[m._v._twigg(auth.),m._v._twigg__(eds.)]_mecha(bookzz.org).pdf

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Volume 2

Mechanisms

of

Inorganic

and Organometallic Reactions


2/456

A Continuation Order Plan

is

available for this series. A continuation order will

bring delivery of each new volume immediately upon publication. Volumes are

billed only upon actual shipment. For further information please contact the

publisher.


3/456

Volume

2

Mechanisms of Inorganic

and Organometallic Reactions

Edited by

M V

Twigg

Imperial Chemical Industries P.

L. C.

Billingham, United Kingdom

PLENUM

PRESS

NEW

YORK

AND LONDON


4/456

Library

of

Congress Cataloging in Publication Data

Main entry under title:

Mechanisms

of

inorganic and organometallic reactions.

Includes bibliographical references and index.

1.

Chemical reactions. 2. Chemistry, Inorganic. 2. Organometallic compounds. I.

Twigg, M.

V.

QD501.M426

1983

541.3'9 83-2140

ISBN-13: 978-1-4612-9659-1 e-ISBN-13: 978-1-4613-2663-2

DOl: 10.1007/978-1-4613-2663-2

1984 Plenum Press, New York

Softcover reprint

of

the hardcover 1st edition 1984

A Division

of

Plenum Publishing Corporation

233

Spring Street, New York, N.Y. 10013

All rights reserved

No part

of

this book may

be

reproduced, stored in a retrieval system, or transmitted,

in any form or by any means, electronic, mechanical, photocopying, microfilming,

recording, or otherwise, without written permission from the Publisher


5/456

Contributors

Dr. f. Burgess

Chemistry Department, The University,

Leicester

LEl

7RH, U.K.

Dr. R. D. Cannon

Chemistry Department, University of East Anglia,

University Plain, Norwich

NR47Tl,

U.K.

Dr. R. f. Cross

Department of Chemistry, The University,

Glasgow G12 8QQ, Scotland, U.K.

Dr. A. f. Deeming

Chemistry Department, University College London,

20 Gordon Street, London

WCIH OAl, u.K.

Dr.

M.

Green

Chemistry Department, The University, York,

North Yorkshire,

YOl

5DD, U.K.

Dr. D.

N.

Hague Chemical Laboratory, The University, Canterbury,

Kent CT2 7NH, U.K.

Dr.

R. W.

Hay

Department of Chemistry, University of Stirling,

Stirling FK9 4LA, Scotland, U.K.

Dr. M. N. Hughes

Chemistry Department, Queen Elizabeth College,

University of London, London W8 7AH, U.K.

Dr. L.

A.

P. Kane-Maguire

Chemistry Department, Wollongong

University, P.O. Box 1144, Wollongong, N.S.W. 2500, Australia

Dr.

A. G.

Lappin

Chemistry Department, University of Notre

Dame, Notre Dame, Indiana 46556, U.S.A.


6/456

vi

Contributors

Dr. P.

Moore

Department of Chemistry and Molecular Sciences,

University of Warwick, Coventry CV47AL, U.K.

Dr.

D.

A.

Sweigart

Department of Chemistry, Brown University,

Providence, Rhode Island 02912, U.S.A.

Dr.

C. White

Department of Chemistry, The University of Sheffield,

Sheffield S3 7HF, U.K.


7/456

Preface

This series provides a continuing critical review of the literature concerned

with mechanistic aspects of inorganic and organometallic reactions in solu

tion, with coverage over the whole area being complete in each volume.

The format of this second volume

is

very similar to that of the first, with

material arranged according to reaction type and compound type along

generally accepted lines. Papers discussed are selected on the basis of

relevance to the elucidation of reaction mechanisms but may also include

results of a nonkinetic nature, such as stereochemical studies and product

ratios, when useful mechanistic information can be deduced.

In this volume extra space has been given to areas concerned with

electron transfer processes and substitution reactions of inert complexes,

and to improve convenience for the reader the text has been further divided

to form three additional chapters. Electron transfer processes are discussed

in three chapters:

"General

and Theoretical," "Reactions between Two

Complexes," and "Metal-Ligand Redox Reactions," while six chapters are

concerned with substitution and related reactions. Here reactions of inert

chromium and cobalt complexes are discussed in separate chapters.

The period of literature coverage

is

January 1981 through June 1982

inclusive and in a few instances, where delays in delivery of journals have

been encountered, the issues not covered will be included in the next volume.

Similarly, some 1980 references that were not available for inclusion in the

previous volume are discussed here. Numerical results are usually reported

in units used by the original authors, except where data from different

papers are compared and conversion to common units

is

necessary.

This series was established as a result of demand from members of the

Inorganic Mechanisms Discussion

Group

(UK), and their continuing sup

port

is

appreciated by the contributors, and by others involved in producing

the series. Comments and suggestions regarding this and future volumes

will

be welcomed.

vii


8/456

Contents

Part 1.

Electron

Transfer Reactions

Chapter 1. Electron Transfer: General and Theoretical

R. D. Cannon

1.1. Reviews .............. .

1.2. The Marcus-Hush Model . . . . . . . .

1.3. Quantum Effects: (1) The

"Normal"

Region

1.4. Quantum Effects:

(2)

The "Inverted" Region

1.5. Optical and Thermal Electron Transfer

1.6. Mixed-Valence Complexes

1. 7. Electron Transfer in the Solid State

Chapter 2. Redox Reactions between Metal Complexes

A.

G. Lappin

2.1. Introduction

. . .

.

2.2. Titanium(III)

. . .

.

2.3. Chromium(II) and (III)

2.4. Iron(II) . . . . .

2.5. Cobalt(II)

2.6. Nickel(II) and (III)

2.7. Copper(I) and (II)

2.8. Molybdenum(IV) and (V)

2.9. Ruthenium(II)

2.10. [*Ru(bipyhf+ . . . .

3

3

6

9

12

16

21

23

23

36

37

42

43

44

45

46

47

ix


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x

Contents

2.11.

Europium(II) . . . . .

2.12. Miscellaneous Reactions

2.13. Metalloprotein Studies .

Chapter 3. Metal-Ligand Redox Reactions

A. G. Lappin

3.1.

Introduction

. . . .

3.2. Ascorbic Acid

H2A

3.3.

Quinols

and

Catechols

3.4. Halogens and Pseudohalogens

3.5. Thiols, Sulfur, Selenium, and Tellurium

Compounds

3.6. Amines

.........

.

3.7. Carbonyls

and

Carboxylic Acids

3.8. Alcohols

and

Diols . . . .

3.9.

Alkenes and

Alkyls

. . . .

.

3.10. Nitrogen

and

Nitrogen Oxides .

3.11. Peroxydisulfate

and

Peroxymonosulfate

3.12. Oxyhalogen

Anions

3.13.

Reactions

of

O2

and

H20 2

3.14. Miscellaneous Reactions

Part 2. Substitution nd Related Reactions

Chapter 4. Reactions of Compounds

of

the Nonmetallic Elements

M. N.

Hughes

4.1.

Introduction

4.2. Boron

4.3. Silicon

4.3.1. Silicon Radicals

4.3.2. Base Hydrolysis

4.3.3. Various Substitutions, Isomerizations, and

Redistributions

..........

.

4.3.4. Reactions of I3-Substituted Organosilicon

Compounds

............

.

4.3.5.

Aqueous

Solutions

of

Silicates

4.4. Nitrogen

.........

.

4.4.1. Nitric Acid and Nitration

4.4.2. Nitrogen Dioxide . . .

48

49

49

53

53

55

56

59

61

62

64

65

66

68

70

71

74

79

79

80

80

81

81

83

84

84

84

86


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Contents

4.4.3. Nitrous Acid and Nitrosation

4.4.4. Trioxodinitrate and Nitrogen Monoxide

4.4.5. Hyponitrite

4.4.6. Dinitrogen Complexes

4.4.7.

Azide

4.4.8.

Nitroamine and Hydroxylamine

4.4.9. Hydrazine

4.5. Phosphorus and Arsenic

4.5.1. Phosphorus(V) Compounds

4.5.2.

Phosphorus(III) Compounds

4.5.3. Phosphorus(l) Compounds

4.5.4. Arsenic Compounds

4.6.

Oxygen

4.7. Sulfur

4.7.1. Oxidation with Peroxo Acids of Sulfur

4.7.2.

Reactions of Oxo Acids of Sulfur

4.7.3.

Decomposition of a Sulfur Nitroso Compound

(S-

Nitrosothiouronium Ion)

4.8. Selenium and Tellurium

4.8.1.

Oxidation of Selenium(IV)

4.8.2. Tellurium Compounds

4.9.

Halogens

4.9.1. Fluoroxysulfate

4.9.2. Chlorine Compounds

4.9.3. Bromine Dioxide

4.9.4.

Iodine Compounds

4.9.5. Oscillating Reactions

4.10. Xenon

Chapter 5. Substitution Reactions

of

Inert Metal

Complexes

Coordination Numbers 4

and

5

R.

J.

Cross

5.1. Introduction .............. .

5.2. Substitution at Square-Planar Palladium(II) and

Platinum(II) .............. .

5.2.1. Palladium(II) Complexes

5.2.2. Platinum(II) Complexes . .

5.2.3. Electrophilic Substitutions .

5.3. Ring Opening and Closing Reactions

5.3.1. Palladium(II) Complexes

5.3.2. Platinum(II) Complexes . .

xi

86

89

90

90

90

91

93

93

93

94

95

95

95

97

97

98

98

99

99

99

99

99

100

100

101

101

103

105

106

106

108

111

113

113

115


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xii

5.4. Five-Coordinate Species . . . . . . . .

5.5. Isomerization of Square-Planar Complexes

5.6. Gold(III) Square-Planar Complexes

5.7.

Miscellaneous . . . . . .

5.7.1. Bridged Complexes

5.7.2. Other Reactions

Chapter

6.

Substitution Reactions

of

Inert Metal

Complexes

Coordination Numbers

6

and Above: Chromium

P. Moore

6.1.

Introduction

6.2.

Aquation and Solvolysis of Chromium(III) Complexes

6.2.1. Unidentate Leaving Groups

6.2.2. Multidentate Leaving Groups

6.2.3. Bridged Dichromium(III) Complexes

6.3.

Formation of Chromium(III) Complexes

6.3.1.

Reactions of [Cr(H

2

O)6]3+

6.3.2.

Formation of Mixed-Ligand Complexes

6.3.3.

Formation of Cr(I1I) Complexes from Cr(II)

or

Cr(O)

6.4.

Chromium(III) Photochemistry

6.4.1.

Ammine Complexes

6.4.2.

Amine Complexes

6.4.3. Other

Chromium(III) Complexes

6.5. Isomerization and Racemization Reactions

6.6.

Base Hydrolysis of Chromium(III) Complexes

6.7.

Solids

6.8.

Other

Chromium Oxidation States

6.8.1.

Chromium(II)

6.8.2.

Chromium(V)

Chapter 7. Substitution Reactions

of

Inert Metal Complexes

Coordination Numbers 6 and Above: Cobalt

R.

W. Hay

7.1.

Aquation . . . . .

7.2. Catalyzed Aquation

7.3. Base Hydrolysis

7.4. Solvolysis

7.5. Anation

Contents

119

122

128

129

129

131

133

133

133

144

144

145

145

145

148

149

149

150

151

151

151

152

152

152

152

153

159

160

165

166


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7.6. Solvent Exchange, Racemization, Isomerization, and

Ligand Exchange

. . . . . . .

168

7.7. JL-Peroxo-dicobalt(l1I) Complexes 170

7.8. Formation

..........

171

7.9. Photochemistry . . . . . . . . 173

7.10. Reactions of Coordinated Ligands 174

7.10.1. Nitrile Hydrolysis 175

7.10.2. Phosphato Complexes 175

7.10.3. Carbinolamine and Imine Formation 177

7.10.4. Coordinated Azides and Nitriles . . 177

7.10.5. Cobalt-Hydroxide-Promoted Hydrolysis and

Lactonization . . . . . . . . . . . . . 178

7.10.6. Peptide Synthesis

...........

181

7.10.7. Dimethylglyoxime Complexes and B12 Models 182

7.10.8. Base-Catalyzed Exchange Reactions . . . . 184

Chapter

8.

Substitution Reactions

of

Inert Metal Complexes

Coordination

Numbers

6 and Above:

Other Inert

Centers

J. Burgess

8.1.

Groups V to VII 187

8.1.1.

Vanadium 187

8.1.2.

Molybdenum 188

8.1.3.

Tungsten 188

8.1.4.

Manganese 189

8.1.5. Technetium

189

8.1.6. Rhenium 190

8.2. Iron 190

8.2.1. Pentacyanoferrates(1I) 190

8.2.2.

Iron(II)-Diimine Complexes

192

8.2.3.

Other Low-Spin Iron(1I) Complexes

197

8.2.4.

Iron(III) Complexes

199

8.3. Ruthenium 200

8.3.1.

Ruthenium(1I)

200

8.3.2.

Ruthenium(III) 202

8.3.3.

Ruthenium(III) / (IV) 204

8.4. Osmium 204

8.4.1. Osmium(II) 204

8.4.2.

Osmium(IV) 204

8.5. Rhodium

206

8.5.1.

Aquation 206

8.5.2.

Base Hydrolysis

206


13/456

xiv

Contents

8.5.3. Reactions in Liquid Ammonia

206

8.5.4. Catalyzed Aquation

207

8.5.5. Formation

208

8.5.6. Solvent Exchange

208

8.5.7. Ligand Replacement

209

8.5.8. Ring Opening and Closing

209

8.5.9. Photochemistry

209

8.5.10. Oxidation States 2+, 2.5+ 210

8.6.

Iridium

210

8.7. Nickel(III) 211

8.8.

Platinum(IV)

211

8.8.1. General 211

8.8.2. Inversion at Coordinated Sulfur and Selenium

212

Chapter 9. Substitution Reactions of Labile Metal Complexes

D. N. Hague

9.1. General

.....................

215

9.2. Complex Formation Involving Un substituted Metal Ions:

Unidentate Ligands and Solvent Exchange 216

9.2.1. Bivalent Ions . . . . . . . . . . . . . . . . 216

9.2.2. Ions of Valency 3 and Higher

. . . . . . . .

219

9.3. Complex Formation Involving Un substituted Metal Ions:

Multidentate Ligands 222

9.3.1. Univalent Ions . . . . . . 222

9.3.2. Bivalent Ions . . . . . . . 223

9.3.3. Ions of Valency 3 and Higher 226

9.4. The Effects of Bound Ligands . . . 227

9.4.1. Reactions in Water . . . . 227

9.4.2. Reactions in Nonaqueous Solvents 231

Part

3.

Reactions of Organometallic Compounds

Chapter 10. Substitution and Insertion Reactions of Organometallic

Compounds

D.

A.

Sweigart

10.1. Substitution Reactions

10.1.1. Introduction

237

237


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Contents

xv

10.1.2. Carbon Monoxide Replacement in Mononuclear

Metal Complexes . . . . . . . . . . . .. 238

10.1.3. Replacement of Other Ligands in Mononuclear

Metal Complexes . . . . . . . . . . . . 245

10.1.4. Substitution Reactions of Polynuclear Metal

Complexes

. . . . . . .

252

10.2. Insertion Reactions . . . . . . . . . . . . . 259

10.2.1. Carbon Monoxide Insertion . . . . . 259

10.2.2. Alkene, Alkyne, and Carbene Insertion 265

10.2.3. Insertion of Other Groups . . . . . . 268

Chapter

11.

Metal-Alkyl Bond Formation and Fission; Oxidative

Addition and Reductive Elimination

M.

Green

11.1. Introduction 271

11.2. Metal-Alkyl Bonds 272

11.2.1. Chromium 272

11.2.2. R-Co(III)[N

4

] and R-Co(III)[N

2

0

2

] Systems 275

11.2.3. Other Elements

. . . . . . . . .

283

11.3. Oxidative Addition and Reductive Elimination 284

11.3.1. Pre transition Metals

. . . .

285

11.3.2. Earlier Transition Metals 285

11.3.3. Cobalt, Rhodium, and Iridium 288

11.3.4. Nickel, Palladium, and Platinum 293

11.3.5. Actinides

. . . . . . . . . .

300

Chapter 12. Reactivity of Coordinated Hydrocarbons

L.

A. P. Kane-Maguire

12.1. Introduction . . . . . . . . . . . .

12.2. Nucleophilic Addition and Substitution

12.2.1. u-Bonded Hydrocarbons

12.2.2.

7T-Bonded

Hydrocarbons

12.3. Electrophilic Attack

12.4. Cycloaddition Reactions . . . . .

301

301

301

303

317

318


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xvi

Chapter 13.

Rearrangements, Intramolecular Exchanges, and

Isomerizations of Organometallic Compounds

A.

J.

Deeming

13.1.

Mononuclear Compounds

13.1.1. Stereochemical Nonrigidity in Metal Carbonyls

and Their Derivatives

13.1.2.

Cis-Trans Isomerism and Exchange in Square

Planar Complexes

13.1.3. Stereochemical Nonrigidity in Five-Coordinate

Compounds

13.1.4.

Other

Examples of Stereochemical Nonrigidity

13.1.5.

Simple Rotation about Metal-Ligand Axes

13.1.6. Ligand Motion Requiring Changes in Hapticity

Contents

319

319

321

322

324

325

329

13.1.7.

Metal Migration between Different Ligand Sites .

330

13.1.8.

Migrations and Interchanges Involving

Hydrogen Atoms 332

13.1.9.

Intraligand Rotations and Rearrangements 336

13.2.

Dinuclear Compounds 337

13.2.1. Migration of Carbonyl Ligands 337

13.2.2.

Hydrogen Migration Reactions 339

13.2.3.

Motion Involving Bridging Organic Ligands

341

13.3.

Cluster Compounds

342

13.3.1.

Migration of Carbonyl Ligands

342

13.3.2.

Hydrogen Migration Reactions 345

13.3.3.

Motion Involving Bridging Organic Ligands 345

Chapter 14.

Homogeneous Catalysis

of

Organic

Reactions

by

Complexes

of

Metal Ions

C. White

14.1. Introduction . . . . . . . . . . . . . . . . . .

. .

351

14.1.1. General Reviews and Elementary Steps in

Homogeneous Catalysis . . . . . . . . . . 351

14.2. Reactions Involving Carbon Monoxide

. . . . . . .

352

14.2.1. Hydroformylation and Hydrocarboxylation of

Olefins

.................

352

14.2.2. Decarbonylation of Aldehydes . . . . . . . 355

14.2.3. Carbonylation and Homologation of Alcohols,

Halides, and Nitro-Compounds, Ethers,

Carboxylic Acids, and Esters . . . . . . . 355


16/456

Contents

14.3.

14.4.

14.5.

14.6.

14.7.

14.2.4. Fischer-Tropsch Reactions . . . . . .

14.2.5. Homogeneous Water-Gas Shift Reaction

(WGSR)

Oxidation

Hydrogenation . . . . . . . . .

14.4.1. Hydrogenation of Alkenes

14.4.2. Hydrogenation of Arenes and Functional Groups

14.4.3. Asymmetric Hydrogenation . . . . . .

14.4.4. Hydrogen Transfer and Dehydrogenation

Reactions . . . . .

Isomerization Reactions . . . . .

14.5.1. Olefin Isomerization

14.5.2. Skeletal Rearrangements

Alkene and Alkyne Metathesis . .

Oligomerization and Polymerization of Alkenes and

Alkynes . . . . . . . . . .

14.8.

14.7.1. Reactions of Alkenes

14.7.2. Reactions of Alkynes

Reactions of Dinitrogen

References

Author Index

Subject Index

xvii

356

358

359

362

362

363

364

366

368

368

369

370

373

373

375

376

377

423

443


17/456

Part 1

Electron

Transfer

Reactions


18/456

Chapter

1

Electron Transfer:

General and Theoretical

1.1.

Reviews

A symposium on radiation chemistry includes short reviews on ion

molecule reactions, 1) redox properties of free radicals, 2) intramolecular

electron transfer from coordinated ligand

radicals(3)

and metal ions in

unusual valency states. 4) The increasingly important(S) field of electron

transfer steps in organic reactions has received two reviews. Eberson(6)

examines the applicability of the Marcus and other equations, [see below,

equations (3), (18), and (19)] and by calculating rate constants from theory

comments on the feasibility or otherwise of postulated mechanisms. Chanon

and Tobe(7) have pointed out analogies between substitution reactions

involving electron transfer in organic (aromatic) and inorganic (Pt and

Au

complex) systems.

1.2.

The

Marcus-Hush Model

In this section we review work which lies within what

is

now accepted

as the classical model of the electron transfer mechanism in solution. We

recall that in this model the second-order rate constant of a general electron

transfer reaction (1) is given by equations (2) and (3). The quantity A

=

A/4,

A+

+B

=A

+B+

k = Z exp(-flO*/RT)

flO* = A(1 + flO*/A)2

(1)

(2)

(3)

3


19/456

4

1

General and

Theoretical

often called the

intrinsic free energy barrier

or

reorganization energy, is

considered as the sum of

inner-sphere

and

outer-sphere

contributions

[equation (4)].

(4)

Calculations of Ao from the electrostatic continuum model and from

quantum mechanical models have been reviewed and compared. 8) The

distinction between electrostatic displacement D and field E is emphasized.

Values of

Ao

are compared for different physical models of the reacting

molecules, e.g., conducting spheres (the model usually considered in pre

vious literature) and cavities of various dimensions. In the electrostatic

model a formula for Ao has been given,

9)

which applies to any system which

has a symmetrical binuclear structure, and from which Marcus' two

sphere(lO) and Cannon's ellipsoidal(11) models can be deduced

as

special

cases.

The two-sphere model gives

e

2

(

1 1

1)(

1

1)

Ao = 47Tco 2

a

1 + 2a2

- R

DOD -

Ds

(5)

where

a1

and

a2

are the radii of the spheres, R is the internuclear distance,

and

DOD'

Ds are the optical and static "dielectric constants" of the solvent.

Experimentally, the R dependence has been tested using the series of

complexes [(H

3

N)sCo(III)LM(II)(CN)s], with L = imidazolate, pyrazine,

or

4,4'-bipyridyl, and M

=

Fe

or

Ru. Values of dG* vary in the expected

way, though the slopes of plots of dG

*

against

R -1

are somewhat less

than predictedY2) The solvent dependence of dG* has been examined

using the reaction [Ru(hfachJ

+

[Ru(hfac)3r (hfac-

=

hexafluoroacetyl

acetonate), and other Ru(III)/Ru(II) systems involving uncharged

reactants. Results agree with equation (5), in contrast to previous

observations, e.g., on

[Fe(CsHsht

IO

,

which had shown unexpectedly small

changes between solvents.(13) Values of Ao for [Co(NH

3

)6]3+/2+ and

[Co(NH

3

)sFf+

l

+ from electrochemical measurements, are

not

in accord

with the Marcus prediction. Nonadiabaticity, and deviations from the

continuum model due to short-range solvent structure are considered

as

possible reasons for this.

(14)

There is growing support for the approximations that, in bimolecular

electron transfer at least, A can be divided into independent contributions

A

(A

+),

A

B)

from the two reactants

(c.f.

an earlier derivation of the Marcus

cross relation on this basis),

15)

and moreover that A

A

+)

=

A

A)

(cf.

Ref. 16). Using these assumptions, Frese(17) has calculated reorganization

energies for a large number of self-exchange and cross-reactions. In many

cases values of A for individual redox couples are consistent from one

reaction to another. Of interest are the different values of A for


20/456

1.2 The Marcus-Hush

Model

5

[HFe(CN)6]3- and

[Fe(CN)6t-,

and the similarity in values for

[Fe(CN)6t

deduced from homogeneous and heterogeneous reactions.

Medium

efiects(18)

and the relative importance of inner- and outer

sphere reorganization energy(9) have been assessed for reaction (6). Using

[Co(bipyh]3+

+

[Co(terpyh]2+

~

[Co(bipyh]2+

+

[Co(terpyht+ (6)

the conducting-spheres-in-contact model (with assumed radii of 7 A),

Ao =

3.3 compared with

Ai =

7.9 kcal

mol-

1

. The same value of

Ao is

obtained

for the [Fe(phenh]3+/2+ self-exchange. Overall second-order rate constants

calculated for the two reactions agree with experiment within factors of 2

and 5, respectively.

The long-standing problem of the failure of Marcus theory to correlate

reactions of cobalt(III) complexes, when one of the self-exchange couples

is [Co(H

2

0)6]3+/2+, is considered by Endicott

et

al.(20) in a comprehensive

review of data on the two reaction series shown in equations (7) and (8)

[Co(NH

3

)6]3+

+

B

~

[Co(NH

3

)6]2+

+

B+ (7)

[Co(H

2

0)6]3+

+

B

~

[Co(H

2

0)6f+ + B+

(8)

where reductants B include aquo ions, macrocycles, and polypyridyl-type

complexes. Both series obey the Marcus equations (when the work terms

are allowed for) except, ironically, the exchange of the hexaaquo complexes

themselves [equation (9)]. For this reaction the rate calculated from the

[Co(H

2

0)6]3+

+

[Co(H

2

0)6]2+

=

[Co(H

2

0)6]2+

+

[Co(H

2

0)6]3+ (9)

correlations is 1O-

12

2

M-

1

S-1;

but experimentally 5.0M-

1

S-1.

Evidently

the self-exchange is facilitated by some extra pathway which is not available

for the cross-reactions. A water-bridged inner-sphere mechanism is sug

gested.

A problem in applying the Marcus relationships to organic systems

is

that of calculating J..G e. For some couples the reduction potentials are

unknown and cannot even be precisely defined since the reduced form

is

in a repulsive state, e.g., a reduced peroxide (ROORT. Good correlations

have, however, been obtained between log k and a free energy change

J..G

e

,calculated from the irreversible polarographic reduction wave of the

oxidant.

(21) On

the

other

hand, values of the

reversible

reduction potential

Be

may be extracted from irreversible electrokinetic data, by applying the

Marcus equations. This has been done with cyclic voltammetric data using

the dependence of the peak potential on the scan rate.

(22)

The principle

is

analogous to the more familiar use of the Marcus equations to calculate

Be

values from a series of homogeneous reactions.(23)

Comparisons of homogeneous and electrochemical rate data continue

to be of interest. In reactions of a series of organometallic compounds with


21/456

6 1 General

and

Theoretical

outer-sphere oxidants (e.g., [Fe(phenh]3+), plots of

I1G

t

(electrochemical)

against I1G

t

(homogeneous) are linear with slope 1.0. Values of A increase

with decreasing coordination numbers of the central metal atom, from

Co (CN

=

6)

through Sn, Pb, and Pt

(4)

to Hg

(2),

as expected for increased

participation of the solvent in the transition state.

22)

Inner-sphere reorganization effects are clearly seen

in

reactions of the

complexes [Co(A

4

)(OH

2

hr+

(A4

= planar macrocyclic tetramine). Only

the

Co-O

bond lengths change appreciably from the 3 + to the 2 + ion,

and rates of self-exchange correlate with this change.(24)

1.3. Quantum Effects:

(1)

The "Normal" Region

Quantum effects which have been introduced to refine the original

Marcus model include nonadiabaticity-according to which a reaction is

slowed by a low probability of transfer at the intersection of the energy

surfaces, and nuclear tunneling which tends to increase the rate by allowing

"horizontal" transitions between the surfaces at points other than the

crossing point. Important parameters are the

tunneling matrix element Hps>

i.e., the resonance integral between "precursor" and "successor" electronic

configurations (A +

. . .

B)

and

(A

B+)

[d.

equation

(l)t],

and the

Franck Condon factors,

or vibrational overlap integrals, between reactants'

and products' nuclear configurations.

The nonadiabatic treatment of Hopfield has been elaborated to include

the possibility that Hps may vary with the binding energy or the transferring

electron. Model calculations are given for both optical and thermal electron

transfer (d. below, p.

12)

and for barriers of different shapes, including

square energy wells.

(25)

Further work on bridged electron transfer includes(26) calculations on

three-atom, symmetrical model systems A-L-A. Using the method of

propagators, time-dependent electron transfer probabilities are calculated

for various energies of the basis orbital of the bridging ligand L. Kuznetsov

and Ulstrup(27) have used perturbation theory to consider the effects of

varying number of bridging atoms, embracing both superexchange and

radical-intermediate electron transfer pathways. Larsson(28) has proposed

rules for predicting relative transfer rates in terms of the occupied and

unoccupied (T and 'IT orbitals. Taking the nonadiabatic model, he calculates

effective interaction matrix elements analogous to the resonance integral

Hps

of the two-state approximation.(29)

t Butler has calculated

Hps

for a number of gas phase reactions such as Ne3+ + H ...... Ne

2

+ +

H+.(92) For reviews dealing with gas phase electron transfer see Refs. 93 and 94.


22/456

1.3 Quantum Effects: (1) The "Normal" Region 7

The main emphasis however of work reported in the period under

review has been on comparisons between theory and experiment, especially

on the difficult question of whether any of the familiar inorganic reactions

are significantly non adiabatic or not. Zawacky and

Taube(30)

have measured

intramolecular electron transfer rates in complexes of the type

[(H3NhCoLRu(II}(NH3}4X], where the bridging groups include isomeric

carboxypyridines. With X = H

2

0,

rates are not very sensitive to the nature

of L, and the reactions are thought to be close to the adiabatic limit; with

X =

[S03f-

however, rates are substantially less. This suggests that the

[S03f- group, a 7T-electron acceptor, decreases the electronic coupling,

which is "tantamount to admitting that electron transfer is strongly non

adiabatic." (On coupling through bridging units, see also section 1.6 below).

Brunschwig

et

al.(31) have reviewed existing non adiabatic theories for

comparison with data on bimolecular reactions. They define the "semi

classical" rate constant ksc

by

equation (10) where kcl is the "classical"

(10)

rate constant k of equation (2), Kel expresses the nonadiabaticity effect,

and

r

n expresses the nuclear tunneling. A full quantum mechanical treat

ment considers the electron transfer as a radiation less transition and

averages the probabilities

W

pv

for transfer from each vibronic level

v

of

the initial or precursor state

p,

to each level w of the final or successor

state s, using the Fermi Golden Rule [equation (11)] where (xpvIXsu) is the

W

pv

= (47T

2

H;s/h)pw

Pw = Iw

l


23/456

8

1 General and Theoretical

rate constants are calculated within a factor of 2 of the experimental values.

The entropy or enthalpy contributions are "clear manifestations" of both

nonadiabatic behavior and nuclear tunneling, but the most remarkable

conclusion

is

that

the bulk of the electron transfer occurs at metal-metal

distances substantially less than the usually accepted contact distance of

6.9 A. It is suggested that in the reacting pair, a vertex of one octahedron

pokes into a face of the other. Further calculations with more detailed

treatment of the interaction forces between the two ions imply distances

as short as 4.5 A. The results are supported by a successful calculation of

the rate of the analogous nuclear spin relaxation reaction (14) where the

(14)

indices m and m' show a change of spin states induced by collision with

the paramagnetic Ni

2

+ ion, and of the ionic strength dependence of reaction

(15). Siders and Marcus(33) have also calculated quantum effects on reaction

(15)

(13), and on the self-exchange reactions [Co(NH

3

)6]3+/2+ and

[Ru(NH

3

)6]3+/2+.

Franck-Condon factors are treated by the Golden Rule,

and the solvent

is

introduced as a harmonic oscillator, but with two frequen

cies instead of one as in most earlier treatments. This refinement is expressed

in the continuum model by introducing a dielectric constant D

ir

for the

infrared frequency region in addition to Ds and Dop of equation (5). These

authors likewise conclude that tunneling and nonadiabaticity are significant

though not large. In reaction (13), with one set of calculations, other things

being equal, inclusion of tunneling effects raises the rate constant by a factor

of 3.5. In

the system

[Ru(NH

3

)6]3+/2+,o4) quantum effects are negligible,

while in [Co(NH

3

)6]3+/2+

(33,34) they are again appreciable but not large,

nuclear tunneling enhancing the rate by a factor of about

7.(34)

The major

difference in rates between these two systems-experimentally the ratio

is

more than 10

15

- i s attributed to reorganization energy differences. (33)

All attempts to calculate absolute values of rate constants depend on

precise knowledge of the difference of the metal-ligand distances in the

oxidized and reduced forms of the complexes. For exchange between

octahedral complexes, such as reaction (13), Sutin(35) obtained them via

equations (16) and (17), where

r2, r3

are metal-ligand distances in the Fe

2

+

*

2r2r3

r

=

r2

+ r3

(16)

(17)


24/456

1.4 Quantum Effects: (2) The "Inverted"

Region

9

and Fe

3

+ complexes, and r* is the value adopted by both complexes in the

transition state. Values of r2,

r3

have hitherto been known only for the

solid

state,t

but now, from X-ray scattering and EXAFS data, they are

becoming available for complexes in solution. For a number of hydrated

transition metal ions, the distances in solution turn out to be smaller than

in the crystal, 36) and the differences (r2 - r3) vary as well. For

[Fe(H

2

0)6]3+/2+, we now have (r2 - r3) = 0.105 A (solution, EXAFS),

0.12

A

solution, X-ray), 0.14

A

(solid). The changes from solid to solution

are enough to change significantly conclusions on the importance of adiaba

ticity of tunneling effects mentioned

above.(31),*

Also significant are the two differences (r2 -

r*)

and

(r*

-

r3). If

these

do not exceed the amplitudes of the metal-ligand vibrations, there will be

no activation energy requirement at all in the classical sense, Le., no

substantial transfer of energy from modes other than the breathing modes

of the two complexes. For the

Fe(H

2

0)2+ system Irn - r*1 are sufficiently

large to require activation, and the new data from solution do not

change this conclusion.(31)

For

the system

[Mn04r12-,

however, the latest

Irn -

r*1

values only just exceed the amplitude and the calculated Ai is only

1.6 kcal

mol-

l

.(37)

The deuterium isotope effect has been proposed as another experi

mental probe for quantum effects. Model calculations on systems such as

[Co(NH

3

)6]3+/2+

indicate that the main effects on replacing H by

Dare

due to inner-sphere reorganization, and that the effect should become very

large at low temperatures. The small values of kH/kD actually observed so

far tend to confirm that quantum effects are not very significant at room

temperature.

38)

1.4. Quantum Effects: (2) The "Inverted" Region

The Marcus equation (3) (above) predicts that when 6.G

8

is more

negative than

-A,

a further decrease causes 6.G

t

to rise: the rate becomes

slower although the driving force

is

increased. A growing body of data

indicate that this either does not happen

or

if the rate does become slower

in this "inverted" or highly "exergonic" region, it does not follow the

t

Of interest

in

this connection

is

an

ab

initio

calculation of the Fe2

+

-OH2 and Fe

3

+

-OH2

bond lengths. Results are given for hypothetical complexes [Fe(OH

2

)mr+ with m = 1-6,

and the latter are in good agreement with the crystal structural values. (95)

t Further X -ray structural data of interest for electron transfer studies include [FeCI

6

]3- and

[FeCI

6

]2- ions, 96) and Cu(I) and Cu(II) complexes with similar tridentate N-S-N ligands.

(97)

See also footnote on p. 16.


25/456


26/456

1.4 Quantum Effects: (2) The "Inverted" Region

11

(20)

Reactions of the type (20) which feature in the quenching of excited

[RuLtf+(L

=

bipy

or

phen) by molecules Q such

as

methylviologen cation,

are highly exergonic, with

tlOB

around

-2

eV. Rates, however, increase

d

. h . . I .

"c

e

(47) Th

slightly rather than ecrease,

WIt

Increasmg y negative

Q , e se

data recall the earlier work from the same laboratory(48.49) in which reactions

of the type in equation (21) were studied with Q

=

Ru(II) and Os(1I)

(21)

complexes, and in which

k

decreases with increasingly negative

tlO

B

,

but

not to the extent predicted by equations (2) and (3). Marcus and

Siders(50)

have discussed these data in the light of modified Marcus treatments which

take account of nuclear tunneling, but they conclude that these theories

too are inadequate to explain the results. They suggest alternative reaction

paths leading to excited-state products, so that tlOBis not

in

fact as negative

as had been assumed.

Another possibility(50) which can explain nonabnormal behavior

(so

to speak) is that electron transfer may take place over longer distances in

the inverted region than in the normal region. Qualitatively, the effect of

increasing R

is

to increase the reorganizational energy barrier

A

[equation

(5)]

and in the

inverted

region, this increases the rate [equations (3) and

(4)], though on the other hand,

if

the reaction becomes non adiabatic,

as

it

presumably m:lst if the distance is great enough, further increase of distance

will

markedly decrease the rate. These considerations are treated in more

detail in another paper.(54) Plots of predicted log

k

against tlOB are given,

using diffusion theory, nonadiabaticity, and

R

-dependent reorganization

energies calculated electrostatically. Even with this theory, however, the

Creutz-Sutin data are only fitted when excited-state products are assumed.

Inclusion of variable electron transfer distance can also raise the frequency

factor Z [equation (2)] by a factor of about 10 and this too improves the

fit

of the data.

(52),t

The possibility of circumventing the effects of diffusion has been

pointed out. (54) I f one of the reagents in the electron transfer process is

generated

in situ

by pulse radiolysis,

as

in reaction (22) at the moment of

B +

hv ..... B*

(22)

excitation species A + and B* are randomly distributed, but when the

reaction (23) proceeds, the distribution changes and the short-distance

(23)

t Electron transfer over varying distances features also

in

a recent discussion of the

photodiffusion of trapped electrons.

99)


27/456

12


pairs are depleted more rapidly, until a steady state is reached. It is this

steady state which is measured in conventional kinetics. The initial rate,

however, is closer to the true electron transfer rate. The theory of this

effect

is

given by Marcus and Siders, and curves of log

k

versus

I:J.O

8

are

plotted for rate constants k measured at different times after the excitation.

The opposite extreme condition of equation

(3) is

highly endergonic

electron transfer (Ll0

8

;:;:

A).

This is found(47) in reactions of the type in

equation (24) where L

=

diamines of the bipyridyl type. Actual values of

[RuLf]2+ + [RhL

3

]3+

....... [RuL

3

]3+

+ [RhL

3

f+ (24)

Ll0

8

are not known but rates decrease with increasingly negative

E

8

(Ru(III)/*Ru(II)) and the slope of a plot of log

k

versus

E8

is

close to

the value 16.9

V-I

expected from equations (18) and (19), or from the

Marcus equation (3) with I:J.0

8

::::: A. The equation actually used to fit the

data was the Marcus equation. For the two couples RuLj+ /*RuLi+ and

RhL

3

+/

2

+ an average self-exchange rate constant was obtained as 2

x

10

9

M-

1

S-I .

1.5. Optical and Thermal Electron Transfer

Interest continues in the problem of relating the optical electron

transfer process (25) to the corresponding thermal process [equation (26)].

+

hv

+

A

.

B ----. (A" . B )* (25)

In these equations the centers A and B are presumed to be linked so that

reaction (26)

is

intramolecular. When the available thermal reaction

is

A

+ ket A +

" . B ----. " . B

(26)

bimolecular [equation (1)] there

is

the problem of calculating a precursor

complex formation constant

Kip

[equation (27)]. The Marcus-Hush model

A

+ +

B A

+

. .

B,

Kip (27)

applied(53) to the processes (25) and (26) leads to equations (28)-(30) where

ii

max

is the wave number and

emax

the extinction coefficient of the absorption

Eth

= ~ p /

4 Eop - E

th

)

k

et

=

l I

e

t

exp(

-Eth/RT)

l I

e

t =

9.76

x 1010(iimax)1/2emaxI:J.1I1/2/r2

(28)

(29)

(30)

maximum in the intervalence charge transfer (IT) spectrum, Eop = hcii

max

,

Ll1I1/2 is the IT bandwidth, and

r

is the internuclear distance A-B. To test


28/456


29/456

T

e

1

1

O

c

a

a

T

m

E

e

r

o

T

a

e

D

a

I

T

a

o

p

o

b

F

m

o

C

m

e

c

a

m

a

e

E

h

k

(o

A

B

K

M

-

1

e

m

M

-1

m

k

m

-

s

[

R

N

sC

2

+

[

R

C

6

2

1

6

2

[

H

N

5

R

N

C

5

1

7

2

[

R

C

6

3

R

N

5

C

+

>

[

2

8

6

>

[

2

4

x

1

2

5

1

1

5

x

1

[

C

1

N

5L

3

2

x

1

2

3

2

4

7

x

1

[

2

5

x

1

1

0

3

[

R

N

5

+

R

C

2

7

x

1

1

5

3

[

R

N

5

+

O

C

:

-

2

9

x

1

1

2

4

F

F

O

O

6

H

O

1

8

1

2

F

F

O

O

6

p

0

4

C

o

o

m

c

F

C

6

m

[

H

N

5

R

N

H

C

H

+

9

0

3

k

(

c

e

S

R

5

5

(

e

1

5

5

1

1

1

1

1

8

8

1 1

.

-

>

.

.

~

;

s

'

~

;

s

$

0

~

'

~ ~

~


30/456

[

H

N

R

N

C

H

F

C

H

3+

[

2

M

y

+

+

[

2

4

X

1

[

N

s

R

R

e

a

[

y

V

y

V

P a

F

m

o

c

a

f

o

o

p

A

+

B

A

B

b

F

o

m

o

c

d

a

s

e

,

R

e

e

1

8

7

3

1

9

4

7

1

6

6

6

7

3

9

1

2

0

1

d

R

s

o

o

e

e

a

h

c

c

a

o

(

R

1

u

n

v

f

o

m

e

o

(

3

n

p

a

o

R

L

,

A

u

m

1

7

x

1

1

6

x

1

O

1

8

x

1

8

x

1

1

9

x

1

f

S

o

d

r

a

e

c

a

(

M

-

S

f

o

m

a

c

o

o

h

M

a

c

c

o

r

e

a

o

,

S

o

d

r

a

e

c

a

(

M

s

)

h

D

a

f

o

s

u

u

e

p

d

v

v

a

s

o

r

e

e

E

c

e

a

e

w

h

H

f

o

h

h

m

e

e

o

a

e

r

e

o

,

2

C

0

1

M

N

O

(

H

b

e

I

P

y

c

a

d

g

m

A

p

e

o

y

r

e

e

I

T

b

s

n

c

m

1

5

1

9

X

1

5

5

2

3

X

1

5

1

9

X

1

1

(

e

5

5 8

8

. ~

~

~

e

;

l

>

.

~

'

~

e

~

~

;

~

;

'

.

.

v


31/456

16

1

General

and Theoretical

ligand, and the final metal-to-metal electron transfer

is

too rapid to fol

low. (57) Its rate constant has been estimated(58)

as

k

-

6 X 10

10

S-1.

The comparison of thermal electron transfer data with optical data

from photoelectron experiments

is

explored by

Delahay.(59.60)

Reorganiz

ation energies derived from the process (33) are compared with those from

(33)

thermal exchange reactions. Agreement is close in some cases (A = Fe

z

+,

Mnz+, Co

z

+),

but

in other cases the optical values are about 0.3 eV below

the thermal(59) (cf. Ref. 61).

1.6.

Mixed-Valence Complexesf

A review by Wong and Schatz(6Z) consolidates earlier work on the

Piepho-Krausz-Schatz (PKS) vibronic coupling model, comparing this

model with the Marcus-Hush and the various nonadiabatic models. The

article, which is a model of clarity, relates the ground state properties of

mixed-valence systems (localized versus delocalized cases), the optical

properties (position and shape of the IT

band), and the thermal electron

transfer properties (rate and temperature dependence of activation energy)

to two critical parameters'\ and e which correspond to the reorganization

energy and the tunneling integral of the Marcus and semiclassical models.

Lindenberg and Ratner have discussed the question of localized versus

delocalized valency states by using a four-site model. In the simplest case,

this

is

the system Hz' . . Hr, in which the two H-H distances can be varied

to provide different values of the coupling parameter. The criteria for

valence trapping, and rates of intramolecular electron transfer are discussed

in terms of the model.

63)

The Creutz-Taube(64) ion (2, L

=

pyrazine) and related systems con

tinue to be of interest. Wong and Schatz(6Z) conclude that it is a class III

[(H

3

N)sRuLRu(NH

3

)s]s+

2

t Some other mixed-valence systems of special interest for electron transfer studies include

the ion

[Y 10026]"'-

containing

ylV05

and

yV

0

4

units, (100) [Cr30(00CCF3)6(Pyh] contain

ing indistinguishable Cr(III) and Cr(II),(lOl) [PtBr2(NH3h][PtBr4(NH3h] (a redetermi

nation of

structure(102);

spectral and electrochemical data on [Cu(III)Cu(II)Cu(II)OL3

]2+

(L = isonitroso

ketimine(103,104);

[Ni(TBP)h[Ni(TBP)tI3" which is a doubly mixed

valent, with metallic conductivity and rapid interconversion between [Ni(II)(TBP+)] and

[Ni(III)(TBP)] tautomers (TBp

2

-

=

tetrabenzoporphyrinate(lOS); Pt(II) doped

in

K

2

Pt(CN)6 (spectra)(llO); [ptX

6

]2 -

and [ptX

4

]2 -

doped

in

Cs

2

ZrX

6

(X

=

Cl or Br:

spectra(lll).


32/456

1.6 Mixed- Valence Complexes 17

compound in the Robin and Day(6S) sense but very close to the class

II-III

boundary. A problem remains however: the observed intervalence band

is markedly unsymmetric and the best fit of the shape to the equations of

the PKS theory implies a very slight degree of valence trapping. That is,

the probability distribution

P(q)

of the normal coordinate q which carries

the electron transfer has two slight maxima rather than one. The PKS

theory thus still predicts far-infrared tunneling transitions due to the

degeneracy of the wave function. These have been searched for, (66) and

not found. Krausz

et

al. propose that the complex is wholly delocalized,

and that the assymmetry of the absorption band must have some other

origin. Wong and Schatz(62) now suggest that the observed band is in fact

a superimposition of two

IT

bands.

In a different approach to the Creutz-Taube ion,

67)

structural data

for [Ru(NH

3

)s(pyz)]3+ and [Ru(NH

3

)s(pYZ)]2+

have been used to predict

the properties of the hypothetical

localized

-valence complex

[(NH

3

)sRu(III)(pyz)Ru(II)(NH

3

)s]5+. The delocalization energy

is

thus

estimated to be 0.4 eV, while the barrier to delocalization in the crystal

state is only 0.2 eV, giving a class III,

or

average-valency structure. In

aqueous solution the delocalization barrier

is

greater, but the ion remains

in class III, though close to the borderline with class II.

Tanner and

Ludi(68)

have extended the PKS calculations to a series of

18

Ru(III)-Ru(II) dimers, calculating the parameters of the model by fitting

the shapes of the

IT

bands. They confirm the delocalized character of the

complexes 2 with e.g., L

=

NCCN, NCCHCN-, and the progressively

weaker coupling along a series such as L = pyz, 4,4'-bipy,

CH

2

(C

s

H

3

Nh,

S(CH

2

CH

2

hS. Bond length changes from Ru(III) to Ru(II) are predicted.

It

is

felt that in general the PKS model provides a simple and consistent

approach.

An

experimental parameter which correlates well with the degree of

delocalization in the mixed-valence complex (A + . . A) is the compropor

tionation constant for the reaction (34). The factors affecting

Kcom have

(34)

been discussed in relation to several systems, some of them newly reported

(Table 1.2). For the complexes 2 Sutton and Taube(69) have considered

solvent interaction, stabilization of the mixed-valence state by resonance,

and

destabilization

of the Ru(II)-Ru(II) species owing to the fact that the

Ru(1I) ions are competing to delocalize

1r

electrons on to the ligand

1r*

system. In a series of complexes with

Kcom

= 7-20 (Table 1.2) the solvent

effect is a major factor. Much stronger coupling

is

found with dicyanamide

ion

as

the bridging ligand, and participation of the Ru(III)-(radical ion)

Ru(III) states is shown by the fact that with nonbridging, 1r-withdrawing


33/456

T

e

1

2

M

x

d

V

e

n

C

m

e

m

a

E

C

m

e

1

c

m

1

M

-

c

m

[

H

N

s

R

N

N

s

4

9

0

2

8

[

P

H

N

4

R

N

N

4

P

+

8

2

2

5

[

s

H

N

4

R

N

N

4

s

4

8

0

2

3

[

9

7

9

M

M

(

H

N

W

U

O

C

-

Q

N

)

1

2

1

[

(

H

3

N

h

R

U

N

Q

-

N

H

-

Q

N

R

U

(

N

H

3

6

S

+

1

8

1

1

[

(

H

3

N

h

R

U

N

Q

-

C

H

:

C

H

-

Q

R

U

(N

H

3

s

S

+

1

4

7

K

R

3

6

1

6

7

6

2

6

9

8

6

2

6

1

4

6

-

0

>

-

~

;

~

; ;

;

~

~

.

;

~


34/456

[

H

N

)

S

R

U

N

Q

-

C

:

C

-

Q

N

R

U

N

H

3

5

5

+

1

8

6

[

H

N

S

R

U

H

N

U

N

S

5

+

1

3

3

[

p

n

R

O

C

H

O

n

R

p

5

+

8

8

[

N

4

R

L

N

4

3

+

d

6

6

4

[

N

4

R

L

b

h

3

+

d

1

4

2

[

b

h

R

L

R

b

3

+

d

[

p

C

R

p

R

C

p

3

+

7

6

6

[

o

C

H

C

S

S

C

C

H

o

]

3

g

g

0

1

M

H

2

C

b

n

=

S

o

S

d

s

a

e

K

p

e

d

L

=

2

S

p

a

n

d

c

b

a

e

D

a

r

e

e

a

s

o

f

o

s

u

u

e

p

d

v

v

f

C

C

G

m

=

2

3

k

m

u

v

s

b

e

sp

a

r

e

e

b

I

T

b

n

a

g

1

8

6

7

1

1

2

x

1

4

9

x

1

1

1

X

1

f

2

6

X

1

6

6

7

1 1

1

1

7

0

~ '

I

,

~

;

s

'

'

g

3

>

'

"

. '

C


35/456

20

1

General

and Theoretical

ligands, the coupling is decreased(69a)

(d.

above, p.

7).

For the complexes

3 [M = Cu(II), M' = Cu(I)] Gagne et al. have included a contribution from

3

magnetic stabilization of Cu(II)-Cu(II) by calculating the singlet-triplet

(70)

separatlon.

In a series of complexes of the type 4 the electronic coupling

is

negligible (Kcorn

-

1) but the IT band

is

observed and must correspond to

an essentially outer-sphere transfer process(71) (the linked-pair

mechanism).(72)

A remarkable series of polynuclear mixed-valence complexes is

exemplified by compound 5

(B

= bipyridyl). Spectra and electrochemical


36/456

1.

7 Electron Transfer in the Solid State

21

data are reported, and Kearn for reactions such as [2,2 . . .

2,2] +

[3, 3

. . .

3, 3]

2[3, 2

. . .

3, 2],

where the numbers denote oxidation

states at the Ru centers.

(73)

The compound 6(74) is a model of the Fe

2

S6

unit found

in

ferredoxin

proteins.

75)

Electrochemical and homogeneous chemical redox reactions

establish the existence of the series Fe(III)Fe(III), Fe(III)Fe(II), and

Fe(II)Fe(II), and epr and Mossbauer spectra show the middle members to

be of the class II mixed-valence type.(74) As yet, however, the IT band has

not been detected.

The comproportionation constants referred to here may be compared

with values ranging from 8 x 10

4

to 1 X 10

9

for directly metal-metal

bonded dimers, as obtained from electrochemical oxidation studies of, e.g.,

[

Cr

2(map

)4]

1

+

/

2+, [

Re

2

CI

4(dppe

h]01+/2+.

76)

1.7. Electron Transfer in the Solid State

t

The conductivity of one-dimensional metal complexes has been

reviewed. 77) The influence of structure is emphasized, as are the interesting

structural changes which occur when the fractional oxidation state is varied.

Measurements of dielectric relaxation frequency have been used to obtain

ac and dc conductivities, the latter of which lead to the rate of hopping

("site-transfer") conductivity. In the double salt K

3

(Mn04)z, these data

give the rate of the outer-sphere(7S) electron transfer reaction (35). A

MnO:;- . . . n O ~ - M n O ~ - . . . MnO:;-

(35)

different technique for measuring the same physical process

is

time domain

reflectometry.(79) Applied to the mixed-valence solid EU

3

S4,

it gives the

rate of Eu(III) + Eu(II) electron transfer in good agreement with previous

Mossbauer work.

The V(V)/(IV) exchange occurs in partially reduced polyvanadic acid

gels. These are class II mixed-valence species, and have been studied

by esr, optical spectroscopy, and electrical conductivity measurements(SO)

(Table 1.1).

t

See also footnote. p. 16.


37/456

22


The mixed-valence ion [P

2

W

S

vIVvi0

62

]10-

exhibits rapid intra

molecular electron hopping as shown by esr line broadening. Moreover,

the esr spectra show partial delocalization of the electrons from V(IV) to

the neighboring V(V), so that the complex must be classed

as

a borderline

case between localized class II and delocalized class

III. (81)

A similar sugges

tion has been

made(82)

for the complex [Fe30(00CCH3)6L3],

(L

=

H

2

0,py). Mossbauer line-broadening data indicate the Fe(III)/Fe(II) elec

tron transfer process, with low activation energies. (83)

The infrared spectrum of Cs

4

[Sb(V)CI

6

][Sb(III)CI

6

] shows some

remarkably temperature-sensitive bands in the region 100-300 cm -1. They

are assigned(84) to vibrations of the Sb(III)CI

6

unit, and it is suggested that

this effect

is

due to the thermal electron transfer process Sb(IV)

+

Sb(III).

It

should be noted, however, that this requires a "hopping" frequency of

the order of 10

12

s -1, whereas Atkinson and

Day(I06)

assigned a much lower

frequency, from conductivity studies.

t

Using the resonance Raman effect, Hester and Nour(8S) have assigned

the Fe(II)

.......

Co (III) intervalence transitions in the complexes

[(NC)sFe(II)CNCo(III)(CN)st- and [(NC)sFe(II)CNCo(III)(edta)]5-.

Detailed analysis of the CN stretching modes also confirms the ground

state valency assignments

as

shown. In the linear chain complexes

[Pt(LLh][Pt(LLhX2]X4 (LL

=

diamine, X

=

Br

(86)

or I

(87))

the

Pt(II)-Pt

Pt(IV) intervalence band is coupled to the symmetrical stretch,

X-Pt-X

of

the Pt(IV) units,

as

in other complexes of this type.

Electronic Raman spectroscopy has attracted increasing attention in

recent

years,(88)

and a theory of the resonance electronic Raman effect has

now been

given.(89)

Wong and Schatz have also discussed in detail the

electronic Raman effect as applied to mixed-valence

systems(90)

particularly

the Pt(IV)-Pt(II) linear chain compounds (following their previous study(91)

of the conventional resonance Raman spectra of these materials).

t The authors of Ref. 84 assumed their data to be in agreement with Atkinson and Day, but

they appear to have identified w, of equation 2, Ref. 106 as the hopping frequency, instead

of

K.


38/456

Chapter

2

Redox Reactions

between

Metal Complexes

2.1. Introduction

In

this chapter, the electron transfer reactions between metal ion

complexes have been reviewed in a systematic fashion which emphasizes

the role of the reductant ion. It has been found convenient in the past to

condense rate data and these are found in Table 2.1. Major theoretical

advances have been made in this area over the period covered but these

have been dealt with in Chapter

1.

Similarly, papers which report no new

rate data but theoretical treatment of existing data have not been included.

In some cases, papers capable of inclusion in more than one section have

been included where the balance of the chapter demands.

2.2.

Titanium (III)

Reduction of

[Ru(enhf+

by TiOH

2

+ is outer sphere(l) and the activa

tion enthalpy for the reaction suggests a closer approach of the reactants

than might be expected. It is thought that the chelate ring directs the

reductant and enhances

t2g/t2g

overlap. Inner-sphere mechanisms are pro

posed(2)

for reduction of [Co(NH3)SX]2+ complexes by Ti3+ where

X-

is

a

good bridging ligand such as F- or OH-. In these cases the rates are close

to the substitution-limited inner-sphere value. Salicylate, salH-, has proved

to be an effective bridge(3)

in

the electron transfer from

Ti3+

to

23


39/456

~

T

e

2

1

R

e

C

a

s

a

T

m

n

m

c

P

a

m

e

o

R

a

o

b

w

n

M

a

I

o

C

m

e

a

2

C

/

L

k

i

H

iS

R

o

M

M

-

8

k

m

c

K

m

R

T

a

u

m

[

R

e

n

+

+

T

O

+

0

1

k

H

5

8

l

1

-

1

1

[

C

N

s

B

2

+

+

T

O

+

1

0

0

1

2

[

C

N

sF

+

+

T

+

1

0

1

x

1

2

c

s

C

e

z

H

O

+

+

T

+

1

0

5

2

t

r

a

[

C

en

H

O

2

+

+

T

+

1

0

2

2

c

s

C

e

H

O

2

+

+

T

O

+

1

0

3

X

1

2

t

r

a

[

C

en

H

O

2

+

+

T

O

+

1

0

4

X

1

2

[

R

N

s

H

2

+

+

T

3

+

1

0

6

X

1

3

[

R

N

sN

+

+

T

H

D

A

2

0

2

8

x

1

5

[

R

N

s

N

+

+

T

3

+

2

0

8

8

5

1

5

[

C

N

4

C

4

+

T

3

+

1

0

0

0

6

[

C

N

4

C

4

+

T

C

4

W

1

0

1

6

[

C

N

4

C

+

T

C

h

1

0

4

6

V

u

m

t

r

a

[

R

e

n

C

+

V

+

0

5

1

2

x

1

8

t

a

n

R

e

n

B

C

+

V

+

0

5

2

2

x

1

8

t

r

a

[

R

e

n

C

B

+

V

+

0

5

2

2

x

1

8

t

r

a

[

R

en

B

+

V

+

0

5

3

0

x

1

8

t

r

a

[

R

en

2

+

V

+

0

5

1

2

x

1

8

t

r

a

[

R

2

3

2

-

e

C

2

+

V

+

0

5

2

0

x

1

8

t

a

R

1

a

N

C

h

+

V

+

0

5

3

6

x

1

8

t

r

a

[

R

[1

a

N

C

h

+

V

+

0

5

3

5

x

1

8

t

a

R

M

e

1

a

N

C

2

+

V

+

0

5

7

7

x

1

8

r

a

R

M

e

6

1

a

N

C

h

+

V

+

0

5

7

6

x

1

8

[

C

N

s0

C

C

H

N

3

+

+

V

+

1

0

1

1

1

[

C

N

s0

C

C

C

H

N

3

+

+

V

+

1

0

1

7

1


40/456

[

C

N

0

C

C

0

C

H

N

3+

+

y

+

1

0

1

6

1

[

C

N

0

C

O

C

H

C

N

H

C

+

+

V

+

1

0

0

8

1

[

C

N

S

C

C

m

C

H

N

3

+

y

+

1

0

0

7

1

[

C

N

s

0

C

+

+

V

+

1

0

1

1

1

[

C

N

0

C

C

2

+

+

y

+

1

0

1

0

1

[

C

N

C

C

C

+

+

V

+

1

0

0

3

1

[

C

N

0

C

C

2

+

+

V

+

1

0

0

2

1

[

C

N

0

C

C

H

+

+

V

+

1

0

0

9

1

[

C

N

S

C

H

0

C

+

+

V

+

1

0

0

6

1

[

C

N

0

C

N

O

C

2

+

+

V

+

1

0

1

4

1

[

C

N

S

C

N

+

+

V

+

1

0

0

8

1

[

C

N

0

C

N

C

+

+

V

+

1

0

0

2

1

[

C

N

s

0

C

N

C

+

+

V

+

1

0

0

1

1

[

C

N

0

C

N

C

3

+

+

V

+

1

0

0

3

1

[

C

N

s

0

C

C

N

+

+

V

+

1

0

0

9

1

[

C

N

s

0

C

C

N

3

+

+

V

+

1

0

0

5

1

[

C

N

s

0

C

3

+

+

V

+

1

0

1

1

1

[

C

N

S

C

H

0

N

+

+

V

+

1

0

0

3

1

C

o

m

u

m

t

a

R

e

C

2

+

+

C

+

0

5

3

2

5

-

3

7

t

a

R

2

3

2

e

C

2

+

C

+

0

5

4

3

6

5

-

3

7

t

a

R

1

a

N

C

+

C

+

0

5

6

4

6

5

-

2

7

t

a

R

e

B

C

+

C

+

0

5

3

0

8

t

a

R

e

C

B

+

C

+

0

5

3

0

8

t

a

R

e

z

B

2

+

C

+

0

5

7

3

8

t

a

R

e

z

2

+

+

C

+

0

5

2

8

t

a

R

1

a

N

C

2

+

C

2

+

0

5

1

8

m

R

M

1

a

N

C

+

C

+

0

5

4

9

x

1

8

r

a

R

M

1

a

N

C

2

+

C

+

0

5

3

7

x

1

8

[

C

N

N

C

H

3

+

+

C

+

0

5

2

3

x

1

1

5

-

1

2

9

[

C

N

N

C

+

+

C

+

0

5

2

1

9

t

[

C

N

N

+

+

C

+

0

5

9

4

x

1

1

3

-

3

1

9

V


41/456

I

J

T

e

2

1

(

c

o

n

d

0

I

L

k

H

ts

R

o

M

M

-

S

1

k

m

c

K

m

R

C

o

m

u

m

c

d

[

C

N

N

C

C

N

+

C

+

0

5

2

5

x

1

9

5

-

3

1

9

[

C

N

s

N

C

+

C

+

0

5

2

6

x

1

9

0

-

3

6

9

[

C

N

N

C

C

H

+

C

2

+

0

5

2

3

x

1

9

5

-

3

2

9

[

C

N

s

N

C

+

C

+

0

5

2

1

1

5

-

1

2

9

[

C

N

s

o

A

3

+

+

C

+

1

0

6

2

x

1

1

2

1

[

C

N

s

o

A

2

+

+

C

+

1

0

1

1

x

1

1

[

C

N

s

o

A

d

+

+

C

+

1

0

4

0

x

1

1

2

1

[

C

N

o

A

c

a

d

f

+

1

0

3

6

1

2

1

[

C

N

o

A

n

2

+

1

0

3

5

1

2

1

[

C

N

m

A

3

+

1

0

3

8

1

3

1

[

C

N

m

A

d

2

+

1

0

3

8

1

3

1

[

C

N

m

A

c

a

d

+

1

0

3

7

9

3

1

[

C

N

m

A

n

2

+

1

0

3

6

9

3

1

[

C

N

s

p

A

3

1

0

4

6

9

3

1

[

C

N

s

p

A

d

2

+

1

0

4

4

9

3

1

[

C

N

s

p

A

c

a

d

f

+

1

0

4

1

1

3

1

[

C

N

p

A

n

2

+

1

0

3

8

1

3

1

[

C

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[m._v._twigg__(auth.),__m._v._twigg__(eds.)]_mecha(bookzz.org).pdf

Documents

[m._v._twigg(auth.),m._v._twigg__(eds.)]_mecha(bookzz.org).pdf