n c o n t r o l l e d r e a c t i o n s . The r e d u c t i o n to form [ P h S n R e ( C 0 ) L ] r e s u l t s i n no net chemical change, s i n c e the back e l e c t r o n t r a n s f e r i s f a s t and the e l e c t r o n added to the Re complex i n equation (15) i s l o c a l i z e d on L . The e x c i t e d s t a t e o x i d a t i o n though r e s u l t s i n net chemistry, s i n c e chemistry according to equation (17) competes with the back e l e c t r o n 1

5

1 6

t

3

[Ph SnRe(C0) J L ] * 3 3 Q

3

— S = solvent

Ph.Sn' + S R e ( C 0 ) L 3 3

Λ

+

(17)

Q

3

1

t r a n s f e r . The unimolecular rate constant k i s >10 s"" from the electrochemistry and from the photochemistry (equation (14)) the value of k could be greater than 10 s"" . The one-electron oxidants lead to production of 18-electron S R e ( C 0 ) L products; + t h i s leads to the c o n c l u s i o n that the cleavage of [Ph ERe(C0) L]* y i e l d s Ph E- and Re(CO) L , not Ph Sn + ^and -Re(C0) L. The experiments with the various^ —M-Re(CO) L s p e c i e s e s t a b l i s h e s that population of the σ l e v e l i s not r e q u i r e d to achieve s u f f i c i e n t M-M bond l a b i l i t y to y i e l d homolytic cleavage w i t h i n the l i f e t i m e of the e x c i t e d s t a t e . However, when the sigma bond order i s reduced from one to approximately one-half by the depopulation of the l e v e l , the rate constant f o r M-M bond cleavage appears to be only ~10 s " . By way o f c o n t r a s t , M-M bond cleavage seems to occur with a rate of >10 s for species J ?

5

1

J 7

+

3

3

+

3

3

3

3

3

5

1

T

1

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3

102

REACTIVITY

such as R e ( C O ) to zero by the 2

Photochemistry

OF

M E T A L - M E T A L

where the sigma bond order i s reduced Ο excitation.

1 0

from

BONDS

one

of T r i n u c l e a r Complexes

Table V summarizes the key photochemistry of t r i n u c l e a r metal-metal bonded complexes. The f i r s t noteworthy photochemical study of t r i n u c l e a r complexes concerns R u ( C O ) . T h i s s p e c i e s was found to undergo d e c l u s t e r i f i c a t i o n to mononuclear fragments when i r r a d i a t e d i n the presence of e n t e r i n g l i g a n d s such as CO, PPhg, *" ethylene. The i n t r i g u i n g f i n d i n g i s that thermal r e a c t i o n of R u ( C 0 ) with PPh r e s u l t s i n the s u b s t i t u t i o n product i n d i c a t e d i n equation (18) whereas i r r a d i a t i o n y i e l d s the mono­ nuclear species given by equation (19) (57). These r e s u l t s 3

1 2

0

3

R

1 2

u

3

(

C

O

3

)

Ru (CO) 3

1 2 m ^

Ru (CO) (PPh )

1 2

Ru(C0) PPh 4

3

(18)

+ Ru(CO) (PPh ) 3

3

2

(19)

suggest that Ru-Rn bond cleavage occurs i n the e x c i t e d s t a t e whereas Ru-CO d i s s o c i a t i o n occurs i n the ground s t a t e . An e l e c t r o n i c s t r u c t u r a l study shows that the o r b i t a l of termination f o r the lowest energy e x c i t e d s t a t e s of Ru3(C0)i2 i s Ο with respect to the Ru3-core (58). The disappearance quantum y i e l d s for R u ( C 0 ) i , F e ( C O ) and R u ( C 0 ) ( P P h ) are a l l i n the range of 10~ f o r e n t e r i n g groups such as CO or PPh and are independent of e n t e r i n g group c o n c e n t r a t i o n (59, 60). The electronic s t r u c t u r e i s c o n s i s t e n t w i t h primary photoreaction as represented by equation (20), and the o v e r a l l low quantum y i e l d s are c o n s i s t e n t 3

2

3

1 2

3

9

3

3

2

3

hv

. \v/-

ι

γ
12

L=P(0Me) , P P h 3

3

' W

C

0

> 1 1

L

(

2

6

)

3

r e a c t i o n has the same quantum e f f i c i e n c y (5 + 1 χ 10" ) f o r L = P(OMe)3 o r PPh3 and f o r a v a r i a t i o n i n concentration of L from 0.01 t o 0.1 M. These observations support the prompt generation o f H u R u i ^ C O ) 1 undergoes s u b s t i t u t i o n the r a t e d r a m a t i c a l l y . Photoexcitatio o ,0s *(C0) 1 does give chemistry and photoreaction may begin with d i s s o c i a t i v e l o s s of CO subsequent to p h o t o e x c i t a t i o n (69). I r r a d i a t i o n of Hi»Rei»(C0) 1 2 does not r e s u l t i n any s i g n i f i c a n t photoreaction (70), The l a c k o f any Re-Re bond cleavage may be a s s o c i a t e d with the f a c t that the Re-Re bonds have m u l t i p l e bond character (71) . The Hi,Rei»(C0) 1 2 e x h i b i t s emission from the lowest e x c i t e d s t a t e . This f i n d i n g prompted a comparison of the e x c i t e d s t a t e decay o f the Dt»Reu(C0) 1 2 . G e n e r a l l y , the highest energy v i b r a t i o n a l modes a r e important i n n o n - r a d i a t i v e decay (72) , and for metal complexes the highest energy M-L v i b r a t i o n s may be most important. The hydrogen atoms i n HwReu(CO) 1 2 a r e b e l i e v e d to be t r i p l y face b r i d g i n g (71) with a Re 3-H s t r e t c h i n g frequency o f -1023 cm" (73). The lUReuiCO) 1 2 complex emits i n hydrocarbon s o l u t i o n o r as the pure s o l i d a t 298 o r 77 K. The emission (-14,300 cm" ) onset overlaps the absorption and thus a l a r g e d i s t o r t i o n o f the complex upon e x c i t a t i o n does not appear to occur. The emission can be quenched by the t r i p l e t quencher anthracene, having a t r i p l e t energy o f ~42 kcal/mol (74), The l i f e t i m e i s i n the range 0.1 - 16 psec depending on c o n d i t i o n s and i s o f the order o f 20-30% longer f o r the H s u b s t i t u t e d complex. Likewise the emission quantum y i e l d s f o r the D Re„(C0) are 20-30% longer than f o r the H s p e c i e s . Thus, r e p l a c i n g U by ^ i n H^Re^CO) has the expected e f f e c t o f reducing the r a t e o f n o n - r a d i a t i v e decay. But the e f f e c t does not lead to a s i t u a t i o n where the e x c i t e d decay i s dominated by the r a d i a t i v e decay r a t e . 2

1

1

2

l

u

1 2

l

1

2

The s t u d i e s of the tetrahedrane-M c l u s t e r s , Table V I , show q u i t e g e n e r a l l y that the importance of d i s s o c i a t i v e processes w i t h i n the e x c i t e d s t a t e l i f e t i m e i s low. Clean, but low quantum y i e l d , photoreactions are detectable i n some cases. The r e s i s t a n c e of the complexes to d e c l u s t e r i f i c a t i o n may be e x p l o i t e d to study and u t i l i z e bimoleaular processes, and i n the case o f the H^Re^CO) the r e a c t i o n s and non-chemical, n o n - r a d i a t i v e decay are s u f f i c i e n t l y low that the e x c i t e d s t a t e l i f e t i m e allows k

1

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

5.

WRIGHTON

Table VI.

E T

AL.

4

107

Complexes

Photochemistr

Complex Co (CO)

Metal-Metal-Bonded

Photoprocess (Ref.) L i k e l y Co-Co Bond Cleavage; Co (CO)g

1 2

0

formed under CO HFeCo (CO) 2

12

(63)

Primary process l i k e l y M-M

Bond Cleavage

Fe,(CO),(n-CJa^)

Inert i n Hydrocarbon; Photooxidation v i a CTTS i n Halocarbon (65)

Ir^(C0)^

D e r i v a t i v e with I r ^ unit r e t a i n e d can be formed (64)

2

H Ru (CO)

1 2

Loss of CO; Low Quantum y i e l d

H Os (CO)

1 2

Loss of CO

H Re (CO)

1 2

P h o t o i n e r t ; Emits at 298 or 77 K (70)

4

4

4

4

4

4

(68)

(69)

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

(63)

108

REACTIVITY

O F M E T A L - M E T A L

BONDS

r a d i a t i v e decay to be observed. Such l o n g - l i v e d e x c i t e d s t a t e s may allow the development o f a bimolecular e x c i t e d s t a t e chemistry of such s p e c i e s . Acknowledgements. We thank the N a t i o n a l Science Foundation and the O f f i c e o f Naval Research f o r p a r t i a l support o f the research described h e r e i n . Support from GTE L a b o r a t o r i e s , I n c , i s a l s o g r a t e f u l l y acknowledged. M.S.W. acknowledges support as a Dreyfus Teacher-Scholar grant r e c i p i e n t , 1975-1980.

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December 1, 1980.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6 Thermal and Photochemical Reactivity of H FeRu (CO) 2

3

and Related M i x e d - M e t a l

13

Clusters G R E G O R Y L . G E O F F R O Y , H E N R Y C . F O L E Y , J O S E P H R. F O X , and W A Y N E L . G L A D F E L T E R Department of Chemistry, Pennsylvania State University, University Park, P A 16802

For the past few year under study i n t h i s l a b o r a t o r y from s e v e r a l viewpoints. First, we have a c o n t i n u i n g i n t e r e s t in developing b e t t e r methods f o r the d i r e c t e d s y n t h e s i s of mixed-metal c l u s t e r s and c o n s i d e r a b l e progress has been made in t h i s area (1-6). We have more r e c e n t l y been e v a l u a t i n g the r e a c t i v i t y f e a t u r e s of mixed-metal c l u s t e r s with a v a r i e t y of s u b s t r a t e s . We have chosen to concentrate our e f f o r t s on one p a r t i c u l a r c l u s t e r , H FeRu (CO) , 1, and to examine its reactivity i n as much d e t a i l as p o s s i b l e . 2

3

13

1

I t has been our aim to t r y to fully understand the k i n d of chemical r e a c t i o n s which t h i s c l u s t e r undergoes as w e l l as t h e i r mechanistic course, with the expectation that t h i s knowledge will prove a p p l i c a b l e to other c l u s t e r systems. Complete d e t a i l s of these v a r i o u s s t u d i e s will be provided i n f u t u r e , separate p u b l i c a t i o n s , but it is the purpose of t h i s article to summarize the more important r e s u l t s and to present the o v e r a l l r e a c t i v i t y p i c t u r e of H FeRu (CO) as we now p e r c e i v e it. 2

3

13

0097-6156/81/0155-0111 $06.00/ 0 © 1981 American Chemical Society

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

112

REACTIVITY OF METAL-METAL BONDS

Results and D i s c u s s i o n R e a c t i v i t y of H FeRu (CO) and Related Mixed-Metal C l u s t e r s With Carbon Monoxide. T r a n s i t i o n metal c l u s t e r compounds are c u r r e n t l y under intense s c r u t i n y as c a t a l y s t s f o r a v a r i e t y of r e a c t i o n s i n v o l v i n g carbon monoxide. These i n c l u d e the r e d u c t i o n of CO to produce hydrocarbons (7,8), a l c o h o l s (9), and ethylene g l y c o l (10), hydroformylation and r e l a t e d o l e f i n r e a c t i o n s (11,12,13), and the water-gas s h i f t r e a c t i o n (13,14,15, 16). With the i n t e r e s t in chemistry of t h i s type so i n t e n s e , it seems e s s e n t i a l that the b a s i c r e a c t i v i t y patterns of c l u s t e r s with CO be understood. We have a c c o r d i n g l y examined the r e a c t i o n s of a s e r i e s of mixed-metal c l u s t e r s with CO with the aim of understanding the types of r e a c t i o n s that can occur as w e l l as t h e i r mechanistic course I t was our f i n d i n g that most of the c l u s t e r s examine under r a t h e r m i l d c o n d i t i o n produce monomeric and t r i m e r i c products, Table I I and eqs. 1-3 (17). 2

H eRu (CO) 2 F

3

1 3

CO

+

13

[ C o R u ( C O ) f + CO 3

2

h >

^

13

c o n v

. > ^(CO)^

+ CO ^ ' S ^ ' c o L '

[HFeRu (CO) r 3

4

3

1 3

CH2

+ Fe(C0)

5

+ ^

(1)

[ H R u ^ C O ^ f + Fe(C0)

^ % y o n v . ' Ru (CO) c

3

1 2

+

5

(2)

[90% of H^FeRu3(CO)^2 ^ converted to products whereas a f t e r 100 h at 50°C only -30% of H 4 R U 4 case, the i n i t i a l l y forme spectroscopy and by a n a l y t i c a l l i q u i d chromatography, but i t s concentration d i d not b u i l d up s i n c e i t r a p i d l y reacts with CO to produce Ru3(CO)i2, R u ( C 0 ) , and H . In order to d e f i n e the mechanistic course of these r e a c t i o n s , we undertook a s e r i e s of k i n e t i c s t u d i e s on H2FeRu3(CO)i3 and f o r comparison, 1^114(00)13. The derived r a t e law which f i t the k i n e t i c data f o r fragmentation of both H Ru4(CO) 3 and H FeRu3(CO) 3 i s that shown i n eq. 7 (17). s

5

2

1

-

d

2

[

c

l

^

t

e

r

2

1

]

- ik

x

+ k [C0]}[cluster] 2

(7)

S p e c i f i c r a t e constants and a c t i v a t i o n parameters are given i n Table I I . Under the c o n d i t i o n s of our experiments the f i r s t order term i s n e g l i g i b l e and the r e a c t i o n s are e s s e n t i a l l y second order o v e r a l l . The mechanism which we b e l i e v e i s most c o n s i s t e n t with t h i s rate law and with the a c t i v a t i o n parameters obtained, Table I I , i n v o l v e s a s s o c i a t i o n of CO with the i n t a c t c l u s t e r concomitant with cleavage of one of the metal-metal bonds to give an H2M4(CO)i4 b u t t e r f l y c l u s t e r as the f i r s t intermediate. The a c t i v a t i o n p r o f i l e envisaged f o r t h i s process i s shown i n Scheme 1. This mechanism implies that a t t a c k of CO on the c l u s t e r i s concerted with metal-metal bond breakage. The l a t t e r i s necessary because the i n i t i a l c l u s t e r i s c o o r d i n a t i v e l y and s t e r i c a l l y s a t u r a t e d . A comparison of the a c t i v a t i o n parameters i n d i c a t e s that the r e l a t i v e degree of CO a s s o c i a t i o n and M-M bond cleavage i n the t r a n s i t i o n s t a t e v a r i e s from H2Ru4(CO)i3 to H2FeRu3(CO) . The l a r g e r values of ΔΗ* (20.0 kcal/mole) and the l e s s negative values of AS* (-25.4 cal/mole-K) imply that i n the t r a n s i t i o n s t a t e , M-CO bond formation occurs to a l e s s e r extent f o r H FeRu (CO) 3 than f o r H Ru4(CO) 3 (ΔΗ* =12.5 kcal/ mole; AS* = -36.6 cal/mole-K). This i s c o n s i s t e n t with the 13

2

3

1

2

1

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

a

4

1 3

1.25 χ 10

2

M.

4

30°C

[ c l u s t e r ] = 6 χ 10'

2

H Ru (CO)

1 3

70°C

3

H FeRu (CO)

2

Temp l

7

4

k 2

1

7 Q

M, [co]

3QO

=

-36.6

12.5

3

2

-25.4

20.0

1

4

[ C 0 ] o = 1.42 χ 10"

5.7 χ l C f

3.0 χ 10~

ΔΗ°*

2

(cal/mole-K)

a

(kcal/mole)

(M^sec"" )

1 3

M, n-heptane s o l u t i o n ;

0

6 χ 10"

(sec~l)

k

2

K i n e t i c Parameters f o r the Reaction of H FeRu3(CO) 3 and H Ru (CO) with C 0 (17)

Cluster

Table I I .

116

REACTIVITY

OF

M E T A L - M E T A L

BONDS

Scheme 1

n o t i o n that Ru i s l a r g e r than Fe and there i s more room on the surface of i n t a c t l ^ R u ^ C O ) ^ to accomodate the incoming CO than i n H FeRu3(CO) 3. The r e l a t i v e degree of CO a s s o c i a t i o n and M-M bond cleavage i n the t r a n s i t i o n s t a t e w i l l s u r e l y vary with the metal composition of the c l u s t e r and may account f o r the g r e a t l y d i f f e r e n t r e a c t i v i t y observed f o r the compounds l i s t e d i n Table I. 2

1

Reactions of H?FeRu (CO) -\ q With T e r t i a r y Phosphines and Phosphites. H2FeRu3(CO)i3 r e a c t s with a s e r i e s of t e r t i a r y phosphine and phosphite l i g a n d s to y i e l d mono- and d i s u b s t i t u t e d c l u s t e r s i n 20-30% y i e l d , eq. 8 (18). 3

H FeRu (CO) 2

3

13

+ PR

H FeRu (CO)

3

2

(PR )

3

3

H FeRu (CO) (PR ) 2

(PR

3

3

n

3

+ (8)

2

= PMe , PMe Ph, PMePh , P E t P h , PPh , P ( i - P r ) 3

2

2

P(0Me) , P ( 0 E t ) , 3

3

2

3

3 >

P(0Et) Ph) 2

These s u b s t i t u t i o n r e a c t i o n s proceed c l e a n l y and give only t r a c e amounts of s i d e products unless the r e a c t i o n s are c a r r i e d out under f o r c i n g c o n d i t i o n s . With the a i d of -4i and 31p N M R spectroscopy, we have been able to take advantage of the low symmetry of these d e r i v a t i v e s and determine the s p e c i f i c s i t e s at which s u b s t i t u t i o n occurs i n H2FeRu3(CO)^3. Figure 1, f o r example, shows the ! H NMR spectrum of H FeRu3(CO) 2(PMe2Ph) at -50°C which i n d i c a t e s the presence of two s u b s t i t u t i o n a l isomers f o r the compound. The intense doublet at τ 28.07 ppm 2

1

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

GEOFFROY

ET

I 28.0

AL.

HuFeRuafCOjis

and

1

Figure 1. *H NMR spectrum at of H FeRu (CO) (PMe Ph) in solution

29.0

Mixed-Metal

2

3

12

Clusters

x

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

111

-50°C CDCl

3

118

REACTIVITY

OF M E T A L - M E T A L

BONDS

(Jp-H = 10.3 hz) i s a t t r i b u t e d to the two equivalent hydrogens of the C isomer shown below while the pseudo t r i p l e t a t τ 28.35 ppm and the doublet o f doublets a t τ 28.93 ppm (Jp-H 9.7 hz; J H - H =2.6 hz) are assigned to and Hg r e s p e c t i v e l y of the isomer. s

=

S i m i l a r s u b s t i t u t i o n a l isomers r e s u l t f o r the monos u b s t i t u t e d d e r i v a t i v e s of the other P R 3 ligands but the r e l a t i v e r a t i o s of these two isomers i s g r e a t l y dependent on the nature of the p a r t i c u l a r l i g a n d . The choice of s u b s t i t u t i o n s i t e f o r a p a r t i c u l a r l i g a n d has been found to depend on both the l i g a n d s s i z e and i t s b a s i c i t y , Table I I I (18,19). With l a r g e ligands such as PPh and P ( i - P r ) , s u b s t i t u t i o n occurs to give only the C isomer regardless of the l i g a n d b a s i c i t y . Presumably there i s l e s s s t e r i c hindrance in this substitution site. For smaller l i g a n d s , b a s i c i t y becomes the c o n t r o l l i n g f a c t o r and the isomer becomes more abundant as the b a s i c i t y of the l i g a n d i n c r e a s e s . This e f f e c t i s best i l l u s t r a t e d by comparing the ligands PMe and P(0Me) , Table I I . The former i s h i g h l y b a s i c and i t gives a Οχ J C e q u i l i b r i u m constant of 0.4 whereas the smaller but l e s s b a s i c P(0Me) gives _ = 10. The increased^abundance of the isomer with i n c r e a s i n g l i g a n d b a s i c i t y presumably occurs because the semi-bridging carbonyl attached to the s u b s t i t u t e d metal can become more f u l l - b r i d g i n g and remove the excess e l e c t r o n density released by the b a s i c phosphine. Consistent with t h i s hypothesis i s the 1

3

3

s

3

3

s

3

C

s

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

GEOFFROY

E T AL.

H FeRu (CO)i3 2

s

and Mixed-Metal

Clusters

119

Table I I I . E f f e c t of Ligand Size and B a s i c i t y on the Ci t C E q u i l i b r i u m of H2FeRu (C0)i2L (18) s

3

Cone A n g l e (degrees)

(cm-1)

160

2059.2

>100

145

2068.9

>100

136

2067.0

11

136

2063.7

11

107

2079.5

10

P(OEt) Ph

116

2074.2

5.5

PMe Ph

122

2065.3

1.8

PMe

118

2064.1

0.4

L P(i-Pr) PPh

3

3

PMePh

2

PEt Ph 2

P(OMe)

3

2

2

a

3

c C

+ C s

l

R e f . 19.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

120

REACTIVITY OF

METAL-METAL

BONDS

increased downfield chemical s h i f t of one of the b r i d g i n g carbonyls i n the C NMR spectrum of H2FeRu3(CO)12(PMe Ph) (6 249 ppm) compared to I ^ F e R u ^ C O ) ^ (6 229 ppm) and a l s o the s h i f t to lower energy of the b r i d g i n g carbonyl i n f r a r e d absorption i n the C^ isomer (-1806 cm"*l vs. -1850 cm- ). In an attempt to understand the mechanism by which these s u b s t i t u t i o n r e a c t i o n s occur, a k i n e t i c study of the r e a c t i o n of PPI13 with H2FeRu3(CO)i3 i n hexane s o l u t i o n was undertaken. The k i n e t i c data obtained implied the r a t e equation shown below, eq. 9, with k± = 6.96 χ 10~ s e r at 50°C (18). 1 3

2

1

4

1

d[H FeRu (CO) ^

] = k^FeRu^CO)^]

(9)

The r a t e of s u b s t i t u t i o but zero order with respec concentrations. The a c t i v a t i o n parameters obtained f o r t h i s s u b s t i t u t i o n are Δ Η ° =25.7 kcal/mole and AS°* =4.8 cal/moleK. The zero-order dependence on [ΡΡΙΊ3], the p o s i t i v e value of AS°*, and the decrease i n the r e a c t i o n r a t e under a CO atmosphere a l l argue f o r a d i s s o c i a t i v e mechanism i n which the rate-determining step i s l o s s of CO l i g a n d from H2FeRu3(CO); 3. This would generate an unsaturated c l u s t e r that could r a p i d l y add PPI13 to give the monosubstituted d e r i v a t i v e , Scheme 2. +

L

Scheme 2 k-

(slow)

H FeRu (CO)

1 3

,

H FeRu (CO)

1 2

+ PPh,

2

2

3

3

H FeRu (CO)

k

2

3

+

u

CO

H^eRu^CO^PPt^

Reaction of H FeRu3(CO)i3 With Alkynes. Isomeric products a l s o r e s u l t from the r e a c t i o n of I^FeRi^(CO)-^ with a s e r i e s of i n t e r n a l alkynes, eq. 10 (20). 2

H FeRu (CO) 2

3

1 3

+ RCECR

1

-> Η

£

+ CO

+ FeRu (CO) 3

(RCECR ) 1

1 2

(10)

(isomers) H2 i s evolved during the course of these r e a c t i o n s and the isomeric FeRu3(CO)i2(^=OR ) products are conveniently separated by l i q u i d chromatography on s i l i c a g e l . With PhCECPh, two isomers of FeRu3(CO) 2(PhCECPh) are obtained i n an o v e r a l l y i e l d of 65%. Reaction with MeC^CMe gives two analogous isomers of F e R u 3 ( C O ) i 2 ( )> * isomers of FeRu3(CO)i2( ) r e s u l t from the r e a c t i o n with MeCECPh. ?

1

M e C E C M e

a n (

t

n

r

e

e

M e C E C P n

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

GEOFFROY

H?JeRu (CO)>,and

E T AL.

Mixed-Metal

0

Clusters

121

n

a

v

The s t r u c t u r e s of the isomers of FeRu3(CO)i2(PhCECPh) been determined by x-ray d i f f r a c t i o n (21, 22). They are isomorphous and d i f f e r only i n the p o s i t i o n i n g of the Fe atom as i n d i c a t e d by the " a x i a l " and " e q u a t o r i a l " l a b e l s given i n the drawings below.

e

/I \ "axial" isomer

"equatorial" isomer

In essence the alkyne has i n s e r t e d across a metal-metal bond to give a closo-FeRu3C2 c l u s t e r . Such a s t r u c t u r e i s f u l l y c o n s i s t e n t with Wade s s k e l e t a l e l e c t r o n counting r u l e s f o r a c l o s o s t r u c t u r e with 6 v e r t i c e s (23,24). The three isomers of FeRu3(CO)i2( h ) a r i s e because i n the e q u a t o r i a l isomer the methyl s u b s t i t u e n t can occupy a p o s i t i o n c i s or trans to the Fe atom. I n t e r e s t i n g l y , we observed that the isomers of these FeRu3(CO)i2( ) c l u s t e r s r e a d i l y i n t e r c o n v e r t upon h e a t i n g to y i e l d an e q u i l i b r i u m mixture, e.g., eq. 11 (20). 1

M e C E C p

R C E C R l

7n°r FeRu (CO) (PhCECPh) ^ * FeRu.(CO)- (PhCECPh) hexane equatorial K q ~ 9.0 axial / u

Q

J

u

10

0

i Z

J

(11)

± Z

e

p h C E C P h

t l i e

For F e R u 3 ( C O ) 2 ( ) e q u i l i b r i u m mixture contains about 90% of the a x i a l isomer. Although d e t a i l e d mechanistic experiments were not conducted f o r the i s o m e r i z a t i o n process, i t was observed that the r a t e of i s o m e r i z a t i o n occurs more slowly i n concentrated s o l u t i o n s than i n d i l u t e s o l u t i o n s and more slowly under an atmosphere of CO than under an N 2 atmosphere. Furthermore, no exchange of coordinated alkyne with f r e e alkyne occurs during the i s o m e r i z a t i o n process. These various experiments i n d i c a t e that the i s o m e r i z a t i o n must be an i n t r a molecular process and l i k e l y proceeds v i a i n i t i a l CO dissociation. I t i s s i g n i f i c a n t to note that although the a x i a l isomer of FeRu3(CO)i2( = h) the more thermodynamically s t a b l e , eq. 11, the e q u a t o r i a l isomer i s formed i n greatest y i e l d i n the 1

PnC

Cp

i s

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

122

REACTIVITY OF

METAL-METAL

BONDS

synthesis of the compound. This observation suggests that the FeRu3(CO)-L2( CR. ) d e r i v a t i v e s may form v i a the sequence of r e a c t i o n s o u t l i n e d i n Scheme 3. The f i r s t step presumably i n v o l v e s d i s s o c i a t i o n of CO from H2FeRu3(CO)i3 to generate H2FeRu3(CO)i2 which r a p i d l y adds the alkyne t o g i v e a s u b s t i t u t e d d e r i v a t i v e . We s p e c i f i c a l l y propose that t h i s i n i t i a l s u b s t i t u t i o n occurs a t the unique Ru atom i n the same s u b s t i t u t i o n s i t e that l a r g e phosphorus l i g a n d s add. From t h i s s i t e , the alkyne can e a s i l y swing under the c l u s t e r to i n s e r t i n t o a Ru-Ru bond to g i v e the k i n e t i c e q u a t o r i a l isomer which then subsequently rearranges to the more thermodynamically s t a b l e a x i a l isomer. Although r a t e data have not been measured, the r e a c t i o n of H2FeRu3(C0)^3 with alkynes i s of the same time s c a l e as the r e a c t i o n of the c l u s t e r with phosphines to g i v e the s u b s t i t u t e d products, i d with th i n i t i a l ste d i Scheme 3. RCE

1

Scheme 3

\ /

Molecular Dynamics of H9FeRu3(C0)^3 and Related MixedMetal C l u s t e r s . Metal c l u s t e r s have been shown to undergo a wide v a r i e t y of f l u x i o n a l processes i n which carbonyls, hydrides, and even the metals themselves undergo rearrangement (25). Mixed-metal c l u s t e r s are i d e a l l y s u i t e d f o r s t u d i e s of f l u x i o n a l processes because of the low symmetry which i s inherent w i t h i n t h e i r metal framework. In such c l u s t e r s , the m a j o r i t y of

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

GEOFFROY

H FeRuj(CO)i

E T AL.

2

:i

and

Mixed-Metal

Clusters

123

ligands are i n chemically non-equivalent p o s i t i o n s and thus are d i s t i n g u i s h a b l e by NMR spectroscopy. Furthermore, a homologous s e r i e s of mixed-metal c l u s t e r s allows one to study the e f f e c t s of metal s u b s t i t u t i o n on the f l u x i o n a l processes and t h e i r a c t i v a t i o n parameters. In t h i s regard, the s e r i e s of c l u s t e r s H2FeRu3(CO) 3, H FeRu Os(CO)i3, and H F e R u O s ( C O ) i n which Ru i s p r o g r e s s i v e l y replaced by Os has been studied i n our laboratory (6). The t r i m e t a l l i c FeRu 0s and FeRuOs c l u s t e r s e x i s t i n the two isomeric forms shown i n F i g u r e 2 which a l s o gives t h e i r corresponding symmetry l a b e l s and carbonyl l a b e l i n g schemes. The low-temperature l i m i t i n g l ^ C NMR spectra of these d e r i v a t i v e s are shown i n Figure 3. The l i m i t i n g NMR spectrum of H FeRuOs (CO)i3 i s p a r t i c u l a r l y i l l u s t r a t i v e . This c l u s t e r e x i s t s i n C^ and C isomeric forms i n s o l u t i o n and examination of the s t r u c t u r e s shown carbonyls i n these two isomers chemical environments. Each should thus show i t s own separate, c h a r a c t e r i s t i c l ^ C NMR resonance. Indeed, 19 separate resonances are r e s o l v a b l e i n the s t a t i c l ^ C NMR spectrum of t h i s d e r i v a t i v e , Figure 3, and e x p l i c i t carbonyl assignments were derived f o r each (5). S i m i l a r assignments were made f o r H FeRu3(CO)i3 and H FeRu Os(CO)i3. S i g n i f i c a n t l y , the terminal carbonyls bound to the d i f f e r e n t metal atoms i n t h i s s e r i e s group together i n c h a r a c t e r i s t i c chemical s h i f t regions. The chemical s h i f t decreases r e l a t i v e to TMS upon descending the t r i a d : Fe (204211 ppm) > Ru (184-180 ppm) > Os (168-177 ppm). Having made the assignments of s p e c i f i c carbonyls to the i n d i v i d u a l resonances i n the s t a t i c spectrum, i t i s b a s i c a l l y a simple matter to analyze the changes i n the NMR spectra as the temperature i s r a i s e d . Three d i s t i n c t l y d i f f e r e n t f l u x i o n a l processes have been found to occur i n each of these c l u s t e r s from such a n a l y s i s ( 5 ) . As the temperature i s r a i s e d from the low-temperature l i m i t i n g spectrum, the f i r s t process to occur i s b r i d g e - t e r m i n a l interchange l o c a l i z e d on i r o n . The mechanism which we have proposed f o r t h i s exchange i s shown i n Scheme 4 and involves opening one of the carbonyl bridges, a subsequent t r i g o n a l twist of the r e s u l t a n t Fe(C0)3 u n i t and f i n a l l y reformation of the CO bridge (5)· The next exchange process to occur at s l i g h t l y higher temperatures i n each of these c l u s t e r s i n v o l v e s m i g r a t i o n of the carbonyls around the Fe-M-M t r i a n g l e which possesses the b r i d g i n g carbonyls. I t seems reasonable to propose that the intermediates i n t h i s c y c l i c movement are the tautomers which have the semi-bridging carbonyls bound mainly to Ru or Os, instead of Fe, as i n d i c a t e d i n Scheme 5. The t h i r d and f i n a l process which occurs i s unique, i n v o l v i n g a s u b t l e s h i f t i n the metal framework. This process and i t s i m p l i c a t i o n s are best i l l u s t r a t e d by c o n s i d e r a t i o n of the drawings shown i n Scheme 6 which d e p i c t the metal framework of 1

2

2

2

2

2

2

13

2

2

s

2

2

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

124

REACTIVITY OF M E T A L - M E T A L

H FeRu (C0) 2

Figure 2.

3

BONDS

l3

Carbonyl labeling schemes for H FeRu (CO) and the C and C mers of H FeRu Os(CO) and H FeRuOs (CO) 2

2

2

13

s

ls

H

2

2

13

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

l

iso-

6.

GEOFFROY

H FeRu3(CO)is

E T AL.

Figure 3. Low-temperature (-95°C); (b) H FeRu Os(CO) 2

and

2

2

limiting C (-90°C); 13

13

Mixed-Metal

Clusters

N MR spectra of (a) and (c) H FeRuOs (CO) 2

2

13

125

H FeRu (CO) (-60°C) 2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3

ja

REACTIVITY OF

126

METAL-METAL

BONDS

Scheme 4

f

H FeRuOs2(CO) 3, the two b r i d g i n g C0 s and the two b r i d g i n g hydrides. The asymmetry of the c l u s t e r i s g r o s s l y exaggerated for c l a r i t y . The process b a s i c a l l y involves movement of the Fe atom away from one metal and c l o s e r to another with a concomitant s h i f t of the b r i d g i n g carbonyls. I t must a l s o i n v o l v e a s l i g h t e l o n g a t i o n or compression of a l l the M-M bonds and i t must be accompanied by a s h i f t i n p o s i t i o n of one of the bridging hydrides. R e f e r r i n g to Scheme 6 and s t a r t i n g with the C i enantiomer, i f the Fe moves away from Os^ towards 0s2> i t generates the Cib enantiomer. Movement of Fe away from Ru i n e i t h e r of the enantiomers and toward both Os atoms gives the C isomer. Each time the c l u s t e r rearranges, the carbonyls execute the c y c l i c process about a d i f f e r e n t Fe-M-M face and hence i n v o l v e d i f f e r e n t carbonyl ligands i n that process. The c y c l i c processes coupled with the framework rearrangement lead to t o t a l exchange of a l l the carbonyl l i g a n d s of the c l u s t e r i n t h i s f i n a l f l u x i o n a l process. The a c t u a l magnitude of the s h i f t s w i t h i n the metal framework must be r e l a t i v e l y small. Although the c r y s t a l s t r u c t u r e of H2FeRuOs2(CO)^3 has not yet been determined, that published f o r the i s o s t r u c t u r a l H2FeRu3(CO)i3 c l u s t e r shows that the greatest change expected i n any one bond length during the 2

1

a

s

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

GEOFFROY

ET

AL.

H FeRu (CO)i3 2

s

and

Mixed-Metal

Clusters

127

Scheme 6

Cfc

Cjb

rearrangement i s 0.11 1 (26). We tend to view the rearrangement process as more of a b r e a t h i n g motion of the metal framework, but one which has coupled to i t motions of the carbonyls and hydride ligands. Although w i t h i n the s e r i e s H2FeRu3(CO)i3, I^FeRt^Os (CO) 13, and H2FeRuOs2(CO)i3 the exchange processes are i d e n t i c a l , the a c t i v a t i o n b a r r i e r f o r each process increases as the Os content of the c l u s t e r i n c r e a s e s . I t i s u n l i k e l y that the a c t i v a t i o n energy increase can be accounted f o r s o l e l y on the b a s i s of the s i z e increase i n the metal involved i n the f l u x i o n a l process s i n c e Ru and Os probably have s i m i l a r atomic r a d i i i n these clusters. In Ru3(C0)]2 and Os3(CO)i2> example, the metal atomic r a d i i are 1.43 A and 1.44 Ä, r e s p e c t i v e l y (27). On the other hand, i t has been demonstrated that Os forms stronger metal-metal bonds and M-CO bonds than does Ru (28) and t h i s greater bond strength could increase the b a r r i e r s f o r the v a r i o u s f l u x i o n a l processes. The i n t r a m e t a l l i c rearrangement process has profound consequences i n the f l u x i o n a l i t y of the phosphine and phosphite s u b s t i t u t e d I ^ F e R ^ i C O ) - ^ d e r i v a t i v e s which were d i s c u s s e d above. Each of the monosubstituted d e r i v a t i v e s can e x i s t i n C^ f

o

r

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

128

REACTIVITY

OF

P M e

METAL-METAL

P h

t h e

C

BONDS

C

and C isomeric forms and f o r H F e R u 3 ( C O ) 2 ( 2 ^ l+ s e q u i l i b r i u m constant i s -1.8. However, these two isomers r a p i d l y i n t e r c o n v e r t as evidenced by the v a r i a b l e temperature 1 H NMR spectra of t h i s d e r i v a t i v e (29). As the temperature i s r a i s e d above -50°C the resonances shown i n Figure 1 broaden, coalesce at ~20°C, and sharpen to a doublet (Jp-H = 9.6 hz) at 70°C. The l a t t e r implies that at t h i s temperature the J C isomerization i s rapid on the NMR time s c a l e and the two hydrogens see an average chemical environment. Computer s i m u l a t i o n of the NMR s p e c t r a l changes gives a r a t e constant of k = 500 s e c ~ l at 20°C for the C i J C i s o m e r i z a t i o n process. This i s an extremely f a s t rate of i s o m e r i z a t i o n f o r a process i n which two s u b s t i t u t i o n a l isomers i n t e r c o n v e r t , e s p e c i a l l y s i n c e the phosphine l i g a n d i s bound to Ru atoms i n d i s t i n c t l y d i f f e r e n t environments i n the two isomers. The question then a r i s e s as to how the i n t e r conversion occurs. Th doublet i n the high-temperatur that P-H s p i n c o r r e l a t i o n i s maintained throughout the i n t e r conversion and hence the i s o m e r i z a t i o n may occur by a purely intramolecular process. This e l i m i n a t e s the p o s s i b i l i t y that the i n t e r c o n v e r s i o n occurs v i a d i s s o c i a t i o n - r e a s s o c i a t i o n of the phosphine. We b e l i e v e that the i s o m e r i z a t i o n occurs by an i n t r a m e t a l l i c rearrangement process analogous to that discussed above f o r the unsubstituted H2FeRu3_ Os (CO)i3 (x = 0-2) c l u s t e r s . T h i s process i s o u t l i n e d i n Scheme 7 below. Consider the C isomer f i r s t i n which the phosphine l i g a n d i s attached to the unique Ru atom (RU3). I f the Fe moves away from Ru^ towards RU3 and the hydride and carbonyls s h i f t a p p r o p r i a t e l y , the C i isomer i s generated. In order to b r i n g the phosphine i n t o the required e q u a t o r i a l p o s i t i o n , r o t a t i o n of the Ru(C0)2(PMe2Ph) u n i t must accompany the rearrangement. Note, however, that the phosphine has remained attached to the same Ru atom but that t h i s Ru has a l t e r e d i t s environment v i a the i n t r a m e t a l l i c rearrangement process. In essence, the phosphine appears to exchange p o s i t i o n s but the exchange occurs by the phosphine s t a y i n g attached to RU3 with the c l u s t e r rearranging around the ligand. s

2

1

s

s

x

x

s

Scheme 7

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

GEOFFROY ET

H FeRui(CO)i:t

AL.

and

2

Mixed-Metal

Clusters

129

Photochemical R e a c t i v i t y of H p F e R u ^ C C C Q i I b F e O s ^ C O ) Λ Τ , and H 2 R 1 1 4 ( C O ) 1 ^ . Monomeric metal carbonyl complexes g e n e r a l l y l o s e CO when i r r a d i a t e d whereas f o r d i n u c l e a r carbonyls, such as Mn2(CO)lO» dominant photoreaction i n v o l v e s cleavage of the M-M bond and fragmentation (30). The question then a r i s e s as to what are the primary photochemical p r o p e r t i e s of metal carbonyl c l u s t e r s c o n t a i n i n g three or more metal atoms; i . e . , w i l l CO l o s s or M-M bond cleavage p r e f e r e n t i a l l y obtain? With t r i n u c l e a r c l u s t e r s , M-M bond cleavage appears to dominate the photo­ chemistry although t r u l y d e f i n i t i v e mechanistic s t u d i e s are lacking. C e r t a i n l y Fe3(CO)i2 (31), Ru (CO)i2 (32), and H3Re3(CO)i2 (33) fragment upon p h o t o l y s i s , although i n the l a t t e r case a d e t a i l e d mechanistic study which we conducted was not able to r e s o l v e whether Re-Re bond cleavage or CO l o s s occurs i n the primary photochemical event Os3(CO)i2 does not fragment upon p h o t o l y s i s but r a t h e obtains (34). Even here v i a a pathway i n v o l v i n g i n i t i a l cleavage of an Os-Os bond. t

n

e

3

Few s t u d i e s of t e t r a n u c l e a r c l u s t e r s have been reported. Johnson, Lewis and coworkers (35,36) have shown that p h o t o l y s i s of H40s4(CO)i2 gives clean s u b s t i t u t i o n chemistry and analogous r e s u l t s were obtained by Wrighton and G r a f f (37) f o r H4Ru4(CO) « The t e n t a t i v e c o n c l u s i o n that may be drawn i s that f o r c l u s t e r s with n u c l e a r i t y _> 4, photoinduced fragmentation does not o b t a i n and the net photochemistry evolves from CO l o s s . Whether t h i s i s because the metal-metal e x c i t e d s t a t e s , which c l e a r l y l i e lowest i n energy (30,34), are too d e l o c a l i z e d to g i v e cleavage of a s i n g l e M-M bond, or whether M-M bond cleavage does indeed o b t a i n but that net fragmentation i s not observed because the remaining M-M bonds h o l d the c l u s t e r together i s s t i l l an unanswered question. In order to t e s t the hypothesis that CO d i s s o c i a t i o n i s the dominant photoreaction f o r t e t r a n u c l e a r c l u s t e r s we have under­ taken a study of the photochemistry of H2FeRu3(CO)i3, a c l u s t e r f o r which we know the e f f i c i e n c y of thermal CO d i s s o c i a t i o n and fragmentation as d e t a i l e d above. For comparison, the c l u s t e r s H2Ru4(CO)i3 and H2FeOs3(CO)^3 have a l s o been examined. With each of these c l u s t e r s CO l o s s i s c l e a r l y the dominant photo­ r e a c t i o n (38). In the presence of PPI13, each c l u s t e r gives c l e a n p h o t o s u b s t i t u t i o n chemistry to y i e l d p r i m a r i l y the monos u b s t i t u t e d d e r i v a t i v e s , eq. 12. 12

H M (CO) 2

4

1 3

+ PPh

h 3

v

> H M ( C O ) ( P R ) + CO 2

4

12

3

(12)

M^ = FeRu , FeOs^, Ru^ 3

Prolonged p h o t o l y s i s a l s o leads to formation of d i - and t r i s u b s t i t u t e d products. P h o t o l y s i s under H2 atmospheres leads to the H4M4 (00)^2 c l u s t e r s i n each case, eq. 13.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

130

REACTIVITY

H M (CO) 2

4

1 3

+ H

h

V

> H M (CO)

2

4

4

OF

M E T A L - M E T A L BONDS

+ CO

1 2

(13)

However, these r e a c t i o n s are not as c l e a n as the PPl^-photos u b s t i t u t i o n r e a c t i o n s s i n c e H ^ F e R i ^ C O ) ^ and Η^ΡεΟββζΰΟ)]^ are themselves p h o t o s e n s i t i v e and r e a d i l y degrade t o other products. H2Ru4(CO)i3, however, can be q u a n t i t a t i v e l y converted to H4Ru4(CO)i2> which i s photochemically i n a c t i v e under H 2 . These v a r i o u s experiments i n d i c a t e that CO l o s s occurs i n the primary photochemical event f o r each c l u s t e r to produce c o o r d i n a t i v e l y unsaturated H2M4(CO)i2 which subsequently adds PPI13 or H , eqs. 14-15. 2

H M (CO) 2

4

H M (CO) 2

4

1 2

h

V

1 3

> CO + H M ( C O ) 2

4

(14)

1 2

+ L —•H

Quantum y i e l d s have been measured f o r PPI13 s u b s t i t u t i o n and these are summarized i n Table IV. The quantum y i e l d s vary s l i g h t l y with the nature of the c l u s t e r but f o r H2FeRu3(CO) 3 are independent of PPI13 c o n c e n t r a t i o n , c o n s i s t e n t with the formation of an H2M4(C0)i2 intermediate which subsequently scavenges PPI13. 1

Table IV.

Cluster

[Cluster]

H FeRu (CO) 2

Quantum Y i e l d Data f o r ΡΡΪ13 S u b s t i t u t i o n i n Isooctane S o l u t i o n

3

13

[PPh ] 3

4

3.65 χ 1 0 ~

4

0.025 ± 0.006

3.65 χ 10"

4

1.83 χ 1 0 ~

3

0.029 ± 0.006

3.65 χ 10~

4

3.65 χ 1 θ "

3

0.030 ± 0.006

2.65 χ 1 θ "

3

0.057 ± 0.012

5.28 χ 1 θ "

Hi RRuu.((CCO0))

3.00 χ 10

oV

o2

4

W

1 13 o

1 o

366

3.65 χ 10~

H Fe0s (C0) 2 ^3 ~ 13 o

p

4

4

1.5 χ 1 0 ~

3

0.016 ± 0.002

We have observed that prolonged p h o t o l y s i s of these c l u s t e r s under CO atmospheres does induce fragmentation, but the quantum y i e l d s are f a r too low to be measured on our apparatus. For H2FeRu3(CO)i3 an upper l i m i t quantum y i e l d of 2.4 χ 1 0 " can be estimated based on our d e t e c t i o n l i m i t s . The very low quantum y i e l d f o r fragmentation of a c l u s t e r which we know can fragment thermally under CO (see above) provides strong evidence that the primary photochemical r e a c t i o n i s not metalmetal bond cleavage, which should r e a d i l y lead t o fragmentation i n the presence of CO, but r a t h e r i n v o l v e s CO d i s s o c i a t i o n . 6

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6.

GEOFFROY

H FeRu (CO),

E T AL.

2

3

!

and Mixed-Metal

Clusters

131

Summary and Outlook One can begin to assemble a composite p i c t u r e of the r e a c t i v i t y of H2FeRu3(CO) 3 with the r e a c t i v i t y information discussed above now a v a i l a b l e . By f a r the most rapid process that t h i s c l u s t e r undergoes i n v o l v e s i n t r a m o l e c u l a r exchange of the carbonyl l i g a n d s and the hydrides and the rearrangement of the metal framework. These processes occur with rate constants ranging from 10-50 s e c " a t 50°C. On a f a r slower time s c a l e , H2FeRu3(CO)i3 undergoes thermal d i s s o c i a t i o n of CO t o y i e l d H2FeRu3(CO)i2* This species can then subsequently add phosphines and phosphites to g i v e s u b s t i t u t e d d e r i v a t i v e s , alkynes to f i r s t y i e l d s u b s t i t u t e d c l u s t e r s followed by r e a c t i o n to give the closo-FeRu3(CO>l2(RCΞCR ) products, C 0 to e f f e c t 13co/l CO exchange, and H2 to give H4FeRu (CO) 2f i r s t step, d i s s o c i a t i o 6.96 χ ΙΟ""* s e c " at 50°C various f l u x i o n a l processes 1,000-10,000 times between each CO d i s s o c i a t i v e event. F i n a l l y , on a much slower time s c a l e , H2FeRu3(CO)i3 undergoes fragmentation with CO to produce Ru3(CO)i2> Fe(C0)5, * 2 by mechanism which we b e l i e v e involves a s s o c i a t i o n of CO concerted with breakage of one of the metal-metal bonds. Recent photochemical experiments i n d i c a t e that CO d i s s o c i a t i o n , but not fragmentation, can be photoinduced, and t h i s r e a c t i o n appears to o f f e r p o t e n t i a l f o r the synthesis of unusual s u b s t i t u t e d d e r i v a t i v e s (38). 1

1

,

1 3

2

T

3

h

e

1

1

a n c

H

a

Obviously there i s s t i l l much to be learned concerning the chemistry of H2FeRu3(CO)i3 and r e l a t e d mixed-metal c l u s t e r s . An important feature f o r which good information i s not a t a l l a v a i l a b l e concerns the r e l a t i v e strength of the metal-metal bonds i n c l u s t e r s of t h i s type. Is an Fe-Ru bond stronger or weaker than Fe-Fe and Ru-Ru bonds? This question could presumably be addressed d i r e c t l y by a s e r i e s of m i c r o c a l o r i m e t r i c s t u d i e s or i n d i r e c t l y by a d e t a i l e d k i n e t i c examination of the r e a c t i o n s of a s e r i e s of i s o - s t r u c t u r a l mixed-metal c l u s t e r s with CO. We know nothing concerning the redox p r o p e r t i e s of these p a r t i c u l a r c l u s t e r s and that should be a subject f o r f u r t h e r study. Will they r e t a i n t h e i r i n t e g r i t y on o x i d a t i o n and r e d u c t i o n or might they open to y i e l d " b u t t e r f l y " c l u s t e r s upon 2 - e l e c t r o n reduction? An i n t e r e s t i n g question concerns e x a c t l y which CO i s l a b i l i z e d i n the CO d i s s o c i a t i o n step: i.e., a CO bound to Fe or Ru; a x i a l Ru or e q u a t o r i a l Ru? Even though we know which s i t e s ligands p r e f e r t o add to give the s u b s t i t u t e d products, t h i s implies nothing about the s p e c i f i c CO i n i t i a l l y l o s t .

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

132

REACTIVITY

OF M E T A L - M E T A L

BONDS

Acknowledgments The research described h e r e i n has been generously supported by the O f f i c e of Naval Research, the N a t i o n a l Science Foundation and the Department o f Energy, O f f i c e of B a s i c Energy Sciences. GLG g r a t e f u l l y acknowledges the Camille and Henry Dreyfus Foundation f o r a Teacher-Scholar Grant, the A l f r e d P. Sloan Foundation f o r a research f e l l o w s h i p . Literature Cited

1. Gladfelter, W. L.; Geoffroy, G. L . , Adv. Organomet. Chem., 1980, 2. Geoffroy, G. L.; Gladfelter 99, 7565. 3.

W L . J Am Chem Soc. 1977

Steinhardt, P. C.; Gladfelter, W. L.; Harley, A. D.; Fox, J. R.; Geoffroy, G. L . , Inorg. Chem., 1980, 19, 332.

4. Burkhardt, E. W.; Geoffroy, G. L . , J. Organomet. Chem., in press. 5. Gladfelter, W. L.; Geoffroy, G. L . , Inorg. Chem., 6. Epstein, R. Α.; Withers, H. W.; Geoffroy, G. L . , Inorg. Chem., 1979, 18, 942. 7. Demitras, G. C.; Muetterties, E. L . , J. Am. Chem. Soc., 1977, 99, 2796. 8. Thomas, M. G.; Beier, B. F.; Muetterties, E. L . , J. Am. Chem. Soc, 1976, 98, 1296. 9·

Bradley, J., J. Am. Chem. Soc., 1979, 101, 7419.

10.

Pruett, R. L.; Walker, W. E., U.S. Patent 2,262,318 (1973); 3,944,588 (1976); Ger. Offen. 2,531,103 (1976).

11.

Laine, R. Μ., J. Am. Chem. Soc., 1978, 100, 6451.

12. Laine, R. M.; Thomas, D. W.; Cary, L. W.; Buttrill, S. E . , J. Am. Chem. Soc., 1978, 100, 6527. 13.

Kang, H. C.; Mauldin, C. H.; Cole, T.; Slegeir, W.; Cann, K.; Pettit, R., J. Am. Chem. Soc., 1977, 99, 8323.

14. Laine, R. M.; Rinker, R. G.; Ford, P. C., J . Am. Chem. Soc., 1977, 99, 252. 15.

Ford, P. C.; Rinker, R. G.; Ungermann, C.; Laine, R. M.; Landis, V.; Moya, S. Α., J. Am. Chem. Soc., 1978, 100, 4595.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6. GEOFFROY ET AL. H FeRuj(CO)i.i and Mixed-Metal Clusters 133 2

16.

Ungermann, C.; Landis, V.; Moya, S. Α.; Cohen, H.; Walker, H.; Pearson, R. G.; Rinker, R. G.; Ford, P. C., J. Am. Chem. Soc., 1979, 101, 5922.

17.

Fox, J. R.; Gladfelter, W. L.; Geoffroy, G. L . , Inorg. Chem., 1980, 19, 2574.

18.

Fox, J. R.; Gladfelter, W. L.; Wood, Τ. G.; Smegal, J. Α.; Foreman, Τ. K.; Geoffroy, G. L . , J. Am. Chem. Soc., submitted for publication.

19.

Tolman, C. Α., Chem. Rev., 1977, 77, 313.

20.

Fox, J. R.; Gladfelter, W. L.; Geoffroy, G. L . , J. Am. Chem. Soc., submitted for publication

21.

Fox, J. R.; Gladfelter Meguid, S.-Α.; Tavanaiepour, I., J. Am. Chem. Soc., submitted for publication.

22. Tavanaiepour, I., Ph.D. Dissertation, University of Nebraska, Lincoln, Nebraska, 1980. 23.

Wade, K., Chem. Britain, 1975, 11, 1977.

24. Wade, K., Adv. Inorg. Chem. Radiochem., 1976, 18, 1. 25.

Band, E.; Muetterties, E. L . , Chem. Rev., 1978, 78, 639.

26.

Gilmore, C. J.; Woodward, P., J. Chem. Soc. A, 1971, 3453.

27.

Churchill, M. R.; Hollander, F. J.; Hutchinson, J. P., Inorg. Chem., 1977, 16, 2655.

28.

Conner, J. Α., Top. Curr. Chem., 1977, 71, 71.

29.

Gladfelter, W. L.; Fox, J. R.; Smegal, J. A.; Wood, Τ. G.; Geoffroy, G. L . , J. Am. Chem. Soc., submitted for publication.

30.

Geoffroy, G. L.; Wrighton, M. S.; "Organometallic Photochemistry," Academic Press, New York, 1979.

31.

Austin, R. G.; Pavnessa, R. S.; Giordano, P. J . ; Wrighton, M. S., Adv. Chem. Ser., 1978, 168, 189.

32. Johnson, B. F. G.; Lewis, J . ; Twigg, M. U., J. Organomet. Chem., 1974, 67, C75.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

134

33.

REACTIVITY OF METAL-METAL BONDS

Epstein, R. Α.; Gaffney, T. R.; Geoffroy, G. L.; Gladfelter, W. L.; Henderson, R. S., J. Am. Chem. Soc., 1979, 101, 3847.

34. Tyler, D. R.; Altobelli, M.; Gray, Η. Β., J. Am. Chem. Soc., 1980, 102, 3022. 35.

Johnson, B. F. G.; Kelland, J. W.; Lewis, J . ; Rehani, S. Κ., J. Organomet. Chem., 1976, 113, C42.

36.

Bhoduri, S.; Johnson, B. F. G.; Kelland, J. W.; Lewis, J.; Raithby, P. R.; Rehani, S.; Sheldrick, G. M.; Wong, K.; McPartlin, Μ., J. Chem. Soc. Dalton Trans., 1979, 562.

37.

Graff, J. L.; Wrighton M S. J Am Chem Soc. 1980 102, 2123.

38.

Foley, H. C.; Geoffroy, G. L . , to be submitted.

RECEIVED December 1, 1980.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7 Kinetic Studies of Thermal Reactivities of M e t a l M e t a l - B o n d e d Carbonyls A N T H O N Y POË J. Tuzo Wilson Laboratories, Erindale College, University of Toronto, Mississauga, Ontario, Canada L 5 L 1C6

The strengths of the metal-metal bonds in dimetal and metal-cluster carbonyls have Mn (CO) was shown to supported by the sort of bridging carbonyls found previously in Fe (CO) (2) and, soon after, in Co (CO) (3). Whereas the lengths of the Fe-Fe and Co-Co bonds in these latter complexes were close to those found in the pure metals the length of the MnMn bond was ca. 0.5 Ålonger than that in manganese metal and this "excessive" length was thought to imply some intrinsic weakness in unsupported metal-metal bonds. An early attempt (4) to obtain a thermochemical measure of the strength of the Mn-Mn bond resulted in a value of 142 ± 54 kJ mol and illustrated the basic d i f f i culty of obtaining precise values. A few estimates of metal-metal bond energies have been made on the basis of mass spectrometric measurements. These involve measurement of appearance potentials as, for instance (5), in eq 1-3. The energy required for eq 3 is the difference between the 2

10

2

9

2

8

-1

+

Mn (CO) + e 2

> Mn(C0) + ·Μη(α)) + 2e

10

5

+

•Mn(C0) + e 5

Mn (CO) 2

5

• Mn(C0) + 2e

(2)

> 2*Mn(CO)

(3)

5

10

(1)

5

+

appearance potentials of Mn(C0)5 through processes shown in eq 1 and 2. It has been argued (6) that there may be a difference between the ·Μn(CO) radicals involved in these two processes and that some ambiguity remains as to the meaning of the value of 80 kJ mol found for eq 3. Alternatively, direct measurement (7) of the value of ΔΗ° for reaction 4, for example, was obtained, the 5

-1

[Mn (CO) ] 2

10

g

^

2[Mn(CO) ]

5 g

(4)

relative concentrations being estimated from ion currents due to Mn (CO) + and Mn(C0) + over the temperature range 210-300°C. 2

10

5

0097-6156/81/0155-0135$08.00/0 © 1981 American Chemical Society

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

136

REACTIVITY OF METAL-METAL BONDS

This estimation depends on the assumption that the cross-sections for formation of the ions are not temperature dependent and led to a value of ΔΗ° = 104 ± 8 kJ mol . Similar measurements on Co (CO) led to a value of ΔΗ° = 61 ± 8 kJ mol for the equilibrium. -1

2

-1

8

More r e c e n t l y a reasonably l a r g e set of thermochemical mea­ surements has been used to o b t a i n metal-metal bond energies f o r a number of dimetal and metal c l u s t e r carbonyls ( 8 ) . These make use of M-CO bond energies determined mass s p e c t r o m e t r i c a l l y and r e l y on the assumption that M-CO bond energies are the same i n mono­ n u c l e a r and a l l polynuclear carbonyls of the same metal. Several d i r e c t l y measured values of ΔΗ° f o r homolytic d i s s o ­ c i a t i o n of a metal-metal-bonded carbonyl i n s o l u t i o n have been obtained (9). This was f o r the complexes [ ( n - C H ) F e ( C O ) L ] where L = CO or a number of d i f f e r e n t P-donor l i g a n d s . The low value ΔΗ° = 56.5 kJ mol" when L = CO was not unexpected f o r such a s t e r i c a l l y crowded molecule the s t e r i c crowding an monomers but the e f f e c t seemed to be c o n t r o l l e d more by AS° than ΔΗ°. In general metal-metal bond energies, however they may have been estimated, are too l a r g e to allow f o r d i r e c t measurement of e q u i l i b r i u m constants i n s o l u t i o n i n t h i s way. Although a l l of these measurements, however u n c e r t a i n t h e i r p r e c i s i o n , have t h e i r own i n t r i n s i c i n t e r e s t , none o f them has any d i r e c t bearing on the r e a c t i v i t y of the complexes concerned. This a p p l i e s not only to the r e a c t i v i t y of the metal-metal bonds that they contain but a l s o to t h e i r r e a c t i v i t y towards d i s s o c i a t i o n of the CO l i g a n d s . The l a t t e r p o i n t i s i l l u s t r a t e d by the r e l a t i v e values of the average M-CO bond energies i n binary mononuclear carbonyls such as M(C0) (M = Cr, Mo, and W) ( 8 ) . Even a f t e r allowance f o r the e f f e c t of d i f f e r e n t valence s t a t e promotion energies the average energies i n c r e a s e monotonically by over 25% along the s e r i e s (8, 10) whereas the a c t i v a t i o n energies f o r CO d i s s o c i a t i o n are 168, 133, and 167 k J m o l " , r e s p e c t i v e l y (11). The metal-metal-bonded carbonyls M ( C O ) (Μ = Mn, Tc, and Re) would have a s i m i l a r l y l a r g e increase i n the average M-CO bond d i s s o c i a t i o n energies and the M-M bond s t r e n g t h i s p r e d i c t e d from thermochemical data almost to double along the s e r i e s ( 8 ) . T h i s c o n t r a s t s with an i n c r e a s e of r e a c t i v i t y (as measured by the a c t i v a t i o n e n t h a l p i e s f o r s u b s t i t u t i o n or thermal decomposition) of only 8% (6_, 12) . This discrepancy i s independent of any uncer­ t a i n t y i n the mechanism s i n c e i t e x i s t s whether CO d i s s o c i a t i o n or homolytic f i s s i o n of the M-M bonds i s rate-determining. Not only do the r e l a t i v e values of the thermochemically estimated bond strengths f a i l to p r e d i c t the s c a l e of the r e l a t i v e r e a c t i v i t i e s but some of the absolute values are g r o s s l y misleading as w e l l . Thus the thermochemically derived value f o r the Mn-Mn bond strength i n M n ( C O ) i s 67 kJ mol" (8). Since t h i s represents the s t r e n g t h of the i n t e r a c t i o n between the two halves of the molecule i n i t s undisturbed ground s t a t e i t must represent an upper l i m i t f o r the enthalpy of a c t i v a t i o n f o r homolytic f i s s i o n 3

3

5

1

6

1

2

1 0

1

2

10

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

2

7.

Metal-Metal-Bonded

pot

137

Carbonyls

of the Mn-Mn bond. (Any adjustment of the c o o r d i n a t i o n around each metal atom as the Mn-Mn bond s t r e t c h e s to form the t r a n s i t i o n s t a t e must decrease the energy r e q u i r e d to form i t . ) The a c t i v a t i o n energy f o r s u b s t i t u t i o n or decomposition i s , however, c l o s e to 153 k J mol" (12). I f t h i s value were to be assigned ( i n c o r r e c t l y ; see below) to the CO d i s s o c i a t i v e process and the value of 67 k J mol"" were assigned to the a c t i v a t i o n enthalpy of homolytic f i s s i o n of the Mn-Mn bond a value of AS^ o f l e s s than - ca. 160 J K mol" would have to be assigned to the homolytic f i s s i o n process i n order f o r CO d i s s o c i a t i o n to be the p r e f e r r e d path. This i s a t o t a l l y unreasonable value f o r a metal-metal bond breaking process that must c e r t a i n l y i n v o l v e a s u b s t a n t i a l l y p o s i t i v e value of Ast. i t t h e r e f o r e seems most probable that the e f f e c t i v e s t r e n g t h of the Mn-Mn bond i s over twice as great as that estimated thermochemically. The r e l a t i o n s h i p o f the thermochemical estimates to th bonyls i s t h e r e f o r e ver make absolute p r e d i c t i o n s , or even c o r r e l a t i o n s , are so u n c e r t a i n as to be almost p o s i t i v e l y misleading. C l e a r l y the only way of o b t a i n i n g r e l i a b l e , p r e c i s e , q u a n t i t a t i v e measurements o f the r e a c t i v i t y o f metal-metal-bonded carbonyls i s to c a r r y out c a r e f u l k i n e t i c s t u d i e s . R e l a t i n g these to the r e a c t i v i t y s p e c i f i c a l l y of the metal-metal bonds i n the complexes r e q u i r e s even more extensive and d e t a i l e d mechanistic s t u d i e s i f the r o l e o f the metal-metal bonds i n determining the e n e r g e t i c s of the r e a c t i o n s i s to be e l u c i d a t e d . 1

1

- 1

1

R e a c t i v i t i e s and Reaction Mechanisms of M? ( C O ) MnRe, and Re?) and some S u b s t i t u t e d D e r i v a t i v e s

10

_(M = Mn?, Tc?, 2

For s t u d i e s o f r e a c t i v i t i e s of such metal-metal-bonded carbonyls to be d i r e c t l y r e l a t e d to the strengths o f the metal-metal bonds r e a c t i o n s have to be found f o r which homolytic f i s s i o n of the metal-metal bond i s the rate-determining step. I f the f i s s i o n of the metal-metal bond i s r e v e r s i b l e then a r e a c t i o n scheme as shown i n eq 5-7 can be envisaged. The r a d i c a l species M i s

M

2

s

v

k-!

2M

(5)

M

• product

(6)

M + X

• product

(7)

allowed to form product by a path that i s e i t h e r zero or f i r s t order i n the c o n c e n t r a t i o n of some added reagent X. The r a t e equation, i n i t s non-integrated form, f o r t h i s scheme (13) i s shown i n eq 8 and 9. Robsd * rate observed a t a given conc e n t r a t i o n , [M ], of complex and Rj i s the l i m i t i n g r a t e that i s , st

n

e

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

138

REACTIVITY

R

k

obsd

•

R

obsd

"

k

k

l - ^

b s d

i - *k|

-l/(k +k [X]) 2

b e d

OF

METAL-METAL

2

BONDS

(8)

3

[M2lk-i/(k2*3[X])

2

(9)

or would be, observed at s u f f i c i e n t l y high c o n c e n t r a t i o n s of X for the reverse of r e a c t i o n 5 not to occur. k i i s an apparent p s e u d o - f i r s t - o r d e r r a t e constant d e f i n e d by Robsd = k o b s d f 2 ] « At very low values of [M ] and/or high values of k + k [ X ] the r e a c t i o n s w i l l be f i r s t order i n [M ] and governed by the r a t e constant k j . However, as [M ] i n c r e a s e s and/or k + k [ X ] de­ creases the r e a c t i o n w i l l approach h a l f order i n [M ] apd be governed by a h a l f - o r d e r r a t e constant k^ = 0.5(^/κ_ι ) ^ (k +k [X]). For a given value of [M ], kobsd should i n c r e a s e with i n c r e a s i n g [X] to a l i m i t i n g value but the rate of i n c r e a s e w i l l depend on [M ] and the r e a c t i o n s proceeding at l e s s than the l i m i t i n g r a t e should b important to note that ing values are observable that p o s i t i v e k i n e t i c evidence becomes a v a i l a b l e i n favor of t h i s mechanism. I f only l i m i t i n g r a t e s are observed then the f i r s t - o r d e r r a t e constants obtained do not d i s ­ t i n g u i s h whether the rate-determining process i n v o l v e s f i s s i o n , formation of some more e n e r g e t i c isomer of M , or even CO d i s s o ­ ciation. E q u a l l y w e l l , of course, observations of l i m i t i n g rates cannot disprove the homolytic f i s s i o n mechanism. Even i f r e a c ­ t i o n s are found to be i n h i b i t e d by CO t h i s does not disprove homo­ l y t i c f i s s i o n nor does i t prove CO d i s s o c i a t i o n . A scheme such as that shown i n eq 10-13 f o r a s u b s t i t u t i o n r e a c t i o n allows f o r O D S (

M

2

2

3

2

2

2

3

2

2

3

2

2

2

[M(C0)] M(C0)

and/or

2

+ L

2MC0

(10)

ML + CO

(11)

ML + M(C0)

•

M (C0)L

(12)

2ML

•

[ML]

(13)

2

2

r e t a r d a t i o n of the rates i n the presence of f r e e CO and only c a r e ­ f u l study of the r e a c t i o n s that proceed a t l e s s than the l i m i t i n g r a t e s can p o s s i b l y d i s t i n g u i s h between the homolytic f i s s i o n and C O - d i s s o c i a t i v e mechanisms (14). The f i r s t f u l l study that showed p o s i t i v e evidence f o r homo­ l y t i c f i s s i o n was made with M n ( C O ) (15). Reaction with X = 0 i n d e c a l i n l e d to decomposition but the k i n e t i c s were q u i t e c l e a n . Reaction at 125°C under 100% 0 occurred a t the l i m i t i n g r a t e over a 500-fold range of [ M n ( C O ) ] but r e a c t i o n under p a r t i a l p r e s ­ sures o f oxygen of 0.21 and 0.053 showed a c l e a r change from almost f i r s t - o r d e r dependence on [Mn (CO)iol a t the lowest concen­ t r a t i o n s to h a l f - o r d e r dependence at the highest c o n c e n t r a t i o n s . Decomposition under 50% 0 at 155°C was unaffected by CO. Decom­ p o s i t i o n under Ar at 155°C or under CO at 170°C showed h a l f - o r d e r 2

10

2

2

10

2

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

7.

Metal-Metal-Bonded

ΡΟΕ*

139

Carbonyls

dependence over a 100-fold range of [Mn (CO)iolDecomposition was unaffected by CO from 80-155°C although an a d d i t i o n a l r e a c t i o n path, i n h i b i t e d by CO, became detectable above 155°C. A l l the extensive data were i n e x c e l l e n t q u a n t i t a t i v e agreement with the r a t e behavior p r e d i c t e d by eq 8-9. Reactions with I under CO a t 125°C l e d to formation of Mn(C0) I i n v i r t u a l l y q u a n t i t a t i v e y i e l d s (6) according to the r a t e equation kobsd ^a [^21· The value of k was unaffected by the presence or absence of CO and was equal to the l i m i t i n g r a t e of r e a c t i o n under 0 from 105 to 125°C. There can be l i t t l e doubt that the r e a c t i o n scheme shown i n eq 5-7 i s being followed and that the a c t i v a t i o n enthalpy of 153.8 ± 1.6 k J mol" found f o r r e a c t i o n under 100% 0 c o r r e ­ sponds to that f o r homolytic f i s s i o n . Reaction with PPh proceeds under Ar at a l i m i t i n g r a t e that i s 35% f a s t e r than tha with 0 wherea r e a c t i o unde 100% proceeds a t the same rat under 0 proceeds at e x a c t l y bonyl product i s observed. This shows that the r e a c t i o n s with PPh and 0 do not proceed v i a two completely independent paths i n which case the r a t e of r e a c t i o n with PPh under 0 should proceed at the sum of the r a t e s observed with each s e p a r a t e l y . I t ap­ pears, t h e r e f o r e , that ca. 25% of the s u b s t i t u t i o n r e a c t i o n occurs v i a a C O - i n h i b i t e d path (probably CO d i s s o c i a t i o n ) but that the remainder occurs v i a homolytic f i s s i o n . The f a c t that no Mn (CO) PPh or M n ( C O ) ( P P h ) i s formed during r e a c t i o n with PPh under 0 i s e x p l i c a b l e because they are both q u i t e unstable to 0 under these c o n d i t i o n s (12, 16). Presumably they are formed i n ca. 25% y i e l d v i a the C O - i n h i b i t e d path but decompose r a p i d l y when formed. Very s i m i l a r , though r a t h e r l e s s extensive, s t u d i e s have been made of decomposition r e a c t i o n s of MnRe(CO) (15), T c ( C O ) (17), and R e ( C O ) (13). A l l the k i n e t i c s are c o n s i s t e n t with the r a t h e r s t r i n g e n t p r e d i c t i o n s of r a t e eq 9 f o r the homolytic f i s s i o n mechanism and are t o t a l l y i n c o n s i s t e n t with a simple C0d i s s o c i a t i v e mechanism. Other workers have s t u d i e d s u b s t i t u t i o n r e a c t i o n s of M n ( C O ) (18) and MnRe(CO) (19) and have com­ mented, c o r r e c t l y , that t h e i r k i n e t i c r e s u l t s are c o n s i s t e n t with the d i s s o c i a t i v e r e a c t i o n . This seems to have l e d to some doubts about the correctness of our c o n c l u s i o n that the r e a c t i o n s of a l l the decacarbonyls proceed mainly or t o t a l l y v i a homolytic f i s s i o n . These doubts do not a r i s e from the k i n e t i c s because the k i n e t i c s observed by these workers (18, 19) are e q u a l l y c o n s i s t e n t with the homolytic f i s s i o n mechanism or, indeed, v i r t u a l l y any f i r s t - o r d e r a c t i v a t i o n of the complexes. The c o n d i t i o n s under which the reac­ t i o n s were followed were simply not those capable of l e a d i n g to a k i n e t i c d i s t i n c t i o n between the various mechanisms proposed. The absence o f any homonuclear products of the r e a c t i o n s of MnRe(CO)io was o f f e r e d (19) as evidence against the homolytic f i s s i o n mech­ anism. This p o i n t had been r a i s e d before (15, 17) when i t was countered by emphasizing the absence of s u f f i c i e n t data on 2

2

5

=

+

a

2

1

2

3

2

3

2

3

2

2

9

3

2

8

3

2

3

2

2

10

2

2

1 0

2

10

10

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1 0

140

REACTIVITY

OF

METAL-METAL

BONDS

d i m e r i z a t i o n of such r a d i c a l species f o r the p o i n t to be s u s t a i n e d . I n i t i a l studies of r e a c t i o n 14 showed (18) that the r a t e increased Mn (CO) (PPh ) 2

8

3

+ P(0Ph)

2

• Mn (C0) (PPh )P(0Ph)

3

2

8

3

3

+ PPh

(14)

3

to a l i m i t i n g value with i n c r e a s i n g [ P ( 0 P h ) ] . A d i s s o c i a t i v e mechanism was proposed although i t was acknowledged that some am­ b i g u i t i e s e x i s t e d . L a t e r s t u d i e s of t h i s r e a c t i o n (14, 20) showed that r e a c t i o n s at l e s s - t h a n - l i m i t i n g r a t e s were i n f a c t h a l f - o r d e r i n [ M n ( C 0 ) 8 ( P P h ) ] and a very extensive set of data was f u l l y i n accord with the q u i t e complicated r a t e equation a p p r o p r i a t e to i n i t i a l homolytic f i s s i o n and t o t a l l y incompatible with a simple d i s s o c i a t i v e process. Other k i n e t i c s t u d i e s have shown that M n ( C O ) { P ( O P h ) } (21) and R e ( C 0 ) ( P P h ) (22) undergo r e a c t i o n i n a way c h a r a c t e r i s t i c of i n i t i a l r e v e r s i b l e homolyti P-n-Bu , and P ( C H ) Mn(C0) ClL i n e s s e n t i a l l y q u a n t i t a t i v e y i e l d s at e x a c t l y the same r a t e s as those f o r r e a c t i o n with 0 (12). S u b s t i t u t i o n r e a c t i o n s of such complexes u s u a l l y proceed at r a t e s s i m i l a r to, though s l i g h t l y d i f f e r e n t from, those f o r r e a c t i o n with 0 (12) but the a c t i v a t i o n parameters are very c l o s e . Thus, although these r e a c ­ t i o n s with 0 seem to go v i a some path i n a d d i t i o n to homolytic f i s s i o n , they do go i n l a r g e part by the homolytic f i s s i o n path and the a c t i v a t i o n parameters are l i k e l y to be g e n e r a l l y very c l o s e to those f o r homolytic f i s s i o n . I t has t h e r e f o r e been con­ cluded (12, 23) that a c t i v a t i o n parameters equal, or very c l o s e , to those f o r homolytic f i s s i o n can be assigned to r e a c t i o n s of the decacarbonyls and many of t h e i r a x i a l l y d i s u b s t i t u t e d d e r i v a t i v e s with CO. These a c t i v a t i o n parameters can be taken as an e x c e l l e n t q u a n t i t a t i v e measure of the r e a c t i v i t y of the metal-metal bonds and the a c t i v a t i o n e n t h a l p i e s are a good measure o f the strengths of the metal-metal bonds. The p o s i t i o n of the monosubstituted complexes Mn (C0)gL i s not so c l e a r . I f they reacted only v i a homolytic f i s s i o n then r e a c t i o n s with 0 , L, and CO should a l l proceed at the same r a t e . However, f o r M n ( C 0 ) ( P P h ) (16) the rates are not the same, nor are the a c t i v a t i o n e n t h a l p i e s very c l o s e . The assignment (12) to homolytic f i s s i o n of the a c t i v a t i o n parameters f o r r e a c t i o n of the monosubstituted complexes with 0 i s t h e r e f o r e l e s s c o n c l u s i v e than i n the other cases. In t o t a l there are 23 complexes of these group 7 metals f o r which a c t i v a t i o n parameters can be assigned to homolytic f i s s i o n (12, 23). Eight of these are based on good k i n e t i c evidence, 9 are based on analogy to very c l o s e l y r e l a t e d complexes i n the f i r s t group (e.g. M n ( C O ) ( P E t ) v s . Mn (CO) (P-n-Bu ), and Mn (C0) {PPh (0Me)} vs. M n ( C O ) ( P P h ) ) or on i n d i r e c t k i n e t i c evidence, and the remaining 6 are based on more d i s t a n t analogy or must be considered t e n t a t i v e . These values are l i s t e d i n Table I and enable the f o l l o w i n g trends to be d i s c e r n i b l e . 3

2

3

2

2

2

8

3

3

8

3

2

6

n

3

i+

2

2

2

2

2

2

9

3

2

2

2

8

2

2

8

3

2

2

8

2

3

8

3

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. 2

8

3

2

2

3

8

3

2

3

± 2.2

138.0 133.1 ± 2.1

± 1.3

155.2

8

3

Q

2

9

133.4

3

± 2.4

2

9

9

6

1 1

3

)

3

Mn (C0) {P(OPh) }

2

Mn (CO) P(C H

Mn (C0) (PBu )

Group C

2

± 0.4

138.7

± 3.3

f

151.2 ± 0.9

141.0 ± 1.5

140.9

152.1 ± 0.6

± 1.1

Assignment of AH£

2

123.6

134.0 ± 0.4

99.1 ± 1.3

132.2 ± 1.6

117.2 ± 0.8

151.0 ± 2.1

Bonds

A s s i g n m e n t o f ΔΗΤ^ u n c e r t a i n .

P o s i t i v e k i n e t i c evidence f o r r e v e r s i b l e homolytic f i s s i o n .

9

2

2

2

Mn (CO) (AsPh )

8

2

2

2

8

2

Mn (C0) {PPh (0Me)}

3

± 0.2

i +

131.9

6

2

2

8

Mn (C0) {PPh(0Me) }

2

± 0.4

2

152.5

2

Mn (CO) {P(p-MeOC H ) }

8

2

136.0 ± 1.6

2

3

Mn (CO) (PEt Ph)

6

162.2 ± 2.1

8

made b y a n a l o g y w i t h v e r y c l o s e l y r e l a t e d c o m p l e x e s .

a

2

Mn (CO) (PPh )

2

Tc (CO) (PPh )

8

MnRe(CO) (PPh )

8

2

2

8

Mn (CO) (PEtPh )

2

2

8

Mn (CO) {P(OMe) }

2

3

Mn (CO) (PEt )

8

2

2

Re (CO) (PPh )

Group Β

2

3

Mn (CO) {P(C Hn) }

1 0

8

8

165.5 ± 0.8

2

2

2

3

2

Re (CO)

2

2

Mn (CO) {P(OPh) }

Group A

Mn (C0) (P-n-Bu )

3

Table I f o r H o m o l y t i c F i s s i o n o f Some M e t a l - M e t a l

162.8 ± 0.8

3

)

± 1.6

- 1

MnRe(CO)! ο

153.8

(inkJ mol

Mn (CO) (PPh )

1 0

Tc (CO)

Enthalpies

160.1 ± 0.8

1 0

Mn (CO)

2

Activation

142

REACTIVITY

OF M E T A L - M E T A L

BONDS

( i ) Dependence on the Metal. The order o f AHfif i s M n 2 ( C O ) < Tc2(CO) < MnRe(CO) < R e 2 ( C O ) . This trend i s i n agreement w i t h the g e n e r a l l y accepted i n c r e a s e of metal-metal bond strengths with i n c r e a s i n g atomic number 0 8 , 24) and there i s a good c o r r e l a ­ t i o n (25) with the f o r c e constants f o r the metal-metal s t r e t c h i n g v i b r a t i o n (26, 27) and a l s o with the values of hv (σ σ * ) , the energy of e l e c t r o n i c t r a n s i t i o n from the metal-metal bonding molecular o r b i t a l to the corresponding anti-bonding o r b i t a l (28, 29). The o v e r a l l change along the s e r i e s i s only about 12 k J mol"" , however.

10

1 0

10

1 0

1

( i i ) The E f f e c t of the Presence o f Two A x i a l PPh^ or_As_Ph Ligands. The presence of two a x i a l PPh s u b s t i t u e n t s r e s u l t s i n a decrease i n AHgf, the decrease being i n the order Mn > Tc2 > MnRe > Re2« This i s suggestive o f a s t e r i c e f f e c t that decreases with i n c r e a s i n g s i z e o f the l i g a n d s i n t o Mn2(CO)i i n t r o d u c t i o n of PPh . This i s compatible with the s m a l l e r s i z e o f A s P h (23). 3

3

2

3

3

( i i i ) The E f f e c t o f Successive S u b s t i t u t i o n of P-donor L i g ­ ands i n t o Mno(C0)Tn. In s p i t e of the u n c e r t a i n t y o f assignment of values o f AHftf f o r the monosubstituted complexes the s u c c e s s i v e i n t r o d u c t i o n o | PPh o r PCy l i g a n d s r e s u l t s i n a p r o g r e s s i v e decrease i n AHftf. The same may be true f o r P-n-Bu but i s not true f o r P(OPh) f o r which no o v e r a l l decrease i s observed. 3

3

3

3

( i v ) C o r r e l a t i o n o f AHfif with Values of hv (σ -> σ ) . The c o r r e l a t i o n found f o r the decacarbonyls (see above) i s a l s o found to i n c l u d e some complexes s u b s t i t u t e d with one or even two smaller P-donor l i g a n d s as shown i n F i g u r e 1. The e l e c t r o n i c e f f e c t s , e i t h e r o f changing the metal or of the presence o f small P-donor l i g a n d s , i s t h e r e f o r e q u i t e small whereas s t e r i c e f f e c t s seem to be much g r e a t e r . This i s confirmed as f o l l o w s . (v) C o r r e l a t i o n o f AHitf with Cone Angles of L i n M n ^ C O ^ L ? . An e x c e l l e n t c o r r e l a t i o n with the s i z e o f the s u b s t i t u e n t i s found when AH^f i s p l o t t e d a g a i n s t the cone angles o f L (12, 23) as shown i n F i g u r e 2. I n t r o d u c t i o n of small l i g a n d s has very l i t t l e e f f e c t but when the cone angle i s £l25° AH^f begins to decrease and when L = Ρ ( 0 Η ) (cone angle = ca. 180°) the value o f AHjjf has been reduced from ca. 154 k J mol" to 100 k J m o l " . A rather d e t a i l e d a n a l y s i s of the s t e r i c e f f e c t s o f l i g a n d s P P h ( 0 M e ) _ and P P h E t _ suggests (23) that v a l u e s of AH^f can even r e f l e c t the more s u b t l e problems caused by the d e t a i l e d mesh­ i n g o f an a x i a l l i g a n d , with e s s e n t i a l l y a 3 - f o l d a x i s o f symme­ t r y , with the manganese atom, surrounded by four e q u a t o r i a l CO ligands. The absence of such strong s t e r i c e f f e c t s on the values of hv (σ σ*) (29) suggests that the strong s t e r i c e f f e c t s on the ease 6

η

3

1

n

3

n

n

3

1

n

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

Metal-Metal-Bonded

POE

143

Carbonyls

-J

I

325

I

350

L

375

400

hv(o—•σ*) / k J m o l

- 1

Inorganic Chemistry Figure

1.

Dependence of enthalpies of activation for homolytic fission, the energies of the corresponding σ —> σ* transitions (12).

ΔΗ;,/,

on

Complexes are: (1) Mn (CO), ; (2) Mn (CO) P(OPh) ; (4) Mn (CO) PBu ; (6) Mn (CO) PPh,; (10) Mn (CO) {P(OMe) } ; (11) Mn (CO) {P(OPh),} ; (19) MnRe(CO) ; (20) MnRe(CO) (PPh ) ; (21) Tc (CO) ; (23) Re (CO) ; (24) Re (CO) (PPh ) . 2

2

8

8

0

2

9

3 2

3 2

3

2

2

10

2

8

9

3

2

2

2

10

l0

2

8

3 2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

9

144

REACTIVITY

100

120

140

OF

160

METAL-METAL

BONDS

180°

Ligand Cone Angle Inorganic Chemistry Figure 2.

Dependence

of

Δ Η

Λ

/

$

for Mn (CO) L 2

8

2

on the cone angle of L (23).

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7. POE

Metal-Metal-Bonded

145

Carbonyls

of homolytic f i s s i o n of the Mn complexes a r i s e s mainly from the r e l e a s e of s t e r i c s t r a i n that occurs on s t r e t c h i n g the Mn-Mn bond, r a t h e r than from a weakening of the Mn-Mn bond strength i n the undisturbed molecule. These s t e r i c e f f e c t s show up not only i n the values of AH^f but i n the rates themselves. Thus, while Mn (CO)io * i t s deri­ v a t i v e s c o n t a i n i n g smaller ligands r e a c t a t convenient r a t e s (convenient, that i s , f o r k i n e t i c or s y n t h e t i c purposes) only a t temperatures s i g n i f i c a n t l y above 100°C, M n ( C 0 ) ( P P h ) r e a c t s smoothly a t ca. 50°C (16), while M n ( C O ) { P ( C H ! χ ) } r e a c t s q u i t e r a p i d l y a t room temperature (12). 2

a n c

2

2

2

8

8

3

6

3

2

2

Reactions with Iodine and Bromine. A l l the r e a c t i o n s d i s ­ cussed above proceed at l i m i t i n g rates that are independent of the concentration and nature o f the r e a c t a n t However r e a c t i o n s of i o d i n e with M n ( C O ) that are f i r s t order i homolytic f i s s i o n paths. P-donor s u b s t i t u e n t s increase the rates of r e a c t i o n with I by s e v e r a l orders of magnitude (31) so that they proceed r a p i d l y even a t room temperature. Thus r e a c t i o n of M n ( C 0 ) { P ( C H ) } i s estimated to occur over 1 0 times f a s t e r than M n ( C O ) by a path f i r s t order i n [ I ] a t 25°C i n cyclohexane. In a l l cases the r e a c t i o n s proceed with f i s s i o n o f the metalmetal bonds to form the mononuclear iodo complexes. D e t a i l e d k i n e t i c s t u d i e s of these r e a c t i o n s have shown that they f r e q u e n t l y proceed according to very complicated r a t e equa­ t i o n s (31). Thus M n ( C 0 ) { P ( 0 P h ) } r e a c t s according to eq 15. 2

2

8

2

8

6

2

1 1

3

2

10

2

2

k

obsd

=

α

χ

3[ 2]

2

8

3

3

+ MI ] }/U 2

2

+ 3 [I ] 2

2

3

+ 3 [I ] }

2

3

(15)

2

3

T h i r t y - f i v e values of kobsd over the range [ I ] = (1-40) x 10" M f i t t h i s equation with a standard d e v i a t i o n of 4.6%. Setting 3 = 0 increases t h i s to 5.9%. Reaction o f R e ( C 0 ) ( P P h ) with I i n c l u d e s a term i n [ Ι ] which appears d e f i n i t e l y to be f i n i t e although i t i s rather i m p r e c i s e l y determined. This k i n e t i c behavior i s a s c r i b e d to the r a p i d preformation of adducts containing up to four I molecules followed by slower f i s s i o n of the metal-metal bonds and subsequent formation o f the mononuclear iodo complexes. The formation o f adducts can i n some cases be complete and i s q u i t e dependent on the nature of the s o l v e n t . A term k [ I ] i s common to most of the r e a c t i o n s and k i s found to increase d r a m a t i c a l l y with i n c r e a s i n g b a s i c i t y of L i n M n ( C 0 ) L . Thus i t v a r i e s from 6.7 x 10" * M~ s when L = P(0Ph) to ca. 10 M" S " when L = PPhEt f o r r e a c t i o n i n c y c l o hexane a t 25°C. Quite a good LFER i s shown by a p l o t of l o g k against Δ(ηηρ), the r e l a t i v e h a l f - n e u t r a l i z a t i o n p o t e n t i a l f o r t i t r a t i o n o f pure s u b s t i t u e n t l i g a n d against p e r c h l o r i c a c i d i n nitromethane. This increase i n r a t e i s a l s o c l o s e l y r e l a t e d to a decrease i n VQQ when L = phosphine. The value of l o g k f o r M n ( C 0 ) { P ( C H ) } deviates by a t l e a s t 3 orders o f magnitude 2

3

2

9

3

2

4

2

2

2

3

2

3

1

2

8

2

_ 1

2

2

3

1

2

3

3

2

8

6

n

3

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

146

REACTIVITY OF

METAL-METAL

BONDS

from these l i n e a r c o r r e l a t i o n s but no such pronounced, and p r e ­ sumably s t e r i c , d e v i a t i o n s are shown by other l i g a n d s . I t i s concluded that the I molecules i n the adduct are attached c l o s e to the 0 atoms of the CO l i g a n d s and that t h e i r r o l e i s to weaken the metal-metal bonds by i n d u c t i v e l y withdrawing e l e c t r o n d e n s i t y from the metal centers through the CO l i g a n d s ( 3 1 ) . Reactions of B r show e s s e n t i a l l y the same type of behavior ( 3 2 ) with s u b s t i t u t e d carbonyls although the extent of adduct formation i s s i g n i f i c a n t l y lower. Reaction of Mn (CO)io with B r i s , however, unique ( 3 3 ) i n proceeding v i a a chain r e a c t i o n , the propagation steps presumably being as i n eq 1 6 and 1 7 . 2

2

2

•Mn(CO) + B r 5

•Br + M n ( C O ) 2

2

>• Mn(CO) Br + · Β Γ

(16)

• Mn(CO) Br + *Mn(CO)

(17)

5

1

2

R e a c t i v i t i e s and Reactio tuted D e r i v a t i v e s R e l a t i v e l y few k i n e t i c s t u d i e s of such complexes have been undertaken and none has suggested that spontaneous homolytic f i s s i o n of the Co-Co bond i s i m p l i c a t e d as a rate-determining step although the importance of metal-centered r a d i c a l s has been shown ( 3 4 , 3 5 ) . Reaction of C o ( C O ) with alkynes to form the alkyne-bridged C o ( C 0 ) ( C R ) has been shown to i n v o l v e r a t e determining d i s s o c i a t i o n of one CO l i g a n d and t h i s i s q u i t e r a p i d even at room temperature ( 3 6 ) . Reaction of the a x i a l l y d i s u b s t i tuted complex C o ( C O ) ( P - n - B u ) with C Pti2 i s very much slower ( 3 7 ) and leads to formation of C o ( C O ) ( P - n - B u ) ( C P h ) i n d e c a l i n at 1 0 0 ° C . A major path i n v o l v e s d i s s o c i a t i v e l o s s f i r s t of a CO l i g a n d and then of a P-n-Bu l i g a n d and t h i s i s followed by a t t a c k of the C P h on the C o ( C 0 ) ( P - n - B u ) intermediate. T h i s i n t e r ­ mediate may w e l l i n v o l v e a CoΞCo t r i p l e bond by analogy with the w e l l - c h a r a c t e r i z e d complexes of the type Cp Mo (CO)i4. (Cp = η C5H5) ( 3 8 ) . The d i s s o c i a t i v e mechanism i s t h e r e f o r e probably favored by the s t a b i l i z a t i o n , due to i n c r e a s e d Co-Co bonding, of what would otherwise be a c o o r d i n a t i v e l y unsaturated intermediate. In t h i s sense the metal-metal bonding i s i n c r e a s i n g the r e a c t i v i t y of the complex towards l i g a n d d i s s o c i a t i o n . The formation of t h i s intermediate proceeds v i a the f i r s t formed C o ( C 0 ) s ( P - n - B u ) and the vacant c o o r d i n a t i o n s i t e on one Co atom may a l s o be s t a b i l i z e d by sideways-on bonding of a CO l i g a n d on the other Co atom i n the way found i n M n ( C 0 ) ( d p p m ) (dppm = Ph PCH PPh ) ( 3 9 ) . Again, the presence of the neighboring Co atom, i . e . of the metal-metal bond, must be h e l p i n g determine the r e a c t i v i t y of the complex towards d i s s o c i a t i o n of one CO l i g a n d . The Co (CO)s(P-n-Bu ) can a l s o be formed by d i s s o c i a t i o n f i r s t of a P-n-Bu l i g a n d followed by l o s s of a CO l i g a n d . This sequence of d i s s o c i a t i o n s i s con­ s i d e r a b l y l e s s important than l o s s of CO f i r s t and then P-n-Bu . An almost i d e n t i c a l p a t t e r n i s found f o r r e a c t i o n of C o ( C 0 ) g { P ( C H ) } with C P h ( 4 0 ) . 2

2

6

2

2

8

2

6

3

2

2

2

5

3

2

2

3

2

2

2

5

3

5

2

2

2

2

5

2

2

3

2

2

2

2

3

3

3

2

6

n

3

2

2

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

Metal-Metal-Bonded

PO£

147

Carbonyls

A t h i r d path followed i n t h i s r e a c t i o n i n v o l v e s h i g h l y r e ­ v e r s i b l e formation of an e n e r g e t i c a l l y e x c i t e d isomer of the com­ p l e x which can be attacked d i r e c t l y by the C 2 P h before subsequent r e v e r s i b l e l o s s of a CO and a P-n-Bu3 l i g a n d . No d i r e c t evidence f o r the nature of t h i s intermediate i s a v a i l a b l e but i t has been speculated (37) to be a CO-bridged complex as shown i n I, the r e a c t i o n i n v o l v i n g a metal m i g r a t i o n analogous to the well-known methyl m i g r a t i o n process (41). The vacant c o o r d i n a t i o n p o s i t i o n on one of the Co atoms provides a s i t e f o r d i r e c t a t t a c k by the C Ph2. A very c l o s e analogue, I I , has been i m p l i c a t e d i n the photochemical r e a c t i o n of Cp2Fe2(C0)i+ with P ( 0 - i - P r ) to form I I I at -78°C i n e t h y l c h l o r i d e o r THF (42). I was a l s o p o s t u l a t e d as the intermediate i n v o l v e d i n the formation of IV by the i n s e r t i o n of S n C l i n t o C o ( C O ) ( P - n - B u ) i n THF (43). This r e a c t i o n pro­ ceeds a t a l i m i t i n g r a t e a t high [ S n C l 2 l and i s much more r a p i d than r e a c t i o n with C 2 P h n u c l e o p h i l i c i t y of S n C l the o r i g i n a l Co (CO)g(P-n-Bu )2 from I when [ S n C l 2 l i s high enough. The k i n e t i c s show that a t t a c k by C2PI12 on I i s slow compared to r e v e r s i o n to the r e a c t a n t complex. This mechanism i s an example of the wider v a r i e t y of mechanisms a v a i l a b l e to metal-metal-bonded complexes, t h i s one being p o s s i b l e because of the s p e c i a l a b i l i t y of a carbonyl l i g a n d to b r i d g e two metal atoms. T h i s type of mechanism has a l s o been proposed f o r i n s e r t i o n of stannous h a l i d e s i n t o other metal-metal-bonded complexes (44). 2

2

3

2

2

6

3

2

2

3

A v e r y s i m i l a r intermediate, V, has been i m p l i c a t e d i n the r e a c t i o n of VI with alkynes (45), and with CO or PPh to form V I I (46). In t h i s case the r e a c t i v i t y of VI i s c o n t r o l l e d by the ease of breaking the Co-Co bond, the conversion of a b r i d g i n g CO i n t o a t e r m i n a l one i n v o l v i n g l i t t l e or no expenditure of energy. Since the r e a c t i o n s with CO o r PPh are s t r i c t l y f i r s t order i n the con­ c e n t r a t i o n of e n t e r i n g l i g a n d (46) the o v e r a l l a c t i v a t i o n enthalpy i s made up of ΔΗ° f o r the h i g h l y r e v e r s i b l e formation of V from VI p l u s the value of A H $ L f o r a t t a c k by CO or PPh on V. The o v e r a l l values of ΔΗ* = ΔΗ° + AH$ are 92.8 ± 1.7 and 72.4 ± 1.4 k J mol f o r L = CO and PPh , r e s p e c t i v e l y . The d i f f e r e n c e between these two values i s r a t h e r l a r g e r than one might expect f o r simple a t t a c k of L on a vacant c o o r d i n a t i o n s i t e and t h i s may be an i n d i c a t i o n that V i s s t a b i l i z e d to some extent by sideways-on b r i d g i n g of a CO group as i n V I I I . This would s t a b i l i z e the intermediate and make i t more s e l e c t i v e towards n u c l e o p h i l i c attack. 3

3

3

L

1

3

R e a c t i v i t i e s and Reaction Mechanisms of Some Metal-Carbonyl Clusters Reactions of Ru (CO)i2-nLn (L = PPh , PBu , and P(OPh) ; n = 0-3). Ru (CO) r e a c t s thermally with CO to form R u ( C 0 ) only at r e l a t i v e l y high temperatures under h i g h pressures of CO, and f u l l 3

3

3

3

1 2

3

5

American Chemical Society Library 1155 16th St. N. w. In Reactivity of Metal-Metal Chisholm, M.; Washington, D.Bonds; C. 20038

ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

148

REACTIVITY OF

METAL-METAL

BONDS

0 C (P-n-Bu ) (OC) Co S

\ o ( CO ) (P-n-Bu )

3

2

0

0

II

II

c

c

/ \ Cp(C0) Fe

*

/ \

Fe(CO)Cp

2

Cp(OC) Fe 2

II

Fe(CO){P(0-i-Pr) )Cp 3

II a.

C I

Sn (P-n-Bu )(0C) Co 3

//

>S

3

'Co(C0) (P-n-Bu ) 3

3

(OO^^Co^

^Co(CO)

3

IV

Ph

Ph

\/

(OC) Co^—^Co(CO) 3

Ph

Ph

X

(OO.Co^ 4

3

^C

Co(C0) L 3 o

VII

II

0 VI

Ph

Ph

(OC) Co^

\o(C0)

VIII *

5 Cp = η - C H 5

5

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

Metal-Metal-Bonded

POE

149

Carbonyls

k i n e t i c data a r e not a v a i l a b l e (47). However, the complexes R113 (CO) χ 2 - n ( h 3 ) (n = 1-3) can undergo thermal fragmentation r e a c t i o n s i n d e c a l i n a t 130-170°C to form the w e l l - d e f i n e d mono­ nuclear products Ru(CO) (PPh ) and/or R u ( C O ) ( P P h ) (48, 4 9 ) . When [CO] and [PPI13] a r e high enough the r a t e s a r e found to be independent of [CO] and [ΡΡΙΊ3] so that spontaneous fragmentation of the complexes i s o c c u r r i n g without p r e l i m i n a r y d i s s o c i a t i o n of CO or PPh and without a s s i s t a n c e from bimolecular a t t a c k by these l i g a n d s . The k i n e t i c measurements are therefore a good measure of the i n t r i n s i c k i n e t i c s t a b i l i t y of the RU3 c l u s t e r s . The a c t i v a ­ t i o n enthalpies increase d r a m a t i c a l l y with n, being 86.4 ± 0.9, 124.2 ± 3 . 6 , and 147.7 ί 5.0 k J mol~*, r e s p e c t i v e l y , when η = 1, 2, or 3. The a c t i v a t i o n parameters f a l l on a good i s o k i n e t i c p l o t of ΔΗ* against AS* with an i s o k i n e t i c temperature of ca. 200°C. The r e l a t i v e r e a c t i v i t i e s a t the temperatures used are t h e r e f o r e e n t h a l p y - c o n t r o l l e d an 100 a t 100°C when η change selves do not d i s t i n g u i s h whether one, two, o r even three Ru-Ru bonds a r e i n the process o f being broken when the t r a n s i t i o n s t a t e i s reached. Concerted fragmentation i n t o three Ru(CO) PPh moieties i s u n l i k e l y s i n c e r e a c t i o n s such as these are r e v e r s i b l e and the aggregation r e a c t i o n would then have to be t r i m o l e c u l a r . Simply breaking one Ru-Ru bond would be u n l i k e l y to lead to the trends i n a c t i v a t i o n parameters observed, a decrease of ΔΗ* with η being more probable due to s t e r i c e f f e c t s of the s o r t discussed above f o r the group 7 metal carbonyls. The a c t i v a t i o n parameters can be r a t i o n a l i z e d (49) i f the complexes break i n t o two f r a g ­ ments, one o f which i s always Ru(CO) 3 ( P P I 1 3 ) , the other being R u ( C O ) , R u ( C O ) ( P P h ) , or R u ( C 0 ) ( P P h ) , r e s p e c t i v e l y , as η changes from 1 to 3. In order to a t t a i n an 18-electron c o n f i g u r a ­ t i o n the R u intermediates would have to c o n t a i n a Ru=Ru double bond of the type which i s now q u i t e w e l l known f o r somewhat analo­ gous complexes (50). The enthalpy of these d i n u c l e a r intermedi­ ates, however, would be expected to increase with i n c r e a s i n g num­ bers of s u b s t i t u e n t ΡΡΙΊ3 l i g a n d s because of l i g a n d - l i g a n d r e p u l ­ sions and so the a c t i v a t i o n enthalpy to form these i n t e r m e d i ­ ates would be expected to increase i n the d i r e c t i o n observed. The i n c r e a s e i n AS* along the s e r i e s probably implies that the Ru-Ru bond strength a c t u a l l y decreases along the s e r i e s to an extent that v i b r a t i o n a l entropy i s much i n c r e a s e d . Whether t h i s mechan­ i s t i c i n t e r p r e t a t i o n i s c o r r e c t or not, the f a c t remains that the a c t i v a t i o n e n t h a l p i e s , and to some extent the r e a c t i v i t i e s , do show a c l e a r dependence on the number of s u b s t i t u e n t s , a depend­ ence that i s opposite to that i n M n ( C O ) χ - n ( P P h ) (n = 0-2) (12). G e n e r a l i z a t i o n of the e f f e c t s of l i g a n d s u b s t i t u t i o n on the r e a c t i v i t y of metal-metal bonded carbonyls i s t h e r e f o r e premature at t h i s stage. p p

n

i+

3

3

3

2

3

3

2

8

2

7

3

2

6

3

3

2

2

2

0

3

n

In a d d i t i o n to spontaneous fragmentation there i s a more r a p i d path that i s i n h i b i t e d by CO and that t h e r e f o r e i n v o l v e s l o s s of CO i n the rate-determining step. This leads to fragmenta-

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

REACTIVITY OF

150

METAL-METAL

BONDS

tion for Ru (CO) L (L = PPh , P-n-Bu , and P(0Ph) ) (51) and simple s u b s t i t u t i o n f o r Ru (CO) _ ( P P h ) (n = 1 or 2) (49). For R u ( C O ) ( P - n - B u ) the product i s simply R u ( C O ) ( P - n - B u ) but f o r R u ( C 0 ) { P ( O P h ) } the mononuclear product was the o r t h o m e t a l l a t e d Ru(CO) {P(0C H ) ( O P h ) } · H^Ru^ (CO) { P ( 0 P h ) } and R u H ( C O ) { P ( O C H i ) ( O P h ) } 2 ( ° n ) were a l s o formed so aggregation and o r t h o m e t a l l a t i o n of intermediate fragments can a l s o occur i n t h i s case. T h i s d i d not appear to have any e f f e c t on the form of the k i n e t i c s . However, f o r both these two s u b s t i t u t e d t r i n u c l e a r c l u s t e r s the k i n e t i c s are more complicated than would be expected from a t t a c k by pure l i g a n d on i n i t i a l l y generated Ru (CO)eL (51). F u r t h e r , the a c t i v a t i o n parameters f o r formation of the interme­ d i a t e s Ru (CO)8L3 depend g r e a t l y on the nature of L v a r y i n g from a low ΔΗ* (81.6 ± 4.6 k J mol" ) and r a t h e r negative AS* (-88.7 ± 4 . 2 JK" mol" ) f o r L = P-n-Buo to a much higher ΔΗ+ (138.1 ± 4.6 k J mol" ) m o l " ) when L = PPh . Ru (CO)8L intermediates v a r i e s considerably with the nature of L. The nature of these intermediates could vary from IX to X I I as Δ Η decreases i n energy. Formation of IX might occur i f the thermal energy cannot simply be concentrated on breaking j u s t an Ru-CO bond. Not only do the a c t i v a t i o n parameters r e q u i r e major d i f f e r e n c e s i n the s t r u c t u r e s of the intermediates but the f a c t that the k i n e t i c s themselves ( f o r L = P-n-Bu and P(0Ph) ) r e ­ q u i r e the existence of two k i n e t i c a l l y d i s t i n c t isomeric forms of Ru (CO)8L to be produced means that the need to s p e c u l a t e on the p o s s i b l e s t r u c t u r e s i s imposed by the form of the k i n e t i c s as w e l l as by the e n e r g e t i c s (51). Such a range of intermediates i s only conceivable by v i r t u e of the c l u s t e r nature of the complexes so the wide range of r e a c t i v i t i e s can be d i r e c t l y a t t r i b u t e d to the polynuclear nature of the complex, i . e . the r e a c t i v i t y towards an e s s e n t i a l l y d i s s o c i a t i v e process i s determined by ener­ g e t i c p r o p e r t i e s of the metal c l u s t e r . In a d d i t i o n to the C O - d i s s o c i a t i v e path the complexes can a l s o undergo fragmentation v i a rate-determining P-donor-ligand d i s s o c i a t i o n and t h i s i s much f a s t e r than e i t h e r the spontaneous fragmentation or the C O - d i s s o c i a t i v e paths (48, 49, 51). In a d d i t i o n to mononuclear products, R u ( C g H ^ ) ( C 0 ) ( P P h ) , R u ( C O ) H{P(C Hi )Ph }(PPh ), and R u ( C 0 ) { P ( C H ) P h } 2 appeared to be formed from R u ( C 0 ) ( P P h ) . Q u a n t i t a t i v e k i n e t i c s t u d i e s were not made f o r the P-n-Bu or P(0Ph) d i s s o c i a t i v e r e a c t i o n s of R u ( C 0 ) ( P - n - B u ) or R u ( C 0 ) { P ( O P h ) } and no data are a v a i l a b l e f o r comparison with the r e s u l t s f o r PPh d i s s o c i a t i o n from R u (CO) (PPh ) . Another, and q u i t e unique, type of k i n e t i c behavior i s shown by R u ( C O ) ( P P h ) i n i t s r e a c t i o n s with 0 i n the presence of s u f f i c i e n t f r e e PPh to suppress the d i s s o c i a t i o n of the complexed PPh (52). This r e a c t i o n i n d e c a l i n at 55-75°C leads to a yellow i n s o l u b l e product that has not yet been f u l l y c h a r a c t e r ­ i z e d . One mole of PPh i s o x i d i z e d to 0PPh f o r every mole of Ru 3

9

3

3

3

3

3

9

3

3

x2

6

n

3

3

3

2

3

6

i+

2

OP

3

3

3

3

9

2

2

n

+

2

9

3

3

p

2

2

3

3

3

1

1

1

1

1

3

3

3

Τ

3

3

3

3

3

6

t

2

3

3

2

9

3

6

9

3

3

4

2

2

3

3

9

3

3

3

3

9

3

3

2

3

3

3

9

3

3

3

9

3

2

3

3

3

7

6

3

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

POE

Metal-Metal-Bonded

151

Carbonyls

/ L(OC) Ru* 3

Ru(CO) L \ -Ru(CO) L Q 3

2

IX

L(OC) Ru

Ru(CO) L

3

2

Ru^CO) L 3

L(OC) Ru

Ru(CO) L

XI

/

L(0C) Ru o

2

Ru(CO) L 3

\/

Ru(C0) L 2

o

II

ο XII

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

152

REACTIVITY

OF

METAL-METAL

BONDS

i n the o r i g i n a l complex. Although the d e t a i l e d natures of the o v e r a l l r e a c t i o n products were not e l u c i d a t e d the k i n e t i c data were c l e a r enough to be s u s c e p t i b l e to d e t a i l e d a n a l y s i s . Of part i c u l a r importance i n t h i s was the p r e c i s e dependence of the r a t e s on the concentrations of R u ( C O ) g ( P P h ) and 0 , there being no dependence on [PPh ] provided t h i s was s u f f i c i e n t to suppress the P P h - d i s s o c i a t i v e path. The a n a l y s i s showed q u i t e c o n c l u s i v e l y that the complex was undergoing r e v e r s i b l e fragmentation i n t o two s p e c i e s , only one of which reacted with 0 at_ a. r a t e p r o p o r t i o n a l to [0?]. I t was concluded that the two fragments were most l i k e l y to be R u ( C O ) ( P P h ) and Ru(CO) (PPh ). The former was envisaged not to undergo r e a c t i o n with 0 , some form of f u r t h e r decomposition being r e q u i r e d f i r s t . The Ru(CO) (PPh ) c o u l d not be i n i t s s p i n - p a i r e d , p l a n a r d form. That intermediate must c e r t a i n l y be i n v o l v e d i n the C O - d i s s o c i a t i v e r e a c t i o n of RuiCO)^(PPh ) with PPh to for i t s r a t e of formation, oxygen (53). I f diamagnetic Ru(CO) (PPh ) were an intermediate R u ( C O ) ( P P h ) should have been observed as a product but i t was not. I t was t h e r e f o r e proposed that the Ru(CO) (PPh ) was i n the h i g h - s p i n , pseudo-tetrahedral c o n f i g u r a t i o n , a form that would be expected to be r e a d i l y s u s c e p t i b l e to a t t a c k by 0 . This c o n c l u s i o n r e q u i r e s the Ru(CO) (PPh ) formed i n the high-temperature fragmentation i n the presence of CO and PPh to be i n the lows p i n , planar form. Intermediates formed i n the r e a c t i o n of 0 with h i g h - s p i n Ru(CO) PPh could w e l l cause the formation of OPPh . The l i m i t i n g r a t e of the low-temperature fragmentation process i s such that, at 100°C, f o r every time spontaneous r e a c t i o n occurs i n the presence of CO and PPh with eventual formation of R u ( C O ) ( P P h ) the fragmentation to form R u ( C 0 ) ( P P h ) and h i g h s p i n Ru(CO) (PPh ) must occur over 10^ times. Because of the absence of 0 t h i s fragmentation leads nowhere and i s always f o l lowed by recombination. S i m i l a r l y , f o r every time CO d i s s o c i a t i o n occurs t h i s fragmentation and i t s reverse must occur 10 times. It must be emphasized that t h i s c o n c l u s i o n i s q u i t e unaffected by the correctness or otherwise of any of the s p e c u l a t i o n s about the d e t a i l e d nature of the intermediates i n v o l v e d and i s a d i r e c t consequence of the k i n e t i c data. I t i s q u i t e c l e a r that the suscepti b i l i t y of t h i s Ru c l u s t e r towards eventual fragmentation depends g r e a t l y on the p a r t i c u l a r path followed. There are four of these a v a i l a b l e , the one a c t u a l l y followed being determined by the part i c u l a r conditions. A s u r p r i s i n g q u a l i t a t i v e o b s e r v a t i o n r e l a t i n g to the s t a b i l i t y of Ru c l u s t e r s was made when the k i n e t i c s of r e a c t i o n of Ru (CO) with P-n-Bu were s t u d i e d (54). R u ( C O ) ( P - n - B u ) can e a s i l y be prepared by t h i s r e a c t i o n but the products i n the k i n e t i c study were Ru(C0)i+(P-n-Bu ) and R u ( C 0 ) ( P - n - B u ) i n the mole r a t i o 2:1. The fragmentation cannot have occurred a f t e r formation of Ru (CO)g(P-n-Bu ) which i s q u i t e s t a b l e under the c o n d i t i o n s p e r t a i n i n g (51, 54). I t was concluded that the sequence of r e a c t i o n s 18-22 best explained the r e s u l t s . At the 3

3

3

2

3

3

2

2

6

3

2

3

3

2

3

3

3

3

8

3

3

3

3

3

3

2

2

3

3

3

2

3

3

3

3

3

3

2

2

3

6

3

2

3

2

3

3

3

3

1 2

3

3

3

3

3

9

3

3

3

2

3

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

3

POE

7.

Metal-Metal-Bonded

5

Ru (CO) 3

+ P-n-Bu

1 2

153

Carbonyls

3

(18)

R u ( C O ) n ( P - n - B u ) + CO 3

3

Ru (CO) (P-n-Bu )

2Ru(CO)4 + Ru(CO) (P-n-Bu )(19)

2Ru(CO)

3

n

3

3

3

+ P-n-Bu

3

2Ru(CO) (P-n-Bu )

(20)

Ru(CO) (P-n-Bu ) + P-n-Bu

3

Ru(CO) (P-n-Bu )

(21)

3

i+

3

Ru(CO) (P-n-Bu ) 3

3

i+

3

3

3

2

1/3 R u ( C O ) ( P - n - B u ) 3

9

3

3

(22)

low concentrations of complex and r e l a t i v e l y high values of [P-nBu ] used i n the k i n e t i c study r e a c t i o n s 20-21 predominate over 22 and the l a t t e r only becomes e f f e c t i v e when the c o n c e n t r a t i o n of complex i s high as i n p r e p a r a t i v e work. This shows q u a l i t a t i v e l y that Ru (C0)χ ι(P-n-Bu ) i s p e c u l i a r l y s u s c e p t i b l e to fragmentation under much m i l d e r c o n d i t i o n other fragmentation r e a c t i o n shows that an apparently simple s u b s t i t u t i o n r e a c t i o n can i n f a c t be going v i a successive fragmentation and aggregation, j u s t as i s observed f o r r e a c t i o n 14 (14). F i n a l l y , i t i s observed that R u ( C O ) i undergoes bimolecular a t t a c k by P-donor l i g a n d s i n a d d i t i o n to simple CO d i s s o c i a t i o n (55, 56). (The absence o f second-order paths i s a s s e r t e d (57), i n c o r r e c t l y , i n the l a t e s t e d i t i o n of a standard t e x t . ) The dependence o f the r a t e s on the b a s i c i t y of the n u c l e o p h i l e s and the magnitude o f s t e r i c e f f e c t s shown by PPh and, more c l e a r l y , by Ρ(0βΗιι) shows that the c l u s t e r i s q u i t e s e l e c t i v e and that the degree of bond-making i n the t r a n s i t i o n s t a t e i s q u i t e h i g h . Bimolecular r e a c t i o n s of t h i s s o r t are very rare with simple b i n a r y carbonyls (11) and i t appears that some degree of ambiguity i n the assignment of o x i d a t i o n s t a t e , i . e . of e l e c t r o n d i s t r i b u ­ t i o n , i s a p r e r e q u i s i t e f o r n u c l e o p h i l i c a t t a c k to be p o s s i b l e (58). Thus the presence o f l i g a n d s such as NO o r n -C5H5 enhances s u s c e p t i b i l i t y to n u c l e o p h i l i c a t t a c k i n mononuclear com­ plexes and v a r i o u s b r i d g i n g groups do t h i s i n b i n u c l e a r complexes (59, 60, 61). I t seems l i k e l y that the metal-metal bonding i n c l u s t e r s i s of a s u f f i c i e n t l y p o l a r i z a b l e nature that i t can a d j u s t r e l a t i v e l y e a s i l y to the approach o f a n u c l e o p h i l e i n such a way as to lower the a c t i v a t i o n energy. Indeed, some r e a c t i o n s can be envisaged (61) as i n v o l v i n g r a p i d adduct formation by donation of e l e c t r o n s i n t o the LUMO i n the metal-metal bonding of the c l u s t e r before subsequent displacementof CO. The nature of the metal-metal bonding i n such c l u s t e r s i s t h e r e f o r e c o n t r o l l i n g t h e i r r e a c t i v i t y towards bimolecular l i g a n d s u b s t i t u t i o n . 3

3

3

3

2

3

3

5

K i n e t i c s of Reactions of Some T e t r a h e d r a l Metal Carbonyl Clusters. ΐΓ^(ΰΟ) has been shown to undergo CO s u b s t i t u t i o n with PPh by a predominantly bimolecular r e a c t i o n (62) and t h i s i s 1 2

3

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

154

REACTIVITY OF

METAL-METAL

BONDS

the case a l s o with other P-donor l i g a n d s (63). When one PPh i s introduced f u r t h e r s u b s t i t u t i o n i s very much more r a p i d and occurs v i a a simple C O - d i s s o c i a t i v e path (62). This was a s c r i b e d to the presence of b r i d g i n g carbonyls, not present i n Ιη+((Χ))ΐ2, which somehow enhance the ease of CO d i s s o c i a t i o n . I t i s not c l e a r that a s s o c i a t i v e r e a c t i o n i s a c t u a l l y d i s f a v o r e d i n an absolute sense since even i f bimolecular r e a c t i o n occurred a t the same r a t e as with Iri+(C0)i2 i t would not have been observed i n competition with the very much enhanced r a t e of CO d i s s o c i a t i o n . I t has, however, been shown that the i n t r o d u c t i o n of a P-donor l i g a n d onto one CO atom i n Co2(C0)e(C2Ph2) s t r o n g l y i n h i b i t s bimolecular a t t a c k a t the other Co atom compared with that on the i n i t i a l u n s u b s t i t u t e d complex (59). Ιη+(00)ΐ2 and i t s d e r i v a t i v e s only undergo f r a g ­ mentation i n t o Ir2 complexes under extreme c o n d i t i o n s and no k i n e t i c data are a v a i l a b l e f o r such processes (64). 004(00)12» on the under only moderately sever d e t a i l e d k i n e t i c study of t h i s r e a c t i o n has shown that i t occurs by paths f i r s t and second order i n [CO] (66). T h i s was i n t e r ­ preted to mean that i n i t i a l , h i g h l y r e v e r s i b l e formation of Co^( C 0 ) i 3 occurred and that t h i s could then undergo Co-Co bond break­ ing, e i t h e r spontaneously or w i t h the a s s i s t a n c e of a t t a c k by an a d d i t i o n a l CO molecule. No d i f f i c u l t y e x i s t s i n p o s t u l a t i n g s t r u c t u r e s a l l o w i n g f o r formal 18-electron c o n f i g u r a t i o n s around the CO atoms. Stepwise formation of intermediates c o n t a i n i n g fewer and fewer Co-Co bonds can be envisaged. This study provides a c l e a r example of c l u s t e r fragmentation being induced by nucleop h i l i c attack. Co4(C0) i s a l s o known to undergo s t r a i g h t f o r w a r d s u b s t i t u ­ t i o n r e a c t i o n s . The r a t e of the displacement of the f i r s t CO l i g a n d i s extremely r a p i d with P-, As-, and Sb-donor l i g a n d s and no k i n e t i c data are a v a i l a b l e (see below). However, i t has been shown that with P(OMe) a t l e a s t one of the subsequent stages of r e a c t i o n i s bimolecular but that i t leads to s u b s t i t u t i o n and not fragmentation (67). Studies of thermal r e a c t i o n s of other t e t r a n u c l e a r metal c a r ­ bonyl c l u s t e r s seem to be r a t h e r r a r e . Reports on some i n t e r e s t ­ ing r e a c t i o n s of complexes such as H^Ru^ (CO) χ 2 (68, 69) have r e c e n t l y appeared as have d e s c r i p t i o n s of fragmentation k i n e t i c s of complexes such as H2Rui (CO)i (70). 3

12

3

+

3

R e a c t i v i t i e s and the Strengths of Metal-Metal Bonds i n Organom e t a l l i c Compounds Estimates of metal-metal bond energies of organometallic com­ plexes i n s o l u t i o n have been made by measurements of ΔΗ° f o r homo­ l y t i c f i s s i o n of ( n - C H ) ^ ^ 2 ( ) k 2 (see above) (9) and a value of ΔΗ* = 96 k J mol" f o r homolytic f i s s i o n of C p F e ( C 0 ) i i n ben­ zene has been reported (71). The l a t t e r was d e r i v e d from nmr monitoring of the scrambling r e a c t i o n s between (n -C5H5)2Fe2(C0) C 0

3

3

5

l ,

2

2

2

t

5

i+

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Metal-Metal-Bonded

POE

7.

Carbonyls

155

and (η -C5H[ C0Me)2Fe2(C0)i , making the assumption that the a c e t y l s u b s t i t u e n t i n the c y c l o p e n t a d i e n y l l i g a n d s d i d not a f f e c t the FeFe bond s t r e n g t h . Mass spectrometric measurements have l e d to the value ΔΗ° = 234.4 ± 3.8 k J mol" f o r the homolytic f i s s i o n of Cp2W2(C0)6 i n the gas phase over the temperature range 200-300°C (72). This i s the strongest s i n g l e metal-metal bond q u a n t i t a ­ t i v e l y c h a r a c t e r i z e d to date ( 8 ) . More needs to be known q u a n t i t a t i v e l y of the strengths of metal-metal bonds i n organometallic compounds and of the f a c t o r s that a f f e c t them. We have r e c e n t l y made s t u d i e s o f some r e a c ­ t i o n s of C p 2 M o 2 ( C 0 ) i n order to see what t h e i r mechanisms are and to see whether a value can be obtained f o r the s t r e n g t h o f the Mo-Mo bond (73). +

+

1

6

R e a c t i v i t y and Mechanisms of Reactions of Cp Mo2(CO)β. Thermal decomposition o proceeds with q u i t e c l e a pressures, the f i r s t order r a t e constants being independent of [ 0 ] and unaffected when the p r o p o r t i o n o f CO i n CO-O2 mixtures above the s o l u t i o n i s v a r i e d from 0 to 95%. The a c t i v a t i o n para­ meters obtained from r a t e constants measured from 70-135°C are ΔΗ' = 135.9 ± 2.2 k J mol" and AS* = 56.2 ± 5.6 J K" m o l . Reaction with C H 3 l occurs very r a p i d l y a t 135°C i n thoroughly deoxygenated d e c a l i n to form CpMo(C0)3l i n high y i e l d . The r e a c t i o n i s , however, i n h i b i t e d by t r a c e s o f O2 when the r a t e i s the same as with O2 alone although the product i s s t i l l CpMo(C0) I. This i s a good i n d i c a t i o n that the rate-determining step i n both the r e a c t i o n s i s homolytic f i s s i o n of the Mo-Mo bond and that the value 135.9 ± 2.2 k J mol" can be taken as a p r e c i s e k i n e t i c measure of the strength of the Mo-Mo bond. T h i s i s con­ s i d e r a b l y higher than the k i n e t i c strength o f the Fe-Fe bond i n Cp2Fe2(C0)i i n s p i t e of the l a r g e r c o o r d i n a t i o n number of the Mo atoms. However, the strength o f the Cr-Cr bond i n C p 2 C r 2 ( C 0 ) 5 i s apparently much lower s i n c e f i n i t e amounts of the monomer are d e t e c t a b l e by esr (74). On the other hand, the value ΔΗ° = 234.4 kJ mol f o r the homolytic f i s s i o n of C p 2 W 2 ( C 0 ) i n the gas phase (72) shows that the strength of the metal-metal bonds increases g r e a t l y with i n c r e a s e of atomic weight o f the metal i n these com­ plexes. Reaction of Cp2Mo2(C0)g with alkynes leads to the alkynebridged complexes CP2M02(CO)^(C2R2) and i t has been suggested that both the photochemical (75) and thermal (76) r e a c t i o n s go v i a i n i t i a l homolytic f i s s i o n . The k i n e t i c r e s u l t s do not support t h i s . The thermal r e a c t i o n with C P h 2 proceeds c l e a n l y a t 145°C i n thoroughly deoxygenated s o l u t i o n s i n d e c a l i n to form CP2M02( C 0 ) i ( C 2 P h ) i n high y i e l d . The r e a c t i o n proceeds v i a two paths, one o f which i s f i r s t order i n [C2PI12] up to high concentrations while the other increases to a l i m i t i n g r a t e . When [C2Ph2l £ 0.1M the observed p s e u d o - f i r s t - o r d e r r a t e constant f o l l o w s eq 23. 2

2

1

1 6

1

- 1

3

3

1

+

1

6

2

+

2

k

obsd

- M

l

i

m

) + k [C Ph ] b

2

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

(23)

REACTIVITY

156

OF

METAL-METAL

BONDS

The v a l u e of k ( l i m ) i s reduced by CO i n such a way as to suggest the mechanism shown i n eq 24-26 f o r which the p r e d i c t e d r a t e behaa

Cp Mo (CO)

6

ν

Cp Mo (CO)

5

Ν

2

2

2

2

k—ι

^

Cp Mo (CO)

+ CO

(24)

^

Cp Mo (C0)4 + CO

(25)

•

Cp Mo (C0MC Ph )

2

2

2

5

2

k— 2 Cp Mo (C0) 2

2

+ C Ph

i+

2

2

v i o r i s described i n eq 27.

2

2

2

(26)

2

At high values of [ C P h ] t h i s would 2

2

kik k [C Ph ] 2

k

3

2

2

=

(27 k_ik_ [CO]

+ ^^ [00][0 Ρη ] + k k [C Ph ]

2

3

2

2

2

3

2

l e a d to values of k ( l i m ) that should f o l l o w eq 28. a

2

A p l o t of

l / k ( l i m ) = 1/ki + (k.i/kik )[CX)] a

(28)

2

l / k ( l i m ) a g a i n s t [CO] i s indeed a good s t r a i g h t l i n e (Figure 3) of p o s i t i v e gradient with a f i n i t e i n t e r c e p t so the data are e n t i r e l y c o n s i s t e n t with the mechanism proposed. T h i s r e a c t i o n scheme c l e a r l y p a r a l l e l s that f o r one path f o r r e a c t i o n of C P h with C o ( C O ) ( P - n - B u ) i n which two l i g a n d s have to be l o s t by r e v e r s i b l e d i s s o c i a t i o n before a t t a c k by the alkyne can occur (37). In the Mo system the intermediate Cp Mo (C0)4. i s , of course, expected to be i d e n t i c a l with the f u l l y c h a r a c t e r i z e d Mo^Mo complex (38) and the energy r e q u i r e d f o r s u c c e s s i v e d i s s o c i a t i o n of two CO l i g a n d s must be lowered by the a b i l i t y of the Mo atoms to form m u l t i p l e bonds. The same f a c t o r has been proposed to be o p e r a t i v e i n the r e a c t i o n of C p ( C O ) g ( P - n - B u ) with C P h (see above). Cp Mo (CO)i+ i s known to undergo r a p i d a d d i t i o n of alkynes (77). An important f e a t u r e of t h i s r e a c t i o n i s that the v a l u e of k i derived from the a n a l y s i s i s ca. 10 x lower than the r a t e constant f o r homolytic f i s s i o n obtained by e x t r a p o l a t i o n . Indeed, during the formation of C p M o ( C O ) ^ ( C P h ) i n high y i e l d under an atmos­ phere of CO homolytic f i s s i o n and i t s r e v e r s e must be o c c u r r i n g ca. 10 times f o r every event l e a d i n g to C p M o ( C O ) ^ ( C P h ) . This i s another example, t h e r e f o r e , of a r e a c t i o n which proceeds by one path a t the same time as another much more f a c i l e one i s o c c u r r i n g , but o c c u r r i n g r e v e r s i b l y and without l e a d i n g anywhere. Another important f e a t u r e of t h i s r e a c t i o n has consequences f o r the s y n t h e s i s of the alkyne-bridged products. As the r e a c t i o n temperature i s lowered the y i e l d of the product decreases so that a t 100°C i t i s n e g l i g i b l e . The product i s thermally s t a b l e under these c o n d i t i o n s and the observations can be explained i f i t i s a

2

2

2

6

3

2

2

2

2

2

3

2

2

2

2

2

2

2

2

2

3

2

2

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

7.

POE

Metal-Metal-Bonded

Carbonyls

157

% CO Figure

3.

Plot of 1/kJlim) against partial pressure of CO Cp Mo (CO) with C Ph in decalin under CO—N 2

2

G

2

2

2

for the reaction mixtures

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

of

158

REACTIVITY

OF

METAL-METAL

BONDS

assumed that the intermediate Cp2Mo2(C0)s can be attacked by C2PI12 i n a r e a c t i o n , l e a d i n g e v e n t u a l l y to decomposition, that has a lower a c t i v a t i o n enthalpy than d i s s o c i a t i v e l o s s of the second CO l i g a n d to form Cp2Mo2(C0)it. This proposal i s not unreasonable s i n c e Cp2Mo2(C0)5(C2Ph2) would c o n t a i n a monodentate, non-bridging C2PI12 l i g a n d and could w e l l be thermally unstable towards decompo­ s i t i o n , probably f o r s t e r i c reasons. The mechanism governing the path that i s f i r s t order i n [ C P h ] i s not w e l l defined but i t seems l i k e l y that i t i s s i m i l a r to the corresponding path f o r r e a c t i o n of C2PI12 with C o 2 ( C 0 ) £ ( P - n - B u ) (37). 2

2

3

2

Recent K i n e t i c Studies of Some Coi+ C l u s t e r s Apart from the r e a c t i o n of Coi+iCO)^ under CO to form C 0 2 (CO)e mentioned above (66 fragmentation of Coi+ c l u s t e r t i o n of ΰοι+(ΰ0)ΐ2 with PPh , AsPh , or SbPh a t room temperature l e d to the mono s u b s t i t u t e d complexes Coi (CO)nL (78) whereas attempts to prepare c l u s t e r s more h i g h l y s u b s t i t u t e d with PPh appeared to lead d i r e c t l y to C o ( C O ) ( P P h ) (79). T e t r a s u b s t i tuted c l u s t e r s Coi^CO^L^ seem to have been i s o l a t e d only with L = P(OPh) (80), P ( 0 M e ) (79), and P(OCH ) CEt (79). Since these r e s u l t s seemed to be a t t r i b u t a b l e to the greater s i z e of PPh a study of the k i n e t i c s of fragmentation of Coi+(CO) χ χ (PPh ) has been undertaken (81). Solutions of Coi+iCO)^ are q u i t e unstable when exposed to a i r or l i g h t . However, stock s o l u t i o n s prepared by weighing under N2 i n very dim l i g h t , and by using "Schlenk-tube" techniques and thoroughly deoxygenated s o l u t i o n s , were s t a b l e f o r weeks when stored i n a r e f r i g e r a t o r under CO. Thoroughly deoxygenated s o l u ­ t i o n s of C o i ( C 0 ) 2 and the a p p r o p r i a t e l i g a n d were mixed i n f o i l wrapped Schlenk tubes under c o n t r o l l e d , i n e r t atmospheres and i n dim l i g h t , and were thermostatted at a p p r o p r i a t e temperatures. Samples f o r s p e c t r o s c o p i c a n a l y s i s were e x p e l l e d through s t a i n l e s s s t e e l tubes by a p o s i t i v e pressure of the i n e r t gas. 3

3

3

+

3

2

6

3

3

3

2

2

3

3

3

+

1

3

Spectroscopic Changes i n S o l u t i o n . Reaction of c a . 10" M Co (C0) with a t e n f o l d excess of PPh or AsPh i n dichloroethane (DCE) or η-heptane a t room temperature l e d immediately to s p e c t r a c h a r a c t e r i s t i c of C o i ( C 0 ) i L (78, 79). An attempt was made to measure the r a t e of r e a c t i o n with PPh by monitoring the s p e c t r o ­ s c o p i c changes at 305 nm with a stopped-flow apparatus but the r e a c t i o n was too f a s t to measure even when [PPh ] was as low as 5 x lO^M. S o l u t i o n s of Coi+(CO) χ χ (AsPh ) underwent no f u r t h e r s p e c t r o s c o p i c changes over 75 min. a t room temperature and a f u r t h e r 2 h a t 45°C. However, C04 (CO) χ χ (PPh ) reacted over 15-20 min. i n η-heptane at room temperature to form a new s p e c i e s . Reaction was somewhat f a s t e r i n DCE and i n both cases the r a t e s were observed to be greater a t higher values of [ P P h ] . The i +

1 2

3

+

3

1

3

3

3

3

3

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

Metal-Metal-Bonded

POE

7.

159

Carbonyls

product could be i s o l a t e d from e i t h e r η-heptane or DCE by evapora­ t i o n of s o l v e n t with a stream of Ar and was r e c r y s t a l l i z e d from benzene-pentane. The s o l i d was not very s t a b l e to l i g h t and a i r but could be kept unchanged i n the dark under an i n e r t atmosphere. A n a l y s i s was i n agreement with that expected f o r C04(CO)s(PPhs)^ ( C a l c : Co, 15.62; C, 63.68; H, 4.00; P, 8.21%. Found: Co, 15.78; C, 62.52; H, 4.40; P, 8.41%). The i n s t a b i l i t y of s o l u ­ t i o n s of the complex other than i n the dark and under a v i g o r ­ ously deoxygenated atmosphere v i t i a t e d attempts to measure the molecular weight. However, the a n a l y s i s was supported by measure­ ment of the IR s p e c t r a of s o l u t i o n s of C o i ( C 0 ) i which had been allowed to come to e q u i l i b r i u m with v a r y i n g amounts of P P I 1 3 . Bands due to C04(CO)γ ι(PPI13) grew i n i n t e n s i t y with i n c r e a s i n g [ P P I 1 3 ] and then decreased as bands due to the product complex grew. Close to 4 times as much PPI13 was r e q u i r e d to remove a l l of the Coi^CO)11(PPh ) as wa of more than t r a c e s of C o i ^ C O ) of the s o l u t i o n s remained unchanged when [PPI13] was i n c r e a s e d above t h i s amount. The IR s p e c t r a of the product i n v a r i o u s s o l ­ vents are shown i n Table I I . i

+

n

D

C

E

2

3

Reaction of C o ( C 0 ) with dppm (Ph PCH PPh ) l e d w i t h i n 40 min a t room temperature i n DCE to a new s p e c i e s , the spectrum of which remained unchanged a f t e r 4 h a t 60°C. A f t e r evaporation of the s o l v e n t and r e c r y s t a l l i z a t i o n from benzene-pentane the product showed a spectrum i n CS very s i m i l a r to that of Rhi* (C0)8dppm (82). An e q u a l l y s i m i l a r spectrum was obtained a f t e r the thermal r e a c t i o n of I r ^ C O ) ^ with dppm i n toluene or t h e i r photochemical r e a c t i o n i n DCE (Table I I ) . The product of the r e a c t i o n with C o i ( C 0 ) i must, t h e r e f o r e , be C04(CO)edppm . I t s spectrum shows bands almost i d e n t i c a l with those assigned to C o i ( C 0 ) 8 ( P P h 3 ) i , a l b e i t with d i f f e r e n t r e l a t i v e i n t e n s i t i e s , and t h i s provides a d d i t i o n a l support f o r the l a t t e r s f o r m u l a t i o n . The bands f o r C o i ^ C O ^ P P h s ) ^ are a l s o i n the expected p o s i t i o n s r e l a t i v e to those f o r C 0 4 ( C 0 ) { P ( 0 M e ) } ^ prepared i n s i t u i n DCE. The s o l u b i l i t y of C 0 4 (CO) 3 (PPh3)i+ i n pentane or heptane was low so that i t u s u a l l y began to p r e c i p i t a t e soon a f t e r being p r e ­ pared i n s i t u . S o l u b i l i t y i n DCE was a p p r e c i a b l y g r e a t e r and f u r t h e r r e a c t i o n s were followed i n t h i s s o l v e n t . I t was s t a b l e at room temperature f o r some time and f u r t h e r r e a c t i o n r e q u i r e d tem­ peratures above c a . 45°C f o r i t to occur at r a t e s convenient f o r k i n e t i c study. This f u r t h e r r e a c t i o n was accompanied by a de­ crease i n the i n t e n s i t y of the bands at 2010 and 1890 cm" and an e r r a t i c growth of the band a t 1955 cm" . This was because the product C o ( C 0 ) g ( P P h 3 ) a l s o absorbs a t t h i s wavelength but begins to p r e c i p i t a t e from s o l u t i o n a f t e r the r e a c t i o n i s c a . 60% com­ p l e t e . The k i n e t i c s were t h e r e f o r e followed by monitoring the decreasing i n t e n s i t y of the band a t 2010 cm" . 4

1 2

2

2

2

2

+

2

2

2

+

+

1

8

3

1

1

2

2

1

PPh K i n e t i c s of the Reaction COJ, (CO) (PPh )i+ > Co (C0) ( P P h ) . The v a r i a t i o n of the observed p s e u d o - f i r s t - o r d e r r a t e constant 3

8

3

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6

3

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8

3

i+

a

8

3

i+

8

8

3

8

2

2

J

e

f

i+

8

8

2

2

2

h

#

2

if

2004vs

2015s

2010 s

2026s

2016s

2030m

2018m

2010s

1972w

1946s

1958vs

1950sh

1960sh

1950m

1950s

1950s

1945s

1848w

189 2w

1910w

1912w

1882s

e

2

1810w

3

In CHC1 .

J . Chem. S o c , D a l t o n Trans., 1974, 328.

υ

ή

Cattermole, P.E.; O r e l l ,

2

1752s

1730w

1765sh

1792m

1780s

+

^Prepared i n

( r e f . 79). •'In

1805m

1792s

1765w

1755w

K.G.; Osborne, A.G.

2

2

1776s

1770s

1790s

1818m

1830w

1820w

1810m

CH C1

1830m

1780m

1812m

1890w

1890s

1880s

In hexadecane ( r e f . 79). in

1979vs

1977s

1980s

1996s

1975s

1992vs

1980s

1970w

1965w

( r e f . 812). ^Prepared i n s i t u by photochemical r e a c t i o n i n C H C l i .

°ln C 2 H 2 C I 4 .

2064vw

2066m

2070w

In heptane.

J

+

2005s 2005s

s i t u by thermal r e a c t i o n i n toluene.

2

^In CS

i n Nugol.

8

#

CS .

a

8

+

Iri (CO) (dppm) ^ Iri (00) (dppe) '

i+

Ir (CO) (dppm)

i+

Rh (CO) (dppm) ^

+

Coi (CO) (dppm)

+

Coi (CO) (etpb)

i+

Co (CO) {P(OMe) }i

i+

Co (CQ) (PPh )

C

Co^CCOQiPPhs)^

+

Coi (CO) (PPh )

IR Spectra o f Some M i ^ C O ^ I ^ Complexes

Table I I

Metal-Metal-Bonded

POE

7.

161

Carbonyls

with [ Ρ Ρ Ι Ί 3 ] are shown i n F i g u r e 4 f o r r e a c t i o n s under N and CO. The i n c r e a s e i n r a t e when [ P P I 1 3 ] i s decreased below ca. 0.04M sug­ gests that a P P h 3 - d i s s o c i a t i v e path i s a v a i l a b l e as a r a t e determining step under both N and CO. The data i n t h i s r e g i o n were not very r e p r o d u c i b l e and the k i n e t i c s were not followed i n detail. When [PPh ] » 0.05M t h i s path i s f u l l y suppressed and r e a c t i o n can occur by a path i n h i b i t e d by CO which presumably i n v o l v e s rate-determining CO d i s s o c i a t i o n . This r e a c t i o n can be completely suppressed by CO, the r a t e s with 0.05M P P I 1 3 under 40 and 100% CO i n C0-N mixtures being the same. The remaining r e a c ­ t i o n i s independent of [CO] and [PPt^] and the r a t e must be c h a r a c t e r i s t i c of spontaneous fragmentation. The a c t i v a t i o n para­ meters ΔΗ* = 146.8 ± 6.5 kJ mol" and AS' = 114.3 ± 19.3 J K" mol are t h e r e f o r e a good k i n e t i c measure of the s u s c e p t i b i l i t y of t h i s Coi+ c l u s t e r towards spontaneous fragmentation. The a c t i ­ v a t i o n parameters f o r th s u b t r a c t i n g the r a t e constant from those found f o r r e a c t i o n under N : ΔΗίοο = 118.4 ± 7.7 k J mol and A s ! = 39.8 * 23.3 J K" mol" . 2

2

3

2

1

1

1

2

- 1

1

1

c o

Although there are no d i r e c t l y comparable data i t i s c l e a r that 0 0 4 ( 0 0 ) 3 (PPh )i+ i s much l e s s s t a b l e than C 0 4 ( C 0 ) d p p m and Coi (CO) {P(OMe) }i which are both v i r t u a l l y u n a f f e c t e d a f t e r s e v e r a l hours a t 60 °C whereas Coi+ (CO) (PPh )i+ has a h a l f - l i f e of only 50 min a t 60°C i n DCE i n an excess of PPh and under N . It would appear that s t e r i c e f f e c t s p l a y an important r o l e i n d e t e r ­ mining the s t a b i l i t y of these C 0 4 c l u s t e r s . 3

+

8

3

8

2

+

8

3

3

2

Summary (1) Thorough mechanistic s t u d i e s are capable of demonstrating when rate-determining fragmentation of metal-metal-bonded c a r ­ bonyls i s o c c u r r i n g . I t i s then p o s s i b l e to o b t a i n a c t i v a t i o n parameters f o r spontaneous fragmentation processes that are e x c e l ­ l e n t estimates of the r e a c t i v i t y of the complexes. The a c t i v a t i o n e n t h a l p i e s provide p r e c i s e and u s e f u l estimates of the strengths of the metal-metal bonds and enable the e f f e c t s of s u b s t i t u e n t s to be determined. Substituents can a c t e i t h e r by weakening the metalmetal bonds i n the reactant complexes (so i n c r e a s i n g the r e a c ­ t i v i t y ) or by weakening bonds i n intermediates (and so decreasing the r e a c t i v i t y ) . (2) Spontaneous fragmentation can a l s o occur by more than one path, i . e . to form d i f f e r e n t types of fragment, and the e n e r g e t i c s of the two paths can d i f f e r g r e a t l y . (3) Fragmentation i s o f t e n preceded by l i g a n d d i s s o c i a t i o n and the e n e r g e t i c s can depend g r e a t l y on the way i n which the metal-metal-bonded system can a d j u s t to the c o o r d i n a t i v e unsaturation. C l u s t e r s have many more ways of doing t h i s than mononuclear carbonyls and t h i s type of behavior can a l s o a f f e c t the e n e r g e t i c s of s u b s t i t u t i o n r e a c t i o n s as w e l l as fragmentation.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

162

REACTIVITY OF

METAL-METAL

BONDS

Figure 4. Values of k for the reaction Co^CO) (PPh ) > Co (CO) (PPh ) in DCE at 59.5°C: (%) reactions under N ; f | j reactions under CO ob8d

8

s 4

2

2

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

6

3 2

7.

Metal-Metal-Bonded

POE

Carbonyls

163

(4) S u b s t i t u t i o n r e a c t i o n s can occur by paths that i n v o l v e fragmentation as rate-determining s t e p s . (5) Metal c l u s t e r s a r e q u i t e s u s c e p t i b l e to n u c l e o p h i l i c a t t a c k , the n u c l e o p h i l e making use of the LUMOs on the c l u s t e r . This type of a t t a c k o f t e n simply leads to s u b s t i t u t i o n but i t can a l s o l e a d to fragmentation o f the c l u s t e r . (6) Metal c l u s t e r s a r e a l s o s u s c e p t i b l e to fragmentation through e l e c t r o p h i l i c a t t a c k (e.g. by halogens). Rates can be very r a p i d indeed but a r e very dependent on the nature of any substituents. (7) Reactions of C p M o ( C O ) i l l u s t r a t e some of the above points. Homolytic f i s s i o n can be shown to occur but the o v e r a l l r e a c t i o n i s very slow i Under these c o n d i t i o n s two CO l i g a n d s by C P h occurs not v i a r a d i c a l intermediates but v i a C p M o ( C 0 ) 4 formed by s u c c e s s i v e d i s s o c i a t i o n of the two CO l i g a n d s b e f o r e a t t a c k by the alkyne. 2

2

2

2

6

2

2

(8) Coi+iCO)^ forms COJ+ (C0)8 ( P P h 3 ) z + r e a d i l y a t room temperature. This complex i s unstable to fragmentation a t £45°C. Spontaneous, C O - d i s s o c i a t i v e , and P P h 3 - d i s s o c i a t i v e paths have been shown to lead to fragmentation and a c t i v a t i o n parameters have been obtained f o r the f i r s t two. They suggest that spontaneous f r a g mentation i n v o l v e s a high degree o f bond weakening i n the Coi* c l u s t e r , and that t h i s i s due to s t e r i c e f f e c t s . C04(CO)3(dppm) and C04(CO)8{P(OMe)3}4 a r e not n e a r l y as e a s i l y fragmented. 2

Literature Cited

1. Dahl, L.F.; Ishishi, E.; Rundle, R.E. J. Chem. Phys., 1957, 26, 1750. 2. Powell, H.M.; Ewens, R.V.G. J. Chem. Soc., 1939, 286. 3. Mills, O.S.; Robinson, G. Proc. Chem. Soc., 1959, 156. 4. Cotton, F.A.; Monchamp, R.R. J. Chem. Soc., 1960, 533. 5. Bidinosti, D.R.; McIntyre, N.S. J. Chem. Soc., Chem. Commun., 1966, 555. 6. Haines, L.I.B.; Hopgood, D.; Poë, A.J. J. Chem. Soc. A, 1968, 421. 7. Bidinosti, D.R.; McIntyre, N.S. Can. J. Chem., 1970, 48, 593. 8. Connor, J.A. Top. Current Chem., 1977, 71, 71. 9. Muetterties, E.L.; Sosinsky, B.A.; Zamariev, K.I. J. Amer. Chem. Soc., 1975, 97, 5299. 10. Battison, G.; Sbrignadello, G.; Bor, G.; Connor, J.A. J. Organomet. Chem., 1977, 131, 445. 11. Angelici, R.J. Organometal. Chem. Rev., 1968, 3, 173.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

164

REACTIVITY OF METAL-METAL BONDS

12. Jackson, R.A.; Poë, A.J. Inorg. Chem., 1978, 17, 997. 13. Fawcett, J.P.; Poë, A.J.; Sharma, K.R. J. Chem. Soc., Dalton Trans., 1979, 1886. 14. Fawcett, J.P.; Jackson, R.A.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1978, 789. 15. Fawcett, J.P.; Poë, A.J.; Twigg, M.V. J. Organomet. Chem., 1973, 51, C17; Fawcett, J.P.; Poë, A.J.; Sharma, K.R. J. Amer. Chem. Soc., 1976, 98, 1401. 16. Fawcett, J.P.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1977, 1302. 17. Fawcett, J.P.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1976, 2039. 18. Wawersik, H.; Basolo, F. Inorg. Chim. Acta, 1969, 3, 113. 19. Sonnenburger, D.; Atwood, J.D. J. Amer. Chem. Soc., 1980, 102, 3484. 20. Fawcett, J.P.; Jackson Commun., 1975, 733 21. Chowdhury, D.M.; Poë, A.J.; Sharma, K.R. J. Chem. Soc., Dalton Trans., 1977, 2352. 22. DeWit, D.G.; Fawcett, J.P.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1976, 528. 23. Jackson, R.A.; Poë, A.J. Inorg. Chem., 1979, 18, 3331. 24. Cotton, F.A.; Wilkinson, G. "Advanced Inorganic Chemistry", Wiley-Interscience, New York, 3rd Edn., 1972, 547. 25. Fawcett, J.P.; Poë, A.J.; Twigg, M.V. J. Chem. Soc., Chem. Commun., 1973, 267. 26. Quicksall, C.O.; Spiro, T.G. Inorg. Chem., 1969, 8, 2363. 27. Spiro, T.G. Prog. Inorg. Chem., 1970, 11, 17. 28. Levenson, R.A.; Gray, H.B.; Ceasar, G.P. J. Amer. Chem. Soc., 1970, 92, 3653. 29. Poë, A.J.; Jackson, R.A. Inorg. Chem., 1978, 17, 2330. 30. Haines, L.I.B.; Poë, A.J. J. Chem. Soc. A, 1969, 2826. 31. Kramer, G.; Patterson, J . ; Poë, A.J.; Ng, L. Inorg. Chem., 1980, 19, 1161. 32. Kramer, G.; Patterson, J.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1979, 1165. 33. Hopgood, D.J. Ph.D. Thesis, London University, 1966. 34. Wegman, R.W.; Brown, T.L. J. Amer. Chem. Soc., 1980, 102, 2495. 35. Brown, T.L.; Forbus, N.P.; Wegman, R.W. Abstract 239, Division of Inorganic Chemistry, Proceedings of the Second Chemical Congress of the North American Continent, Las Vegas, 1980. 36. Ellgen, P.C. Inorg. Chem., 1972, 11, 691. 37. Basato, M.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1974, 607. 38. Klingler, R.J.; Butler, W.; Curtis, M.D. J. Amer. Chem. Soc., 1975, 97, 3535. 39. Colton, R.; Commons, C.J. Aust. J. Chem., 1975, 28, 1673. 40. Cobb, M.A.; Poë, A.J. Unpublished observations.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

7.

POË

Metal-Metal-Bonded

Carbonyls

165

41. Ref. 24, p. 777. 42. Tyler, D.R.; Schmidt, M.A.; Gray, H.B. J. Amer. Chem. Soc., 1979, 101, 2753. 43. Barrett, P.F.; Poë, A.J. J. Chem. Soc. A, 1968, 429. 44. Barrett, P.F. Can. J. Chem., 1974, 52, 3773, and references therein. 45. Basato, M.; Fawcett, J.P.; Fieldhouse, S.A.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1974, 1856. 46. Basato, M.; Fawcett, J.P.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1974, 1350. 47. Bor, G.; Dietler, U.K. Unpublished work. 48. Keeton, D.P.; Malik, S.K.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1977, 233. 49. Malik, S.K.; Poë, A.J. Inorg. Chem., 1978, 17, 1484. 50. E.g., Calderon, J.L.; Fonatana S.; Frauendorfer E.; Day V W.; Iske, S.D.A. J 51. Malik, S.K.; Poë, A.J Inorg , , , 52. Keeton, D.P.; Malik, S.K.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1977, 1392. 53. Malik, S.K.; Poë. A.J. Unpublished observations. 54. Poë, A.J.; Twigg, M.V. Inorg. Chem., 1974, 13, 2982. 55. Candlin, J.P.; Shortland, A.C. J. Organomet. Chem., 1969, 16, 289. 56. Poë, A.J.; Twigg, M.V. J. Chem. Soc., Dalton Trans., 1974, 1860. 57. Cotton, F.A.; Wilkinson, G. "Advanced Inorganic Chemistry", Wiley-Interscience, New York, 4th Edn., 1980, p. 1204. 58. Basolo, F.; Pearson, R.G. "Mechanisms of Inorganic Reactions", Wiley, New York, 2nd Edn., 1967, pp. 571-578. 59. Basato, M.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1974, 456. 60. Cobb, M.A.; Hungate, B.; Poë, A.J. J. Chem. Soc., Dalton Trans., 1976, 2226. 61. Aime, S.; Gervasio, G.; Rossetti, R.; Stanghellini, P.L. Inorg. Chim. Acta, 1980, 40, 131, and references therein. 62. Karel, K.J.; Norton, J.R. J. Amer. Chem. Soc., 1974, 96, 6812. 63. Sonnenburger, D.C.; Atwood, J.D. Abstract 124, Division of Inorganic Chemistry, Proceedings of the Second Chemical Congress of the North American Continent, Las Vegas, 1980. 64. Drakesmith, A.J.; Whyman, R. J. Chem. Soc., Dalton Trans., 1973, 362. 65. Adkins, H.; Krsek, G. J. Amer. Chem. Soc., 1948, 7Ό, 383. 66. Bor, G.; Dietler, U.K.; Pino, P.; Poë, A.J. J. Organomet. Chem., 1978, 154, 301. 67. Darensbourg, D.J.; Incorvia, J.J. J. Organomet. Chem., 1979, 171, 89. 68. Ungermann, C.; Landis, V.; Moya, S.A.; Cohen, C.; Walker, H.; Pearson, R.G.; Rinker, R.G.; Ford, P.C. J. Amer. Chem. Soc., 1979, 101, 5923.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

166

REACTIVITY OF METAL-METAL BONDS

69. Walker, H.W.; Kresge, C.T.; Ford, P.C.; Pearson, R.G. J. Amer. Chem. Soc., 1979, 101, 7428. 70. Fox, J.R.; Gladfelter, W.L.; Geoffroy, G.L. Inorg. Chem., 1980, 19, 2574. 71. Cutler, A.R.; Rosenblum, M. J . Organomet. Chem., 1976, 120, 87. 72. Krause, J.R.; Bidinosti, D.R. Can. J. Chem., 1975, 53, 628. 73. Amer, S.; Kramer, G.; Poë, A.J. Unpublished data. 74. Adams, R.D.; Collins, D.E.; Cotton, F.A. J. Amer. Chem. Soc., 1974, 96, 749. 75. Ginley, D.S.; Bock, C.R.; Wrighton, M.S. Inorg. Chim. Acta, 1977, 23, 85. 76. Curtis, M.D.; Klingler, R.J., J. Organomet. Chem., 1978, 161, 23. 77. Bailey, W.I.; Chisholm M.H.; Cotton F.A.; Rankel L.A J Amer. Chem. Soc., 1978 78. Cetini, G.; Gambino, O.; , ; Stanghellini, Inorg. Chem., 1968,100609. 79. Labroue, D.; Poilblanc, R. Inorg. Chim. Acta, 1972, 6, 387. 80. Sartorelli, U.; Canziani, F.; Martinengo, S.; Chini, P. "Proceedings of 12th International Conference on Coordination Chemistry", 1970, p. 144. 81. Huq, R.; Poë, A.J. Unpublished work. 82. Carré, F.H.; Cotton, F.A.; Frenz, B.A. Inorg. Chem., 1976, 15, 381. RECEIVED December 11, 1980.

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8 Metal-Metal Bond Making and Breaking in Binuclear Complexes with Phosphine Bridging Ligands ALAN L. BALCH Department of Chemistry, University of California—Davis, Davis, CA 95616 Polyfunctional phosphine ligands make useful bridging ligands for constructing binuclea cerned with the use of tw (dpm) and 2-diphenylphosphinopyridine (Ph Ppy)-in promoting metal-metal bond forming and breaking reactions. Three specific topics will be covered. These are the ability of dpm to modify the stability of Rh-Rh bonds in interrelated compounds of Rh(I), Rh(II) and Rh(III), the utility of dpm bridging ligands in allowing novel metal-metal bond opening and closing in low valent rhodium and palladium complexes, and the usefulness of PhPpy in the stepwise construction of binuclear complexes. 2

2

Modification of Metal-Metal Bonding in Rhodium Complexes by a Bridging Diphosphine. The yellow, planar complexes, (RNC) Rh , undergo novel self-association reactions in concentrated solution to form the blue or violet dimers, (RNC) Rh2 , via reaction (1) (1,2). The equilibrium constants for this reaction are strongly +

4

2+

8

L (1)

+

2L—Rh—L L

L \

L / Rh---

/

\

l_

2+

/

L = RNC influenced by solvent and by the size of the substituent on the isocyanide ligand. Not unexpectedly (t-BuNC) Rh with its bulky substituents is most reluctant to dimerize. Substitution of the bidentate dpm for half of the isocyanide ligands produces the blue, dimeric cation 1. (3) These dimers, which can be prepared with various isocyanide substituents including t-butyl, show no tendency to dissociate into monomers. Similar stabilization of the dimeric structure can be achieved by the use of bifunctional isocyanides like 1,3-diisocyanopropane. (4,5) These form qua+

4

0097-6156/81/0155-0167$05.00/0 © 1981 American Chemical Society In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

REACTIVITY

168

OF

METAL-METAL

BONDS

2 +

P—Ph

Ph-P 'CH ' 2

druply bridged cations 2. The c o l o r and s t a b i l i t y o f the Rh-Rh bonds in ( R N C ) R h , 1 and 2 may be understood by reference to 2 +

8

2

2 +

2 the molecular o r b i t a l diagram i n Figure 1. This diagram shows the i n t e r a c t i o n o f the o u t - o f - p l a n e o r b i t a l s , which are the o r ­ b i t a l s of the complex most s t r o n g l y a f f e c t e d by s e l f - a s s o c i a t i o n . The occupied o r b i t a l s of the dimer are s t a b i l i z e d s i n c e the upper and lower sets of dimer o r b i t a l s , which are derived from the mono­ mer aig and a u o r b i t a l s r e s p e c t i v e l y , have the same symmetries. Consequently, the net Rh-Rh i n t e r a c t i o n i s bonding; although the f o r m a l l y antibonding Rh-Rh σ o r b i t a l i s doubly occupied. The blue c o l o r r e s u l t s from a lowering o f the HOMO-LUMO gap upon d i merization. The spectra of a r e l a t e d p a i r of complexes, monomer­ i c ( M e N C ) R h ( P P h ) and dimeric ( M e N C K R h ( d p m ) , are compared i n Figure 2. Thg lowest allowed t r a n s i t i o n in the monomer r e s u l t s from the d + Lnr (ag + l b ) e x c i t a t i o n . In the dimers, the t r a n ­ sition l b + 2ag i s i n v o l v e d , and the energy gap i s smaller. Not only does the i n c o r p o r a t i o n of the dpm l i g a n d i n t o 1_ i n ­ h i b i t d i s s o c i a t i o n , i t a l s o p r o h i b i t s f u r t h e r Rh-Rh bond forma­ tion. The c a t i o n s (RNCKRh can f u r t h e r a s s o c i a t e to f o r m t r i m e r s v i a r e a c t i o n (2). A d d i t i o n a l l y the isocyanide bridge complex 2 also s e l f a s s o c i a t e s v i a r e a c t i o n (3) to form a tetrarhodium species. However the dpm-bridged dimers, 1_, show no evidence f o r s e l f a s s o c i a t i o n even at the highest c o n c e n t r a t i o n s . Undoubtedly 2

+

2

Z 2

3

2+

2

2

2

l u

l u

+

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

8.

BALCH

Binuclear

Complexes

with

Phosphine

Bridging

Ligands

3biu

2b

2 b,,

1u 3a

(

2b,,, ib,. 2a

ibi

u

a , d 2-

^

R,P\

/CNR

1a

q

R,P\

/CNR

R

3

P ^

Rh

Rh RNC

•d 2 7

7

PR

3

RNC R

3

ι P

X

^CNR Rh

^PR

3

RNC

PR

3

j /CNR Rh

RNC^

Figure 1.

Partial molecular

X

P R

3

orbital diagram for the interaction complexes

of two planar Rh(I)

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

169

170

REACTIVITY OF M E T A L - M E T A L

BONDS

WAVELENGTH (nm)

Figure 2. Electronic NC),,Rh (dpm) ][PF ], 2

2

6

spectrum of (A) a 0.20-mL acetonitrile solution of [(CH and (B) a 0.40-mL acetonitrile solution of [(CH NC) Rh(PPh ) [PF ] . 3

s

3 2

2

e 2

Each solution is contained in a 1.0-mm pathlength cell. The markers show the position of the low-energy maxima for the cases where the isocyanide substituent is changed to η-butyl, cyclohexyl, and X-butyl. Increasing the bulk of the isocyanide substituent in­ creases the Rh - - - Rh separation and increases the energy of the b -» 2a transition. 1

lu

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

g

Binuclear

BALCH

8.

Complexes

L

L

1

(2)

\

+

L-Rh-L

+

L

L

X

L

/

Rh-

with

L

I

Rh

\

Phosphine

/

L

Bridging

L

2 +

I /

L

L

L

I /

L

3

+

Rh----Rh

/' L L

L L

L

I /

Rh

171

Ligands

/· L L

/' L L

the bulky phenyl groups at each end o f these dimers p r o h i b i t s the approach of a u n i t along the Rh-Rh v e c t o r .

(3)

2

The same pattern of dimer s t a b i l i z a t i o n at the expense of monomeric or more h i g h l y polymerized forms i s a l s o e v i d e n t i n the behavior of these rhodium complexes toward o x i d a t i o n . Treatment of the dimer 1_ with i o d i n e , bromine, or t r i f l u o r o m e t h y l d i s u l f i d e r e s u l t s i n t r a n s - a n n u l a r , o x i d a t i v e - a d d i t i o n to form the brown dimers, 3_, v i a r e a c t i o n (4) (3,6). As i n s p e c t i o n o f Figure 1 shows Ph

Ph

Ph—Ρ

P—Ph

I

L (4) Ph—Ρ

X = I, 2

+ X

Ρ—Ph

Pf/' ^ C H Br , 2

2

^

Ph—P

^ Ph

(SCF ) 3

Ph

2

X

I/

L

X

~~Rfr

XH x

2

1/

X—Rh-

Rh—X

Ph—Ρ Ph^ ^ C H

P—Ph

.Ρ—Ph 2

^

Ph

5

such two-electron o x i d a t i o n should strengthen and shorten the Rh-Rh bond by removing e l e c t r o n s from the p r i m a r i l y anti bonding l b i orbital. These dimers, once formed, are s t a b l e to the p r e s ­ ence of a d d i t i o n a l q u a n t i t i e s o f oxidant. That i s , they cannot be r e a d i l y converted i n t o Rh(III) complexes. In c o n t r a s t o x i d a ­ t i o n of (RNCKRh gives a v a r i e t y of products whose formation can be c o n t r o l l e d by l i m i t i n g the amount of oxidant used. (7-10) The r e a c t i o n s involved are shown i n equations 5-7. The d i n u c l e a r and t r i nuclear rhodium c a t i o n s have been s t r u c t u r a l l y c h a r a c t e r i z e d by X-ray c r y s t a l l o g r a p h y . (8,10) The s t r u c t u r e of the t r i nuclear c a t i o n i s shown i n Figure 3. These metal-metal bonded products u

In Reactivity of Metal-Metal Bonds; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

172

REACTIVITY

L

L

I

L

I

L

L

1/

I

V/ I'

L

L L 1/ X—Rh

L-Rh-L

2 +

Rh—

L/ ' L

L

L

1/