Comprehensive chemical kinetics Bamford

7/25/2019 Comprehensive chemical kinetics Bamford

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C O M P R E H E N S I V E

C H E M I C A L

K I N E T I C S


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Volume 1

Volume 2

Volume

3

Volume 4

Volume 5

Volume

6

Volume 7

Volume

9

C O M P R E H E N S I V E

Section

1.

THE

PRACTICE

AND THEORY

OF

KINETICS

The Practice of Kinetics

The Theory of Kinetics

The Formation and Decay of Excited Species

Section 2. HOMOGENEOUS DECOMPOSITIONAND ISOMERISATION

REACTIONS

Decomposition of Inorganic and Organometallic Compounds

Decomposition and Tsomerisation

of Organic Compounds

Section 3.

INO RG ANIC

REACTIONS

Reactions of Non-metallic Inorganic Compounds

Reactions of Metallic Salts and Complexes, and Organometallic Com-

pounds

Section 4. ORGANIC

REACTIONS

(6 volumes)

Addition and Elimination Reactions

of

Aliphatic Compounds

Volume I0 Ester Formation and Hydrolysis and Related Reactions

Volume 13

Reactions

of

Aromatic Compounds

Section

5.

POLYMERISATION R EACTIONS (2 volumes)

Section 6.

O XIDATIO N

AND COMBUSTION REACTIONS ( 2 volumes)

Section 7. SELECTED ELEMENTARY REACTIONS ( 2 volumes)

Additional Sections

HETEROGENEOUS REACTIONS

SOLID STATE REACTIONS

KINETICS AN D TECHNOLOGICAL PROCESSES


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CHEM ICAL K IN ETICS

EDITED

BY

C.

H. BAMFORD

M.A. , Ph.D. , Sc.D. (Cantab.),

F.R.I.C.,

F.R.S.

Camp bell-Brown Professor

of

Industrial Chemistry,

Unioersity

of

Liverpool

A N D

C.

F. H.

TIPPER

Ph.D. (Bristol),

D.Sc.

(Edinburgh)

Senior Lecturer in Physical Chemistry,

Uniuersity of Lioerpool

VOLUME 7

REACTIONS

OF

METALLIC SALTS A N D COMPLEXES,

A N D ORGANOMETALLIC COM POUND S

E L S EV IE R P U B L I S H I N G C O M P A N Y

A M S T E R D A M

-

L O N D O N

-

NEW

Y O R K

1972


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E L S E V I E R P U B L I S H I N G C O M P A N Y

336

I A N V A N G A L E N S T R A A T

P.O. BOX

211 A M ST E RD A M, T H E N E T H E R L A N D S

A M E R I C A N E LS EV IE R P U B L I S H I N G C O M P A N Y, I N C .

52 V A N D E R B I L T A V E N U E

N E W Y OR K, N E W Y O R K

10017

L I B R A R Y OF C O N G R E S S C A R D

N U M B E R

76-151731

ISBN

0-444-40861

W I T H 35

I LLUSTR ATI ONS

A N D 228

T A BL E S

C O P Y R I G H T @

1972

B Y

E L S E V I E R

P U B L I S H I N G

COMPANY,

A M S T E R D A M

A L L R I G H T S R E S ER V ED

N O P A R T

OF

T H I S P U B L I C A T I O N M A Y BE R E P R O D U C E D ,

S T O R E D I N A R E T R I E V A L SY ST EM , O R T R A N S M I T T E D I N A N Y F O R M OR B Y ANY M EANS

E L E C T R O N I C, M E C H A N I C A L , P H O T O C O P Y I N G , R E C O R D I N G , O R O T H E R W I SE ,

W I T H O U T T H E P R I O R W R I T T E N P E R M I SS I O N O F T H E P U BL I S HE R ,

E L SE V I ER P U B L I S H I N G C O M P A N Y, J A N V A N G A L E N S T R A A T

335,

AM S TER DAM

P R I N T E D I N T H E N E T H ER L A ND S


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C O M P R E H E N S I V E C H E M I C A L

K I N E T I C S

A D V I S O R Y

B O A R D

Professor

S.

W. BENSON

Professor SIR

FREDERICK DAINTO N

Professor G . G E E

the late

Professor

P.

GOLDFINGER

Professor

G . s.

HAMMOND

Professor W. JOST

Professor

G.

B.

KISTIAKOWSKY

Professor

v. N. K O NDRAT I E V

Professor K. J .

LAIDLER

Professor M. MAGAT

Professor SIR

HARRY MELVILLE

Professor G. NATTA

Professor R . G.

W.

NORRISH

Professor

s.

OKA M U R A

Professor

SIR

ERIC

RIDEAL

Professor N.

N.

SEMENOV

Professor z. G. SZABO

Professor 0. WICHTERLE


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Contributors to Volume 7

D.

BENSON Department of Chemistry,

Widnes Technical College,

Widnes, Lancs., England

L. J. C S ~ N Y I

Institute of Inorganic and Analytical Chcniistry,

J6szef Attila University,

Szeged, Hungary

T.

J.

KEMP

School

of

Molecular Sciences,

University of

Warwick,

Coventry, England

C. H. LANGFORD

Department

of

Chemistry,

Carleton University,

Ottawa, Canada

M . P A R R I S Department of Chemistry,

Carleton University,

Ottawa, Canada

P. J.

PROLL

Department of Chemistry,

Widnes Technical College,

Widnes, Lancs., England


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Preface

The rates of chemical processes and their variation with conditions have been

studied for many years, usually for the purpose of determining reaction mecha-

nisms. Thus, the subject of chemical kinetics is a very extensive and important

part of chemistry as a whole, and has acquired an enormous literature. Despite

the number of books and reviews, in many cases it is by no means easy to find

the required information on specific reactions or types of reaction or on more

general topics in the field. It is the purpose of this series to provide a background

reference work, which will enable such information to be obtained either directly,

or from the original papers or reviews quoted.

The aim is to cover, in a reasonably critical way, the practice and theory of

kinetics and the kinetics of inorganic and organic reactions in gaseous and con-

densed phases and at interfaces (excluding biochemical and electrochemical kinet-

ics, however, unless very relevant) in more or less detail. The series will be divided

into sections covering a relatively wide field; a section will consist of one or more

volumes, each containing a number of articles written by experts in the various

topics. Mechanisms will be thoroughly discussed and relevant non-kinetic data

will be mentioned in this context. The methods of approach to the various topics

will, of necessity, vary somewhat depending on the subject and the author(s) con-

cerned.

It is obviously impossible to classify chemical reactions in a completely logical

manner, and the editors have in general based their classification

on

types of chem-

ical element, compound or reaction rather than on mechanisms, since views on

the latter are subject to change. Some duplication is inevitable, but it is felt that

this can be a help rather than a hindrance.

Section 3 deals with reactions in which at least one of the reactants is an in-

organic compound. Many of the processes considered also involve organic com-

pounds, but autocatalytic oxidations and flames, polymerisation and reactions of

metals themselves and of certain unstable ionic species, e . g . the solvated electron,

are discussed in later sections. Where appropriate, the effects of low and high ener-

gy radiation are considered, as are gas and condensed phase systems but not fully

heterogeneous processes or solid reactions. Rate parameters of individual elemen-

tary steps, as well as of overall reactions, are given if available.

In volume 7 reactions of metallic salts, complexes and organometallic com-

pounds are covered. Isomerisation and group transfer reactions of inert metal

complexes and certain organometallics (not involving a change

in

oxidation state)

are considered first, followed by oxidation-reduction processes (a) between dif-

ferent valency states of the same metallic element (b) between salts of different


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VIII P R E F A C E

metals and (c) involving covalent inorganic or organic compounds. Finally,

induced reactions are discussed separately.

The Editors desire to record their sincere appreciation of the continuing advice

and support from the members of the Advisory Board.

Liverpool

October,

1971

C. H. BAMFORD

C. F. H. TIPPER


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Contents

Preface

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

VI

i

Chapter I

(C

.

H

. LANGFORD

N D M .

PARRIS)

Reactions of inert complexes and metal organic compounds

. . . . . . . . .

1

1

. NTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

2. STOICHIOMETRIC MECHANISMS . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. REACTIONS OF

CO(II1) COMPLEXES

. . . . . . . . . . . . . . . . . . . . . . . 7

4

. Cr(ll1). Rh(ll1).

RU(II1 ) . I T ( I I 1 )

AND Pt(1V)

C O M P L E X E S

17

5

.

COMPLEXES

OF

Pt(1I). Pd(11).

AU(II1) AND

Rh(1) 20

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

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

6. COMPLEXES

WITH

B

CLASS LIGANDS: THE

BINARY CARBONYLS . . . . . . . . . . . . 25

7.

COMPLEXES

WITH B CLASS LIGANDS:

TH E

SUBSTITUTED CARBONYLS. . . . . . . . . .

31

8

.

THEORETICAL

CONSIDERATIONS

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

43

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

52

Chapter 2

(P.

J

. PROLL)

Reactions in solution between various metal ions of the same element in dif-

ferent oxidation sta tes

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

56

1

. INTRODUCTION

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

2.1

The exchange reaction between

Cu(II)

and

Cu(1)

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

2.2 The exchange reaction between Ag(I1) and Ag(1)

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

2.3 The exchange reaction between Au(II1) and Au(1) . . . . . . . . . . . . .

2.4 The exchange reaction between Au(II1) an d Au(I1) . . . . . . . . . . . . .

2.5 The disproportionation

of

Au(1I)

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

3.1 The exchange reaction between Hg(l1) and Hg( l); the disproportionation reac-

2. COPPER. SILVER AND

GOLD

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

3. M E R C U R Y

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

tion

of

Hg(I1)

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

3.2 The exchange reaction between Hg(I1) and Hg(1) in non-aqueous media . . . .

4.1 The exchange reaction between TI(III) and TI(1)

4

. THALLIUM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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

56

58

58

58

59

60

60

60

60

62

62

62

5

.

TIN AND LEAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

68

70

70

71

5.1 The exchange reaction between Sn(IV) and Sn(I1) in aqueous media

. . . . .

68

The exchange reaction between Sn(IV) and Sn(I1) in non-aqueous media

. . .

The exchange reaction between Pb(lV) and Pb(I1) in aqueous media

. . . . .

The exchange reaction between Pb(IV) and Pb(1l) in non-aqueous media

. . .

5.2

5.3

5.4


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X CONTENTS

6. A R S E N I C

A N D

A N T I M O N Y . . . . . . . . . . . . . . . . . . . . . . . . . . .

6.1 The exchange reaction between As(V) and As(II1)

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

6.2 The exchange reaction between Sb(V) an d Sb(II1) in aqueous media . . . . .

6.3

The exchange reaction between Sb(V) and Sb(II1) in non-aqueous media

. . .

6.4 The Sb(II1)-catalysed hydrolysis of Sb(V) . . . . . . . . . . . . . . . . .

7.1 The exchange reaction between Te(V1) and Te(1V)

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

8.1 Vanadium and tantalum

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

8.1.1 Th e exchange reaction between V(II1) and V(I1) . . . . . . . . . . .

8.1.2 The exchange reaction between V(1V) and V(11I)

. . . . . . . . . .

8.1.3 The exchange reaction between V(V) an d V(1V)

. . . . . . . . . . .

8.1.4 React ions between vanad ium ions . . . . . . . . . . . . . . . . .

8.1.5 React ions between tanta lum cluster ions

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

Chromium, molybdenum and tungsten

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

8.2.1 Th e exchange reaction between Cr(II1) and Cr(1 l)

. . . . . . . . . .

8.2.2 The exchange reaction between Cr(V1) and

Cr Il1)

. . . . . . . . . .

8.2.3 Th e reaction between Cr(V1) and Cr(I1)

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

8.2.4 Cr(1l)-catalysed substi tution and isomerisation reactions of Cr(I1l) . . .

8.2.5 Th e exchange reaction between Mo(V) and Mo(1V)

. . . . . . . . .

8.2.6 Th e exchange reaction between W(V) and W(IV) . . . . . . . . . .

8.3 Manganese

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

8.3. The exchange reaction between Mn(1l) and Mn(1)

. . . . . . . . . .

8.3.2 The exchange reaction between Mn(lI1) and Mn(I1)

. . . . . . . .

8.3.3 Th e exchange reaction between Mn(VI1) an d Mn(V1) . . . . . . . .

8.3.4 The exchange reaction between Mn(VI1) and Mn(Il 1)

. . . . . . . .

8.3.5 Th e exchange reaction between Mn(VI1) and Mn(1l) . . . . . . . .

8.3.6 Th e reaction of Mn(VI1) and Mn(I1)

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

8.4 Iron. ruthenium and osmium

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

8.4.

The exchange reaction between Fe(II1) and Fe(I1) in aqueous media . .

8.4.2 The effect of inorgani c ions on the exchange reaction between Fe(Il1)

and Fe(I1) . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.4.3 Th e effect of organic ligands o n the exchange reaction between Fe(I11)

and Fe(1l) . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.4.4 Th e exchange reaction between Fe(1Il) an d Fe(I1) in non-aqueous an d

mixed solvents . . . . . . . . . . . . . . . . . . . . . . . . .

8.4.5 Th e exchange reaction between hexacyanoferrate(lI1) and hexacyanofer-

rate(I1) . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.4.6 Reactions of Fe(II1) with Fe(I1)

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

8.4.7 The Fe(I1)-catalysed aquat ion of Fe(II1) . . . . . . . . . . . . . .

8.4.8 The reaction between Fe(IV) an d Fe(I1)

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

8.4.9 The exchange reaction between Ru(VI1) an d Ru(VI) . . . . . . . . .

8.4.10 Ru(I1)-catalysed substitution reactions

of

Ru(II1)

. . . . . . . . . .

8.4.1 1 Th e exchange reaction between Os(1II) and Os(I1) . . . . . . . . . .

8.5 C obalt, ruthenium and iridium . . . . . . . . . . . . . . . . . . . . . .

8.5.1 The exchange reaction between Co(1lI ) and Co(l1 ) in aqueous media ;

the effect of inorganic anions

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

8.5.2 Exchange reactions involving complexes of Co(II1) and Co(I1) with am-

monia an d organic ligands . . . . . . . . . . . . . . . . . . . .

8.5.3 Th e exchange reaction between Co(1II) and Co(I1) in non-aqueous media

8.5.4 Co(I1)-catalysed substi tution reactions

of

Co(1II) . . . . . . . . . .

8.5.5 The reaction of Co(IV) with Co I1) . . . . . . . . . . . . . . . .

8.5.6 Rh(1)-catalysed substitution reactions of Rh(II1)

. . . . . . . . . .

8.5.7 The exchange reaction between Ir(1V) and

Ir(Il1)

. . . . . . . . . .

7. T E L L U R I U M

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

8. T R A N S I T I O N M E T AL S . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8.2

71

71

71

74

15

75

75

75

75

75

76

17

78

80

80

80

83

84

85

91

92

92

92

92

93

94

94

95

96

96

98

103

105

106

108

109

110

110

110

111

111

1 1 1

114

119

119

121

121

122

8.5.8

The reaction between complexes of Ir(1V) and

Ir(II1) . . . . . . . . .

122


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CONTENTS

XI

8.6 Pla t inum

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

122

122

9.1

T he exchange reaction between C e(IV) and C e

111)

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

128

9.2 T he exchange react ion between Eu(II1) an d Eu( l1)

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

130

8.6.1 T he exchange reaction between Pt(1V) and Pt( 1l); Pt(I1)-catalysed sub-

sti tutio n reactions of Pt(1V) . . . . . . . . . . . . . . . . . . . .

9. CERIUM AND EUROPIUM

128

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

10. URANIUM . NEPTUNIUM. PLUTONIUM AN D AMERICIUM

10.1 Exchange reactions between uranium ions .

10.2 React ions between uran ium ions . . . . .

10.3 Exchange reactions between neptunium ions .

10.4 Reactions between nep tuniu m ions . . . . .

10.5 Exchange reactions between plutonium ions

10.6 Reactions between plutonium ions

. . . . .

10.7 Exchange reactions between americium ions

10.8 Reactions between americium ions

. . . . .

REFERENCES

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

. . . . . . . . . . . . . . . 13 0

. . . . . . . . . . . . . . . 13 0

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

132

. . . . . . . . . . . . . . . 133

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

13 5

. . . . . . . . . . . . . . . 138

. . . . . . . . . . . . . . . 138

. . . . . . . . . . . . . . . 141

. . . . . . . . . . . . . . . 141

. . . . . . . . . . . . . . . 142

Chupter 3

(D

BEN S ON )

Oxidation-reduction reactions between complexes

of

different metals

. . . . .

153

. INTRODUCTION

153

2.

OXIDATIONS BY VANADIUM

. . . . . . . . . . . . . . . . . . . . . . . . . . 154

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

2.1 Oxidations by vanadium(V) . . . . . . . . . . . . . . . . . . . . . . . 154

2.2 Oxidations by vanadium(1V)

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

157

2.3 Oxidations by vanadium(l1l) . . . . . . . . . . . . . . . . . . . . . .

15 9

3. OXIDATIONSY C H R O M I U M AND M O L Y B D E N U M . . . . . . . . . . . . . . . . . 162

3.1 Oxidations by chromium(V1) . . . . . . . . . . . . . . . . . . . . . . 162

3.2 O xidations by chromium (l l1)

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

167

3.3 Oxidations by molybdenum(\/) . . . . . . . . . . . . . . . . . . . . . . 169

4

. O X I D A T I O N S

B Y

MANGANESE

AND R H E N I U M . . . . . . . . . . . . . . . . . . . 169

4.1 Oxidations by manganese(V I1) . . . . . . . . . . . . . . . . . . . . . .

169

4.2 Oxidations by manganese(lI1)

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

172

4.3 Oxidations by rhenium(VI1) . . . . . . . . . . . . . . . . . . . . . . . 175

176

5.1

Oxidations by iron(II1) . . . . . . . . . . . . . . . . . . . . . . . . . 176

5.2 Oxidations by ruthenium(Il1) . . . . . . . . . . . . . . . . . . . . . . 188

6.

OXIDATIONS

B Y C O B A L T ( I I I ) . . . . . . . . . . . . . . . . . . . . . . . . . .

18 8

6.1 Inorg anic bridging ligands in oxidations by cobalt(II1) complexes . . . . . . . 188

6.2 Organic bridging ligands in oxidations by cobalt(II1) complexes

. . . . . . . .

206

6.3 Oxidations by aq uo complexes

of

cobalt(II1) . . . . . . . . . . . . . . . . 213

7

. O X I D A T I O N S

B Y

P L A T I N U M ( I V )

. . . . . . . . . . . . . . . . . . . . . . . . 227

8

.

O X I D A T I O N S BY C O P P E R ( I I )

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

228

9

.

OXIDATIONS BY

M E R C U R Y ( I I )

. . . . . . . . . . . . . . . . . . . . . . . . . 228

10

.

O X I D A T I O N S

B Y T H A L L I U M ( I I I )

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

230

11. OXIDATIONSB Y L E A D ( I V )

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

241

12. OXIDATIONS B Y C E R I U M ( I V ) . . . . . . . . . . . . . . . . . . . . . . . . . .

243

13

.

OXIDATIONS B Y

URANIUM, NEPTUN IUM

AND P L U T O N I U M . . . . . . . . . . . . . . 253

13.1 Oxidations by uranium(V1) . . . . . . . . . . . . . . . . . . . . . . . 253

13.2 Oxidations by neptunium

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

257

13.3 Oxidations by plutonium . . . . . . . . . . . . . . . . . . . . . . . . 261

R E F E R E N C E S

267

5

.

OXIDATIONSB Y I R O N ( I I I ) AND R U T H E N I U M ( I I I ) . . . . . . . . . . . . . . . . . .

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


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XI1 CONTENTS

Cliupter 4

(T. J. KEMP)

Oxidation-reduction reactions between covalent compounds and metal ions

274

1. INTRODUCTION

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

1.1

Scope and pattern of this chapter

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

1.2 Categorisation of oxidants as one- or twoequivalent . . . . . . . . . . . .

2. OXIDATION BY cr(vr1) AND Mn(vII)

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

2.1 General features

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

2.1.2

Solution equilibria of oxy-anions of Cr(V1) and Mn(VI1) . . . . . . .

2.2 Oxidation of inorganic covalent species

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

2.2.1 Halide ions . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.2 Cyanide ion . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.3 Oxides of hydrogen . . . . . . . . . . . . . . . . . . . . . . .

2.2.4 Oxy-acids of sulphur . . . . . . . . . . . . . . . . . . . . . .

2.2.5 Nitrite ion . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.6

Trivalent phosphorus compounds

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

2.2.7 Arsenious acid . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.8 Carbon monoxide . . . . . . . . . . . . . . . . . . . . . . . .

2.2.9 Molecular hydrogen . . . . . . . . . . . . . . . . . . . . . . .

2.3 Oxidation

of

monofunctional organic molecules . . . . . . . . . . . . . .

2.3.1 Aliphatic hydrocarbons . . . . . . . . . . . . . . . . . . . . . .

2.3.2 Olefins and acetylenes . . . . . . . . . . . . . . . . . . . . . .

2.3.3 Alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.3.4 Aldehydes . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.3.5 Phenols

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

2.3.6 Ketones

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

2.3.7 Monocarboxylic acids . . . . . . . . . . . . . . . . . . . . . .

2.3.8 Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.3.9 Nitroalkanes

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

2.4 Oxidation of polyfunctional organic molecules . . . . . . . . . . . . . .

2.4.1 Glycols. . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4.2 Allylic alcohols . . . . . . . . . . . . . . . . . . . . . . . . .

2.4.3

Ketols, keto-aldehydes and keto-acids

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

2.4.4 Dicarboxylic acids

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

2.4.5

Hydroxy-acids . . . . . . . . . . . . . . . . . . . . . . . . .

2.4.6 Boronic acids . . . . . . . . . . . . . . . . . . . . . . . . . .

2.4.7

Furfurals . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.5 Some metal-ion catalysed reactions of chromic acid . . . . . . . . . . . .

3.

OXIDATION BY

Pb(Iv),

Tl(III) ,

Hg(II), Hg(I), Bi(V), AU(III),Pt(IV), Pd(II), Rh(III),

R U (1 II )

AND

MO(V1)

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

3.1 General features . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.2 Oxidation of inorganic species . . . . . . . . . . . . . . . . . . . . .

3.2.1 Halide and pseudohalide ions . . . . . . . . . . . . . . . . . . .

3.2.2 Oxy-acids of sulphur . . . . . . . . . . . . . . . . . . . . . .

3.2.3 Hydrazine

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

3.2.4 Nitrite ion . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.2.5 Hypophosphorous acid . . . . . . . . . . . . . . . . . . . . .

3.2.6 Carbon monoxide . . . . . . . . . . . . . . . . . . . . . . . .

3.2.7 Molecular hydrogen . . . . . . . . . . . . . . . . . . . . . . .

Oxidation of monofunctional organic molecules . . . . . . . . . . . . . .

3.3.1

Olefins

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

3.3.2

Arylcyclopropanes

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

3.3.3

Alcohols

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

3.3.4 Hydroperoxides . . . . . . . . . . . . . . . . . . . . . . . . .

3.3

274

274

276

278

278

279

279

279

283

284

285

287

287

288

290

291

292

292

298

300

309

313

313

316

318

319

320

320

322

322

322

324

326

327

327

329

329

330

331

332

332

333

334

334

335

336

336

342

343

344


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C O N T E N T S XI11

3.3.5 Formic acid . . . . . . . . .

3.3.6 Higher carboxylic acids . . . .

3.3.7 Ketones

. . . . . . . . . .

3.3.8 Ethers . . . . . . . . . . .

3.4

Oxidation of polyfunctional molecules

3.4.1 Glycols

. . . . . . . . . . .

3.4.2 a-Hydroxycarboxylic acids

. .

3.4.3 Dicarboxylic acids . . . . .

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

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

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

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

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

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

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

4

.

OXIDATION

BY

Ag(I1). Ag(1II). CO(III). e(rv).

m ( i I I ) .

v(v). rr(1v). NP(VI) N D P U ( V I ) .

4.1 Inorganic chemistry of these oxidation states . . . . . . . . . . . . . . .

4.1.1 Silver species . . . . . . . . . . . . . . . . . . . . . . . . . .

4.1.2 Cobalt(II1) . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.1.3 Cerium(1V) . . . . . . . . . . . . . . . . . . . . . . . . . .

4.1.4 Manganese(II1) . . . . . . . . . . . . . . . . . . . . . . . . .

4.1.5 Vanadium(V) . . . . . . . . . . . . . . . . . . . . . . . . . .

4.1.6 Iridium(1V) as IrCI6*-

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

4.2

Oxidation of inorganic species

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

4.2.1 Chloride ion . . . . . . . . . . . . . . . . . . . . . . . . . .

4.2.2 Bromide ion

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

4.2.3 Iodide ion . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.2.4

Hydrazoic acid

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

4.2.5 Bromine . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.2.6

Chlorine dioxide

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

4.2.7 Hydrazine and methylhydrazines

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

4.2.8

Hydroxylamine. 0-methylhydroxylamine and nitrous acid . . . . . . .

4.2.9 Oxides of hydrogen

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

4.2.10

Sulphur compounds

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

4.2.1 1 Hypophosphorous acid

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

4.2.12

Arsenious acid

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

4.2.1 3 Antimony(II1) . . . . . . . . . . . . . . . . . . . . . . . . .

4.3 Oxidation of monofunctional organic molecules . . . . . . . . . . . . .

4.3.1 Alkanes

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

4.3.2 Activated alkyl groups and polynuclear aromatics

. . . . . . . . . .

4.3.3 Olefins . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.3.4 Benzene . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.3.5 Alcohols

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

4.3.6 Hydroperoxides

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

4.3.1 Aldehydes

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

4.3.8 Ketones . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.3.9 Ethers

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

4.3.10 Carboxylic acids . . . . . . . . . . . . . . . . . . . . . . . . .

4.4 Oxidation of polyfunctional organic molecules

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

4.4.1 Unsaturated and benzylic alcohols . . . . . . . . . . . . . . . . .

4.4.2

Glycols

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

4.4.3 Unsaturated aldehydes . . . . . . . . . . . . . . . . . . . . . .

4.4.4

Unsaturated carboxylic acids

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

4.4.5 Hydroxy ketones

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

4.4.6

Hydroxy acids . . . . . . . . . . . . . . . . . . . . . . . . .

4.4.7 a-Mercaptocarboxylic acids . . . . . . . . . . . . . . . . . . . .

4.4.8 a-Keto acids . . . . . . . . . . . . . . . . . . . . . . . . . .

4.4.9 Oxalic acid

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

4.4.10 Malonic acid . . . . . . . . . . . . . . . . . . . . . . . . . .

4.4.1 1 Other dicarboxylic acids . . . . . . . . . . . . . . . . . . . . .

4.4.12

Phenols and hydroquinone

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

4.4.13 Aromatic ethers and amines . . . . . . . . . . . . . . . . . . . .

4.4.14 Thioureas . . . . . . . . . . . . . . . . . . . . . . . . . . .

345

346

347

348

349

349

352

352

353

353

354

355

355

355

355

356

356

356

356

358

360

362

362

363

364

365

369

370

371

371

372

373

373

374

375

376

378

378

380

383

384

387

387

388

390

391

391

392

394

395

396

399

402

402

404

406


15/632

XIV C O N T E N T S

5 . OXIDATION BY Fe(II1). Ag(1). CU(I 1) . NP(V) AND MO(V)

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

5.1 Oxidation of inorganic molecules

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

5.1.1 Halide ions

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

5.1.2 Pseudohalides; cyanide ion . . . . . . . . . . . . . . . . . .

5.1.3 Pseudohalides; thiocyanate ion

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

5.1.4 Pseudohalides; azide ion

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

5.1.5 Hydrogen peroxide . . . . . . . . . . . . . . . . . . . . .

5.1.6 Thiosulphate ion

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

5.1.7 Sulphurous acid . . . . . . . . . . . . . . . . . . . . . . .

5.1.8 Hypophosphorous acid . . . . . . . . . . . . . . . . . . .

5.1.9 Persulphate ion . . . . . . . . . . . . . . . . . . . . . . .

5.1.10 Phosphorothioic acid

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

5.1.1 1 Hydrazine . . . . . . . . . . . . . . . . . . . . . . . . .

5.1.12 Hydroxylamine

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

5.1.13 Carbon monoxide . . . . . . . . . . . . . . . . . . . . . .

5.1.14 Molecular hydrogen

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

5.1.15 Antimony(II1)

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

5.1.16 Borohydride ion

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

5.2 Oxidation of organic molecules . . . . . . . . . . . . . . . . . . .

5.2.1 Thiols

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

5.2.2 Carbonyl and nitro compounds

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

5.2.3 Formic acid . . . . . . . . . . . . . . . . . . . . . . . . .

5.2.4 Unsaturated alcohols . . . . . . . . . . . . . . . . . . . . .

5.2.5 Hydroxy-ketones (a-ketols. acyloins) . . . . . . . . . . . . . .

5.2.6 Ascorbic acid . . . . . . . . . . . . . . . . . . . . . . . .

5.2.7 Phenols . . . . . . . . . . . . . . . . . . . . . . . . . .

5.2.8 Amines

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

5.2.9 Phenylhydrazine and its sulphonic acids . . . . . . . . . . . .

5.2.10 Ortho-aminoazo compounds

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

5.2.11

Dichlorophenolindophenol

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

5.2.12 Ethylenediaminetetraacetic acid (EDTA)

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

5.2.13 Thiourea and thioacetamide . . . . . . . . . . . . . . . . . .

6. REDUCTION

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

6.1 Introduction

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

6.2 Simple electron acceptance by inorganic molecules . . . . . . . . . . .

6.2.1 Perchlorate ion

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

6.2.2 Chlorate and bromate ions . . . . . . . . . . . . . . . . .

6.2.3 Chlorite ion and chlorine dioxide

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

6.2.4 Molecular oxygen . . . . . . . . . . . . . . . . . . . . . .

6.2.5 Water

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

6.2.6 Sulphur dioxide

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

6.2.7 Xenon trioxide

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

Simple electron acceptance by organic molecules . . . . . . . . . . . .

6.3.1 Acetylenes . . . . . . . . . . . . . . . . . . . . . . . . .

6.3.2 Quinones

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

6.3.3 Nitro compounds . . . . . . . . . . . . . . . . . . . . . .

6.3.4 Carbonyl compounds . . . . . . . . . . . . . . . . . . . .

6.3.5 Unsaturated dicarboxylic acids

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

Electron acceptance followed by cleavage . . . . . . . . . . . . . . .

6.4.1 The Fenton reaction . . . . . . . . . . . . . . . . . . . . .

6.4.2 Hydroperoxides

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

6.4.3 Halogens. cyanogen iodide. hypohalous acids and hydrogen fluoride

6.4.4 Hydroxylamine. hydrazine. hydrazoic acid and azide ion

. . . . .

6.4.5 Nitrite

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

6.4.6 Nitrate

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

6.4.7 Peroxodisulphate ion (also called persulphate and peroxydisulphate)

6.3

6.4

. .

407

. . 408

. .

408

. .

410

. .

411

. .

412

. .

412

. .

414

.

. 415

. .

416

. . 416

. .

417

. . 417

. .

419

. . 419

.

.

420

. . 422

. .

422

. . 423

. .

423

.

. 425

.

.

428

. . 428

.

.

430

.

. 432

. .

433

.

.

435

. . 436

. .

436

.

.

437

.

. 437

. . 438

. .

439

. .

439

.

. 440

. .

440

. . 441

. .

442

. . 443

.

.

452

. . 452

.

.

452

.

.

453

. .

453

.

.

455

. .

456

. . 456

. . 457

. . 458

. . 458

.

. 464

.

. 466

. . 470

.

.

471

. .

473

.

.

475


16/632

C O N T E N T S xv

6.4.8 Peroxomonosulphate ion (Caros acid)

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

482

6.4.9 Organic halides . . . . . . . . . . . . . . . . . . . . . . . . . 482

6.4.10 p-Substituted alkyl halides

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

486

6.4.1

1

Carb on tetrachloride

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

487

6.4.12 Aromatic sulphonyl chlorides . . . . . . . . . . . . . . . . . . . 488

7.

R E D O X REACTIONS BETWEENR A D I C A L S AND METAL IONS . . . . . . . . . . . . . .

488

7.1 Stable radicals

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

489

7.2 Growing polymer radicals . . . . . . . . . . . . . . . . . . . . . . . 490

7.3 Transient simple radicals

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

491

ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

493

REFERENCES

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493

Chapter

5 (L

.

J. C S ~ N Y I )

Induced

reactions

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

510

I

.

INTRODUCTION

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 0

1.1

Definitions

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

510

1.2 Types of induced reactions

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

511

1.2.1 Coupled reactions

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

512

1.2.2 Induced chain reactions . . . . . . . . . . . . . . . . . . . . . 516

2.

EXAMPLES OF INDUCED REACTIONS

. . . . . . . . . . . . . . . . . . . . . . . 519

2.1 Chromium(1V) and chromium(V) species as coupling intermediates

. . . . .

519

2.1.1 Reaction between arsenic(II1) and chromium(V1) . . . . . . . . . . 521

2.1.2 Reaction between isopropyl alcohol and chromium(V1)

. . . . . . .

525

2.1.4 Oxidation of aldehydes and organic acids by chrornium(V1) . . . . . 529

2.1.6 Oxidation of vanadium(I1) and vanadium(1V) by chromium(V1)

. . . . 533

2.1.8 Properties of the chromium(V) and chromium(1V) intermediates . . . 536

2.1.3 Oxidation of other alcohols by chromic acid . . . . . . . . . . . . 528

2.1.5 Reaction between iron(1I) and chromium(V1)

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

532

2.1.7 Chromium(V1) as indicator in the induced oxidation of arsenic

111)

by

molecular oxygen . . . . . . . . . . . . . . . . . . . . . . . . 534

2.2 Induced reactions caused by arsenic (IV) intermediates . . . . . . . . . . . 538

2.2.1

Iron l1)-arsenic II1)-peroxydisulphate

system . . . . . . . . . . . . 538

2.2.2 Iron(1I)-hydrogen peroxide-arsenic(II1) system

. . . . . . . . . . .

542

2.2.3 Arsenic II1)-peroxydisulphate reaction catalyzed by iron(II1) and copper(I1) 543

2.2.4 Polarographic behaviour of the system containing peroxydisulphate,

arsenic(II1) and copper(I1) . . . . . . . . . . . . . . . . . . . . 547

2.2.5 The induced reduction of chlorate by arsenic(II1) . . . . . . . . . . 550

2.2.i Properties of arsenic(1V) intermediate

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

552

2.3 Induced reactions involving H 0 2 and

OH

radicals . . . . . . . . . . . . . 554

2.3.1 Induced reactions occurring in the

H 2 S Z 0 8 - H 2 0 2

ystem

. . . . . . .

554

2.3.2 Induced reactions involving other peroxy compounds . . . . . . . . . 563

2.3.3 The Fenton reaction

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

564

2.4 Induced reactions involving sulphate radicals

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

567

2.5 Induced reactions involving intermediates produced by partial oxidation

of

thiocyanate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569

2.6

Induced reactions effected by reduction

of

permanganate ions

. . . . . . . .

573

2.7 Induced reactions involving tin(II1) intermediate . . . . . . . . . . . . . 575

2.7.1 Reaction between iron(II1) and tin(I1)

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

576

2.7.2 Reaction between tin(I1) and chromate . . . . . . . . . . . . . . 576

2.7.3 Reaction between tin(I1) and perrnanganate . . . . . . . . . . . . 576

3.

CONCLUSIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

577

REFERENCES

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

577

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

581


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This Page Intentionally Left Blank


18/632

Chapter

I

Reactions

of

Inert Complexes and Metal

Organic Compounds

C. H. L A N G FO R D A N D M. P A R R I S

1. Introduction

This chapter is concerned with the simplest reactions of inert transition metal

complexes. Fig.

1

shows a typical compound . T his is

Co(II1)

coordinated to six

N H , molecules to form a triply positive cation

[CO NH,)~]~+.

t is indicated in

Fig.

1

to be in aqueous solution w here water molecules occupy positions in what

Outer sphere /

Fig.

1.

The C O ( N H ~ ) ~ ~ +on in aqueous solution. The inner sphere contains six ammonia

ligands strongly bonded to Co(II1). The outer sphere contains several water molecules.

may be called the outer o r second coordination sphere. The reactions in question

are typified by the replacement

of

one of the

NH,

molecules by a water molecule

from th e outer sphere. Inert will be considered t o mean an y complex whose

reactions occur slowly enough for conventional experimental kinetic techniques

tG

be

applied; in general this means half times longer than

10

sec. The inert

complexes have som etimes been called robu st bu t this term seems to be more

suggestive of therm odyn am ic stability th an kinetic non-lability

.

This chapter is restricted to treatment of the inert complexes because these

References

pp 52-59


19/632

2

I N E R T C O M P L E X ES A N D M E T AL O R G A N I C C O M P O U N D S

provide an extraordinary range of experimental results to examine. The richness

of results arises because a wide variety of closely related structures are synthetical-

ly available when ligand substitution does not occur too rapidly.

It

is also of

theoretical importance to consider reactions of inert complexes because the range

of closely related structures allows careful examination of small structure changes

and correspondingly detailed information about the mechanisms of reactions.

The basic ideas about the mechanistic pathways of ligand substitution that arise

from study of inert complexes also serve as an excellent starting point for analysis

of all ligand substitutions including fast ones. In fact, two cases to be given de-

tailed treatment, Co(II1)

and Pt(lI), have served admirably as paradigms for the

general study of ligand substitution. The systems to be considered here include the

octahedral complexes of Co(IIl), Rh(III), Cr(III), Ru(11I) and Ir(IJ1) which

contain principally amine ligands and square planar complexes of Pt(II), Pd(I1)

and Au(II1) which contain similar ligands. For lack of a more preLise characteriza-

tion of the situation we shall describe these as complexes of the harder ligands

in the sense of Pearsons distinction between hard and soft acids and bases2.

Consideration will also be given to some organometallic systems, principally the

metal carbonyls. These, by contrast with the other inert complexes, essentially

involve ligands which are soft bases in the Pearson classification. A cautious

reading of the evidence suggests separating consideration of these two types of

complex since their reactions occur under quite different circumstances

e.g.

type

of solvent), but there is actually no strong evidence that the major generalizations

about mechanism do not extend over both classes of system.

There are essentially two distinct types of experiment which have been utilised

by students of the mechanisms of ligand substitution. One type involves the

detailed analysis of the rate law governing the reaction (including stereochemistry)

and this type can yield (under favorable circumstances) insight into the number

of elementary steps of the reaction and the stoichiometric composition of the

transition state3. However, the experimental rate law for a single reaction cannot

reveal the energetic role played by the various groups in the complex. The value

of

the rate coefficient itself reveals the energy difference between two points along

the reaction coordinate. the initial state and the transition state4. The role that

any particular substituent (or component of the environment) plays in determining

that energy difference may only be assessed from a variation of the group (or factor

of

the environment). Thus, the energetics of activation must be inferred from a

second type of experiment in which rates of a series of reactions presumed to have

related mechanisms are compared. It is this question of the energetics of activation

which demands our consideration first.

In a ligand substitution reaction, two groups must always receive attention.

There is a bond to the leaving group to be broken and a bond to the entering group

to be formed. The relative importance of these two processes provides a basic

dichotomy for the classification of substitutions. If a reaction rate is sensitive to


20/632

2 S T O I C H I O M E T R I C

M E C H A N I S M S

3

the nature of the entering group, it is clear that the energy of the bond being

formed is important to the activation process; and its influence must be

in

the

nature of an entering group assistance to activation. If otherwise, then the entering

group could not influence the rate, and the minimum requirement for a sub-

stitution reaction is that the bond to the leaving group be broken.

The question of the sensitivity of the rate of substitution to the nature of the

entering ligand provides a basis for the mechanistic dichotomy. A reaction which

is clearly insensitive to the nature of the entering group must reach its transition

state principally by the internal accumulation of the energy to break the bond to

the leaving group within the ground state complex because any significant as-

sistance (in the sense of formation of a new bond) should be

selective

with respect

to the ligand assisting. Such a reaction, insensitive to the nature of the entering

ligand, will be said to have a

dissociative mode

of

activation. Reactions which are

sensitive to the nature of the entering group will be characterized as having an

associative mode of activation because the assistance of the entering ligand does

play an important part in the determination of the free energy of activation,

although not necessarily t o the exclusion of the leaving ligand. As the discussions

of particular cases will suggest, the dichotomy between

dissociative

and

associative

activation does encompass the secondary effects on the energy of activation by

other groups in the complex or by the factors of the environment of the complex

e.g. solvent effects).

A discussion of ligand exchange reactions of organometallic compounds as-

sociated with oxidation-reduction processes leading to free-radical formation

will be found in Volume 14 (Free-radical polymerization).

2. Stoichiometric mechanisms

So far nothing has been said about rate laws. This has been intentional, for there

is no simple relationship between the modes of activation and the concentration

dependencies that determine the rate law. The form of the rate law for a reaction,

that is, the dependence of rate on concentrations over the widest possible varia-

tions, tests hypotheses concerning the stoichiometric composition of the transition

state and the number of elementary steps of the reaction. These aspects may be

called the stoichiometric mechanism3. There

I S

enough variation in activation

mode among reactions of similar stoichiometric mechanism to recommend clas-

sifying reactions in two separate ways, first according to stoichiometric mechanism,

then according to activation mode. Moreover, these two distinguishing classifi-

cations follow the natural evolution in experiment. Stoichiometric mechanism is

inferred from analysis of rate laws; mode of activation is inferred from con-

sideration of relative rates.

Note that the three transition states,

a ,

b and c , in Fig. 3 all contain both the

Referen ces p p . 52-55


21/632

4

INERT COMPLEXES AN D METAL ORGANI C COMP OUND S

Reaction coordinate

Fig. 2. Free energy vs. reaction coordinate

for

the concerted

( I )

and two-step A , D ) reactions.

Y

a

b C

d

+ X

M

\Y

Fig.

3.

Possible transition states in the reaction

M X + Y M Y + X .


22/632

2 S T O I C H I O M E T R I C M E C H A N I S M S

5

entering and leaving ligands, Y and X in addition to the remaining metal complex

moiety. All three of these must be reached

uia

an initial encounter with the en-

tering groups. Since we consider here only slow reactions of non-labile com-

plexes we might write a simple mechanism in all three cases as

K

MX +Y

+

MX, Y

(fast equilibrium)

MX, Y MY

+ X

(slow)

(1)

Here MX, Y designates an

outer sphere

or

second sphere

complex. There is every

reason to suppose that formation and dissociation of M X, Y occurs at rates

ap-

proaching the diffusional-control limit so that the slow conversion to MY is a

negligible perturbation o n the equilibrium of the first step. There is

a

similarity

here the Langmuir, the Michaelis-Menten and the Lindemann-Hinshelwood

schemes.

Two common limiting forms of the rate law for mechanism (1) are encountered

experimentally. In the event that the equilibrium constant, K , for outer sphere

complexation is small in relation to the concentration of MX and Y, the rate law

becomes

d[MX1 -

k,,,[MX][Y]

dt

where k o b s ,

he second-order rate coefficient, is

k K . If K

is large, the outer sphere

equilibration may become saturated and the rate law reduce to

dCMX1 - k:,,[MX]

dt

(3)

where kAbs now equals k , the rate coefficient for the slow step. It has not always

beerl realised that these two forms may arise from the same mechanism. Choosing,

as is usual, Y to be in excess, a general expression under mechanism

(1)

may be

written

Recall that three transition states m:ght be considered as falling within the

pattern (1). Transition state a of Fig. 3 involves strong binding of both X and Y.

In this case, it is quite possible that an

intermediate

of increased coordination

number is formed during the reaction. Since the initial attack of Y determines

the stereochemical course of any reaction obeying the rate law 4) there is no

Referencespp, 52-55


23/632

6 I N E R T C O M P L E X ES A N D M E T AL O R G A N I C C O M P O U N D S

simple way to identify an intermediate of increased coordination number save

its actual accumulation to the extent that it can

be

detected. But, should such an

intermediate be detected, the pattern of reaction clearly differs in a qualitative

way from others following the scheme (1).

A

substitution involving an intermediate

of increased coordination number is a reaction with at least three elementary

steps:

( i )

encounter,

( i i )

addition of entering group,

( i i i )

loss of leaving group.

Recognising that such a path must involve an a transition state it may be usefully

labelled an

A ,

associative, stoichiometric pathway.

The other two transition states that are consistent with mechanism (1) are not

expected to involve an intermediate. They may accomplish an interchange between

the outer and inner coordination spheres in a single elementary step. Such processes

may be felicitously labelled interchange, I , reactions. Note that interchange

reactions may have either

a

or

d

transition states. That is, there may be

I ,

or

I ,

reactions. It is interesting to note precisely how these cases are distinguished. If

a wide enough concentration range can be studied so that the rate coefficient,

k , for the slow step can be found, it is a straightforward matter. However, in-

terpretation of information from the range covered by equation

(2)

must normally

be attempted. It must then be determined whether variations in the observed

second-order rate coefficients as

Y

is varied reflect only variations in the outer

sphere association constant

K ,

or

if

there are significant variations ink . If variations

in kobsare attributable to

K

only, k is constant and the reaction employs the d

activation mode. Definite information concerning values

of

K

is unfortunately

scarce.

Simple examples of A ,

I

and I reactions are seen to have the same rate law.

There remains another important mechanism that differs. This is the pathway

through an intermediate of reduced coordination number that is possible if a

transition state like d of Fig. 3 occurs. This path may be called dissociative,

D .

The mechanism may be represented as

k-,

kx

MX + M+X (slow)

M + Y

5

M Y

(fast)

Mechanism

(5)

gives rise to the rate law

as long as the rate of capture of the intermediate

M

by Y is large compared to

its rate

of

recapture by

X.

If recapture by X becomes important compared to

reaction with

Y

the expression becomes


24/632

2

S T O I C H IO M E T R I C M E C H A N I S M S 7

which is not easily distinguished experimentally from expression 4) for mech-

anism

1 ) .

Table

1

summarizes the mechanistic categories described and Fig.

2

indicates their relationships on an energy-reaction progress diagram.

TABLE 1

C L A S S I F I C A T I O N

O F

L I G A N D S U B S T I T U T I O N M E CH A N IS M S

Mode of activation Stoichiornetric niechanisrn

Interriiediaie of increased One-step process Interniediute

of

reduced

roordinotion number coordination number

Associative activation A

1,

Dissociative activation

Id

D

The categorization just described was proposed recently3.

In most of the

literature, substitution reactions have been characterized according to the scheme

introduced by Hughes and Ingold

for

organic reactions. This scheme has been

critically refined by Basolo and Pearson. Following Basolo and Pearson, the fol-

lowing rough equivalence may be listed

s N 2 (lim)

= A

s N 2 = 1,

s N 1

=

1,

s N 1 (Iim)

=

D

SN

denotes substitution, nucleophilic and

1 or

2 the molecularity of the process.

The designation s N 1 seems an objectionable usage since it describes as uni-

molecular a process which requires the entering group as a stoichiometric

component of the transition state and suggests a too sharp distinction from the

s N 2 or

I ,

process. This could be deemed reasonable if molecularity were defined

as the number of ligands changing covalence but such a definition is probably

no longer an operational one, and it seems unfortunate to diminish the clear

operational significance which molecularity has for gas-phase reactions.

3.

Reactions

of Co(II1)

complexes

The most extensively studied family of non-labile complexes is the cobalt(II1)

ammine series. These are octahedral systems and all those to be considered are

low spin d 6 systems. The subtle variations that can be achieved synthetically make

References pp.

52-55


25/632

8

I N E R T C OM P LE XE S A N D M E T AL O R G A N I C C O M P O U N D S

them especially attractive for the systematic kineticist and the reactions of Co(II1)

systems have served as the model for development of most of the key concepts

about substitution reactions of octahedral systems.

A

good starting place for discussion

is

the rate law for substitution reactions

in acidic aqueous solutions. One typical reaction is

CO(NH,)~CI'+

H 2 0

C O ( N H , ) ,O H , ~ + C l -

8 )

The rate law for this solvolytic process is

which is of course expected in this case from almost any mechanistic picture. When

the solvent is a potential ligand, the perpetual encounter between solvent molecules

and complex severely limits the cases in which the rate equation is directly

in -

formative. However, one limit of mechanism is available. The general reaction

(where Y is an arbitrary anionic ligand)

and related ones, in which

CI-

is replaced by another leaving group, or the

NH ,

ligands are replaced by others inert to substitution, all follow a rate law similar

to equation (9)'-'. Significantly, the Concentration of

Y -

does not appear

i n

the

rate law in any case except when Y - = O H - . Overlooking the exception of O H -

for the moment, Y - is not a stoichiometric participant of transition state(s)

leading to its entry and there must be an intermediate in the reactions. There are

two choices of pathway, viz .

I / slow 1

co x

I

-

/?o

+ x

I /

I

yco + Y faSt > ~ o - Y

I /

slow

I /

-Co- X + HO

-

CO-

O H 2 + X

'I / I

-Co-0H2 +

Y

fa5 _ 7 o-Y+H20

/ I

I / I,

The first possibility, 1 l), clearly is concerned with a dissociative mode of activa-

tion ( d ) . The second, (12), might beassociative led by water attack. But, this in-

terpretation of (12) involves commitment to the proposition that no nucleophile

(possibly excepting OH-) has been discovered which is better than water, i.e. the

associative attack must always involve water in the first instance. The proposition


26/632

3

R E A C T I O N S

O F

CO(II1) C O M P L E X E S 9

is unattractive and there is an appealing alternative. Pathway

(12)

might also

represent a case of dissociative activation without a stable intermediate. Since

water will be the predominant component of the second coordination sphere a

dissociative interchange reaction

( I d )

would lead to the aquo complex as the im-

mediate product. The main point at the moment is that either of the attractive

interpretations of the absence of [Y ] in rate laws leads to a mod el involving the

dissociative mode of activation.

It

is interesting that pathway (12) has been clearly

demonstrated to be common in Co(II1) chemistry6, . In the

Co(lI1)

case, as

i n

all others in which reaction with solvent predominates, it is fruitful to adopt the

tentative view that activation is dissociative and seek support for this from other

criteria.

It is pertinent, then, to seek a dependence of substitution rates on i ) leaving

group,

( i i )

solvent,

iii)

steric crowding,

iu)

charge,

( v )

nature of non-labile

substituents including stereochemistry, consistent with this picture of the activa-

tion mode. If these tests generally support

d

modes

i t

will be desirable to examine

rate laws closely to attempt a distinction between

D

and

I ,

stoichiometric path-

ways.

In a dissociative process the reaction rate is expected t o decrease a s the strength

of the metal to leaving ligand bond increases. This trend is generally observed in

Co(1II) ammine complexes. As can be seen in Table 2,

a

partial leaving group

order is

This is t o be compa red to Y atsimirskiis bond energy order estimated for the

gas phase

Perhaps the closest definition of the role of the leaving group emerges from cor-

relation of rates with equil ibria in reactions of the family C O (N H ~ )~ X . n a

dissociative mode the leaving group in the transition state strongly resembles

the leaving group in the product state. If X is an anionic ligand, in the transition

state it should resemble th e free an ion . Th e activation free energy should respond

to changes in leaving group in much the same way as the free energy difference

for the overall reaction responds.

A

linear free energy relationship (see Vol.

2,

Chapter 4) is suggested between the activation energy AG an d the free energy of

reaction

AGO

of the form A(AG) =

j?A(AGo)),

where

A(AG)

denotes change in the

free energy qu an tity with change of

X .

In the ideal dissociative case, j? would be

unity. This has been realised fo r the C o( N H ,),X 2+ family as shown in Fig. 4.

A

persuasive reason for preferring the dissociative over the associative inter-

pretation of equation (12) has emerged from recent work on reactions of Co(II1)

Ref erencespp . 52-55


27/632

10 I N E R T C O M P L E X E S A N D M E TA L O R G A N I C C O M P O U N D S

T A B L E

2

R A T E S

O F

A C I D H Y D R O L Y S I S OF S O M E

C o l l 1 )

C O M P L E X E S

A T

2 5 0

C

The

labile ligand

is

italicized.

Complex*

k(sec- )

AHX(kca1)

ASt(eu) ReJ

Co NH3)5

op(ocH3)1

Co NH3)s

NO3

Co NH3)5

1

Co

(N

Ha)

Br

C O N H ~ ) ~H 2 +

Co NHj)5 CI2+

C O N H ~ ) ~O4+

CO N H3)

5 O P 0 3H 2

Co NH3)s N O z Z

Co NH3)5 N C S 2

Co NH3)S O H Z +

CO N H3)63

trans Coen 20H CI+

cis

CoenzOH Cl+

t r a n s Coen,BrCI+

cis

Coen,BrCI+

trans Coen,CI2+

cis

CoenzCIz

trans

CoenzN3CI+

cis

CoenzN3CI+

t r a n s

CoenzNCSCI+

cis CoenzNCSCl+

trans Coen,NH3CIZC

cis Coen,NH3C12+

trans Coenz0HZClZ+

cis Coen20H2CI2+

trans Coen,CNCI+

trans CoenZNOzCI+

cis Coen,NO,Cl+

t r a n s Coen2NOzEr+

t r a n s

Coen,

Br 2

t r a n s C O N H ~ ) ~ C I ~

c i s

CO NH,),CI,

2.5 x 10-4

2.7 x

1 0 - 5

8.3 x I O ~

6.3 X ~ O - ~

5.8

x I O ~

1 . 7 x 1 0 - 6

1 .2 x10-6

2.6 X I O - 7

1.15

x

5.0

x I O - O

very

slow

- l O - * O

1.2 x10-2

1.6 X I O 3

4.5

X I O - 5

1 .4 X I O 4

3.5 X I O - 5

2.4

x

10-4

2.2

x

10-4

2.0 ~ 1 0 - 4

1.1

X I O - 5

3.4

X I O - 7

5 X I O - 7

2.5

x I O - ~

1.6

x I O ~

8.2

X I O - 5

9.8 x

10-4

1 . 1

X I O 4

4.0

~ 1 0 - 3

1.8 X I O - 3

5 x 1 0 - 8

1.2 x10 6

fast

-

25.5

23.5

27

23

19

-

-

-

31

-

-

26.2

23.1

24.9

23.5

26.2

21.5

22.5

21.3

30.2

20. I

23.2

24.5

-

-

22.5

20.9

21.8

-

-

-

-

-

+ 6

- 4

+ 6

-9

- 4

-

-

-

0

-

-

+ 0

+

10

+ 2

+

14

+ I 4

- 5

0

- 4

+9

- 4

- 1 1

- 6

-

-

- 2

- 2

- 3

-

-

-

-

~~-

13

14

14

14

15

14

14

13

16

17

14

18

18, 19

18, 19

18,

19

18, 19

18, 19

18, 19

18, 19

18, 19

18, 19

18, 19

18, 19

18, 19

20

20

21, 19

18,

19

18, 19

18, 19

18

23

23

*

en represents ethylene diarnine.

complexes in solvents other than water. Work of Tobe, Watts, Langford and their

respective collaborators has demonstrated that these solvents, dimethyl form-

amide, dimethyl sulphoxide, dimethyl acetamide and methanol also function as

preferred nucleophiles. This reinforces the suggestion that it is the high solvent

concentration and not solvent nucleophilicity that is important. Furthermore, it

is found that in these solvents some direct replacement by an anion may be

observed but that such replacement is always associated with ion pair formation

and that reaction rates show very little sensitivity to the nature of the entering

ion12.


28/632

3

-3

-

- -4 -

I

C

.-

E

X

-5-

R E A C T I O N S O F cO(III) C O M P L E X E S

1 1

LOP

H eq

Fig.

4.

Linear free-energy relationship for the reaction, C O ( N H ~ ) ~ O H ~ ~ +

X -

* C O ( N H ~ ) ~ X * +

+ Hz O.

Log k

rate coefficient)

us. log K

equilibrium constant).

The interpretation of the next factor, steric crowding,

is

quite straightforward

i f its effects can be isolated. Steric crowding should inhibit an associative reaction

but accelerate a

dissociative

one. It is frequently difficult to isolate the steric effects

for a reaction in solution since the structure variations that result

in

crowding

of the reaction site may also modify the surrounding solvent structure i n an un-

controllable way

or

be associated with important electronic effects. There is at

least one clear cut experiment concerned with the steric effect

o n

a Co(II1) complex

that supports the dissociative mode. This is the comparison of the acid hydrolysis

rates for

d , 1

and

meso

Cobn,Cl,+ (bn

=

2,3-diaminobutane). It can be seen

that the methyl groups on the chelate rings must be opposed in the meso form and

staggered in the d , form. The

meso

form hydrolyzes about thirty times faster24.

The other available data are consistent with this suggestion of acceleration by

steric crowding and its implication of dissociation.

The effect of overall charge on the complex is perhaps even more difficult to

isolate than the steric effects. Each change of charge type is accompanied by an

important change in the electronic arrangement about the metal which the discus-

sion (below, p. 12) of electronic effects of non-labile ligands shows to be quite

important. However, an assessment of the rates for the 2' and 3 + charged species

cited in Table 2 suggests that substitution rates at Co(II1) decrease as the overall

charge on the complex is increased. This conforms to expectation for the dis-

References

pp.

52-55


29/632

12

I N E R T C OM P LE XE S A N D M E TA L O R G A N I C C O M P O U N D S

sociative model when the leaving group is anionic. For the associative model, one

might expect opposite or at least very small charge effects (see the discussion of

In this catalogue of structure variation experiments

to

test the hypothesis of

a dissociative activation mode, the last is the role of non-labile ligands. This

question has been examined using the hydrolysis reactions of the family of com-

plexes

cis

and

trans

Coen,ACI+ where CI- is the leaving group and A is a variable

non-labile substituent (see rates in Table 2). The first approach taken to the

analysis of the data was to classify the ligands A as electron donors or electron

acceptors on the basis

of

organic chemical precedent and then to plot the observed

25 rate coefficients as a function of decreasing electron donor-increasing electron

acceptor proper tie^^^. The two branches of the curve (Fig.

5)

were given a two-

Pt(II), p. 20).

-1

-

-

-2-

r n

P)

a- 3 -

m

J

- 4 -

- 5 -

-6-

- t

NHZ OH- N

CI -

NCS- NHJ H,O CN- NO,

Fig. 5. Rates

of

hydrolysis

of

a series

of

Coen,ACI complexes. The abscissa represents the

electron-donating or -accepting power

of

A .

mechanism interpretation. Good electron donors were supposed to replenish the

depleted electron density at Co(II1) in a dissociative transition state. Electron

acceptors (NOz-, CN-) were supposed to drain away the excess electron density

at Co(1II) in an associative transition state. A difficulty for this attractive hypo-

thesis is that there

is

no correspondingly simple pattern in the values of AH .

The extended effort to obtain some evidence for nucleophilic discrimination

in the reactions supposed to involve an associative transition state have been


30/632

3 R E A C T I O N S O F CO(II1) C O M P L E X E S

13

reviewed. 26. No direct support has emerged for the postulation

of

an associative

transition state. Fortunately, there is an alternative account of the situation. The

dissociative transition state is an incipient five coordinate complex. In the extreme

case there are two possible geometries, trigonal bipyramidal and square pyramidal

and different ligands may stabilize geometrically different transition states. Note

(Fig. 6 ) that the square pyramidal form cannot lead to cis-trans isomerization

a-M

-

r a n s

c i s

t r a n s

=2.1

c i s

A

Fig. 6 . Stereochemical changes accompanying the dissociative reaction MA4BX

MA4B

MA4BY.

whereas the trigonal bipyramidal form can. A fairly satisfactory correlation

between stereochemical rearrangement emerges following the suggestion that

the formation of a strongly trigonal form is accompanied by a positive

AS.

The

two-geometry uniformly dissociative model seems to give the most consistent

account of the effects of non-labile ligands.

The hypothesis of dissociative activation in Co(II1) reactions stands the avail-

able tests well. It is therefore profitable to attempt to distinguish the D from the

I d

pathways. Fig. 7 summarizes the two pathways consistent with d activation,

and the general methods for establishing the stoichiometric mechanism

I d

are

illustrated by the example of CO ( N H ~ ) ~ O H , ~ + .

First, a case against the

D

path may be constructed. A knowledge of the rate

References

pp. 52-55


31/632

14

I N E R T C O M P L E X E S A N D M E T A L O R G A N I C C O M P O U N D S

-M

Fig.

7.

The

I d us.

the

D

mechanism.

ofwater exchange of CO(NH,) ,OH,~+and the rate of hydrolysis of Co(NH3),X2+

under concentration conditions where the reaction goes to completion gives

k - - H 2 0 nd k - x . These may be combined with the overall equilibrium constant

for the reaction to give the ratio k + H 2 0 / k + X ,he competition ratio for the inter-

mediate CO(NH,),~+.A number of these competition ratios were calculated by

Haim and TaubeZ7 ncluding the case X = SCN-. The assumed mechanism may

be tested by generating the intermediate from a different source and checking

the competition ratio by evaluating the immediate product distribution. This was

done by Pearson and Moore6 who generated the intermediate by hydrolysis of

the labile nitrato and bromo pentaamines of

Co(II1)

in the presence of a large

concentration of SCN- ion. In conflict with the explicit predictions from the

D

mechanism27, they found

no

evidence for capture of the intermediate by

thiocyanate. Unless very small concentrations of

Br-

or NO3- in solution affect

the reactivity of the intermediate, it is necessary to conclude that it is not there.

Now we can proceed to assemble the positive evidence for the I d path

(I -+ I

-+

IV, Fig.

7).

Once the outer sphere complex, (11), is formed, all replace-

ments of water should occur at the same rate, k -H 20 . f the ion pairing constant

K, is known,

or

a limiting rate of anion entry corresponding to saturation of the

association is observable, the rates of conversion of

11)

into (IV) may be compared

for various X . All should be equal to k - H z Of the activation mode is d , but they

will not equal the rate of water exchange which wasidentified with k-H20on the

D path. The reason is that species (11) has a number of solvent molecules in its


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3

R E A C T I O N S O F CO(lI1) C O M P L E X E S 15

outer coordination sphere as well as the ligand X . Even if the rate of dissociation

of water is unaffected by the presence

of X

in the neighborhood, the most probable

result of water

loss

will be water exchange and not X entry. Thus on the path we

expect all ion pairs tc show closely similar rates

of

conversion to aniono com-

plexes, but we expect these rates to lie below the solvent exchange rate by the ap-

propriate statistical factor for the population of the outer sphere. Recently

9 ,

l 3

the rates of formation of Co(NH,),X from Co(NH,),OH,

. . .X

have been

reported relative to the water exchange rate for X = SO4, C1-, SCN-, and

H2 P0 4- . The values are

0.24, 0.21,

0.16 and

0.13,

respectively. The values span

a range of a factor

of

two which must be admitted to be a little larger than the

experimental uncertainty and also easily within the differences among the anions

in their probability of occupancy of the crucial outer sphere site adjacent to the

leaving water molecule. All are nearly a factor of five below the water exchange

rate. These results conform neatly to the I d predictions.

Examining the relationship between the probability that a ligand occupies an

outer sphere site, the rate of ligand incorporation, and the solvent exchange rate,

appears to be the most general method for identifying the I pathway. It has

been applied to several other systems recently. Rates of anion entry into cis-

[Coen2NO2(DMS0)I2+ n dimethyl sulphoxide (DMSO) have been compared

to the DMSO exchange rate obtained from the deuterium tracer

NMR

experiments

of Lantzke and Watts29. The pattern is very akin to that for CO(NH,),OH,~

in water. Similarly the rate

of

sulphate anation in the pair ~is-[Coen,(OH~)~l~+

.

. .

SO4- has been found to be 0.25 times the water exchange rate of the free

Several authors have suggested that the Id pathway may prove to be the most

common mechanism in substitution reactions of octahedral complexes generally.

However, the

D

path can be clearly demonstrated in some cases including at least

two examples from Co(II1) chemistry. The path (I

-

111 - IV, Fig.

7)

through the

fivecoordinate intermediate would lead, in the case

of

rate studies in the presence of

excess anionic ligand, to observed first-order rate constants governed by equation

ion

9 a .

(13)

This form

of

[ X - ] dependence was observed by Haim et ~ 1 . ~ ~ 9 ~ ~n studies of

the anation of Co(CN),OH,-. Equation

(13)

predicts a limiting rate at high

[X- equal to the solvent exchange rate, and allows substantial variation in reac-

tivity of X- roups. These features are realised i n the Co(CN),OH,- system.

The reactivity order toward the intermediate Co(CN),- is: OH- >

1,- >

NH,

>

SCN-

>

thiourea

>

NH,

>

Br-

>

S 2 0 3 -

>

NCO-

>

H20, spanning

about four orders of magnitude. A parallel case3 has evolved

for

the intermediate

References

p p .

52-55


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16 I N E R T C O M P L E X E S A N D M E T AL O R G A N I C C O M P O U N D S

Co(NH,),SO,+. The order

of

reactivity toward this five coordinate species is:

O H - > NO,- > CN- > NH, > H,O and spans six orders of magnitude.

These two five coordinated Co(II1) species are, to date, the only ones clearly

established from detailed knowledge of the rate law for substitution (a situation

very clearly subject to change). Strongly suggestive evidence for others has been

accumulated from another approach. Loeliger and Taube3, and Sargeson et al.,,

have examined reactions of a complex where there is a distribution of products

(i .e . 6O and l 8 O aquo complexes

or

stereoisomerically different complexes). They

argue that a constant product ratio strictly independent of the nature of the

leaving group implies product formation

ufler

the leaving group is removed from

the scene of reaction and the existence of an intermediate. The key word here

is

strictly. Leaving group effects may often be quite subtle. In

I ,

processes,

for

example, they would appear only to the extent that they modified outer sphere

populations. Some persuasive indication of

D

reaction has been presented for

some of the so-called induced aquations of Co(II1) amine complexes. Induced

aquations include Hg2+-catalyzed halide loss and rapid azide loss catalyzed by

nitrous acid.

Before leaving Co(II1) chemistry we must consider the base hydrolysis reaction

Co(NH,),X+OH-

--+

Co(NH,),OH+X-

14)

The usual rate law is typified by

-d CCo(N 3)5x

= k [

Co( N H,) X] [OH-]

dt

It is important to appreciate that the values of

k

in equation (15) are often quite

large when compared to the rates summarized in Table 2. Some of these values

appear in Table

3.

The simplest interpretation of equation (15) would assume a nucleophilic at-

tack on Co(II1) by OH-. This, however, would put OH- in an extraordinary

category of nucleophilicity. G a r r i ~ k , ~as the first to note that an alternative

explanation for the role of OH- was available. In the alternative, the conjugate

base of the initial complex ammine is presumed to be formed in small amount

and to function as the actual reactive species

Co(NH3),CI2 +OH- o(NH,),(NH,)CI+ +H,O(fastequilibrium)

Co(NH,),(NH,)Cl+ Co(NH3),(NH2)

fC1-

(slow)

(16)

C O ( N H ~ ) ~ ( N H , ) ~ +H,O Co(NH,),OHZf (fast)

The mechanism allows for a slow step analogous to the d process observed in


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4 Cr(III), Rh(III), Ru(III), Ir(II1) A N D Pt(IV) C O M P L E X E S 17

TABLE 3

R A T E S O F B A S E H Y D R O L Y S I S O F S O M E

cO III )

C O M P L E X E S

The labile ligand is italicized.

Cottiplex*

k o H ( l .

mole- '.s ee -' ) T( C) E,,(kcal. mole-

')

AS(eu) Ref.

Co NH3)512+

Co N H

3

5Br

Co N H

Cl

Co NH3) N 3 2

C O N H ~ ) ~ N O ~ * +

trans Coen2CI2+

trans

Coen,OHCI+

trans Coen2NO2CI+

cis Coen2C12+

cis Coen20HCl+

cis Coen2N02Cl+

trans C o ( d ,I-bn)2C12+

trans

Co Meso-bn), C12

23

7.5

0 . 8 5

4.2

x

0.017

0.080

0.37

0.03

2100

9800

3

x 10-4

85

15.1

25

25

25

25

25

0

0

0

0

0

0

25

25

29

28

29

3 3

38

23.2

22.8

24.4

24.6

22.4

23.

-

-

+42

+40

+36

+ 3 5

+30

-

35

35

35

35

35

36

36

36

36

36

36

3 1

37

*

bn represents 2,3-diaminobutane.

acid hydrolysis, and the extensive evidence of a parallel between acid and base

hydrolytic reactivity has been reviewed3'. It has also been established by mea-

surement of H-D exchange on the ammine li g a n d ~~ ' . ~ 'hat the conjugate base

can be formed sufficiently rapidly. The most telling experiments, though, are those

that establish that product formation occurs in a step independent of the initial

hydroxide involvement. Green and T a ~ b e ~ ~ave shown that the

160 /180

sotope

fractionation factors in base hydrolysis are more easily explained assuming in-

corporation of

0

from H 2 0 than from O H - , and Sargeson et

u I . ~ ~

ave shown

that other anions ( Y - ) can effectively compete with O H - in the base catalyzed

pathway to give products

C O ( N H , ) ~ Y ~ +

n addition to

Co ( NH 3 ) , O H 2 + .

Thus,

there seems little doubt that direct O H - attack on Co(I1I) is excluded.

The reason for high reactivity on the base-catalyzed pathway remains something

of a puzzle. The simplest, but not entirely convincing interpretation, suggests

that the N H 2 - ligand functions as an electron donor similar to O H - .

has suggested, on spectroscopic and stereochemical grounds, that the conjugate

base species may be labile because it is a high spin d 6 complex. This view is rendered

more attractive by Watt and Knifton's recent report45 of a paramagnetic solid

Co(II1) conjugate base species isolated from liquid

N H , .

Gillard46 has made the

interesting suggestion that O H - may not function as a base but as an electron

donor, to produce a transient OH radical and a labile Co(I1) species.

4.

Cr(III), Rh(III), Ru (III), Ir(II1) and Pt(1V) complexes

The related octahedral non-labile complexes which have received some at-

tention will be grouped together simply because there

is

much less information

References p p . 52-55


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18

INERT COMPLEXES AN D METAL ORG ANIC COM POUND S

available than there is with respect to Co(II1) complexes. Reactions of Pt(1V)

complexes are very slow and attempts to isolate simple thermal substitutions from

Pt(I1)-catalyzed redox pathways and photochemical reactions have not yet been

v e r y s u c ~ e s s f u 1 ~ ~ - ~ ~ .nformation on

Ir(II1)

complexes is very limited

to

date

and probably not adequate for mechanistic analysis'0*

A

reasonable account

of Cr(III), Rh(II1) and Ru(1II) behavi

Comprehensive chemical kinetics Bamford

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