Inorganic reaction mechanisms vol 4

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A Specialist Periodical Report

Inorgan ic Reactio n
Mechanisms
Volume 4
A Review of the Literature Published between
July 1973 and December 1974

Senior Reporter
A. McAuley, Department of Chemistry, University of Victoria,
Victoria, British Columbia, Canada
Rep0 rters
J. Burgess, University of Leicesfer
J. S. Coe, King’s College, London
D. N. Hague, Universify of Kent
R. D. W. Kemmitt, University of Leicesfer
P. Moore, Universify of Warwick
K. L. Scott, Ross foods, Grimsby
M. A. R. Smith, Bruker-Physik AG, Karlsruhe, West Germany
G. Stedman, Universify College, Swansea

0Copyright 1976

The Chemical Society
Burlington House, London WIV

OBN


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ISBN: 0 85186 285 3
ISS N :0305-8255
Library of Congress Catalog No. 73-642977

Printed in Great Britain by
Adlard & Son Ltd.
Bartholomew Press, Dorking


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Foreword

The format, coverage and approach to this fourth volume dealing with Inorganic
Reaction Mechanisms are similar to those for the previous volumes of the series.
Rather than attempting to refer to all articles in which there is mention of mechanism, we have chosen (with an eye on volume size and cost!) to concentrate on papers
in which kinetics and mechanisms in solution form the principal interest. This policy
inevitably leads to mention of some papers being cursory with a consequent loss of
subtlety. We hope, however, that any resultant oversimplificationor misrepresentation is minimal. Review articles cited are those which should be accessible to the
great majority of readers.
The period of coverage of the literature is from July 1973 to December 1974. The
boundaries at both ends are somewhat vague since not all libraries have issues available at the same time. Care has been taken to ensure that an overlap with Volume 3 is
maintained in all areas. Material which is derived from Chemical Abstracts rather
than from the original source is indicated by inclusion of a Chern. Abs. citation in the

reference quoted. References to the Russian literature derived from this source
quote the page number of the original article whereas those giving the title of the
English translation quote the page number of the translation.
As in previous volumes, kinetic data are reported in the form (Ea and log A or
A H * and AS*)or units (SI or other) used in the original article, although some
conversions have been made both in the text and in the tables where comparisons are
required.
The Reporters are grateful to many of their colleagues for helpful comments. We
are particularly indebted to Professor W. C. E. Higginson and Drs A. G. Sykes,
G. Davies, R. J. Cross, and A. G. Lappin for their critical reading of sections of the
manuscript.
A. MCAULEY


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Contents
~~

Part I Electron Transfer Processes
Introd uction
Chapter 1 Reactions Between Two Metal Complexes
By K. L. Scoff

3
5

1 Reducing Agents
Chromium(n)
Vanadium@)
Iron@)
LanthanidHn) Ions

5
5
11
13
17

2 Oxidizing Agents
Cerium(1v)

23
23
23

Iron(ru)

3 Miscellaneous Redox Reactions

26

4 Electron Exchange Reactions

32

Chapter 2 Metal Ion-Ligand Redox Reactions

By A. McAuley
1 Chromium(v1)

36

36
41

3 Mangan@u~) and Manganesdvu)

45

4 CoWt(m)

48

5 Vanadium(v)

53

6 Thallium(m)

54

7 Cerium(1v)

55


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vi

Contents

9 Miscellaneous Redox Reactions
10 Halogens and Halogenate Ions

65

11 Metal Ion Reductions

71

12 Pulse Radiolysis Studies

75

Chapter 3 Reactions of Water and Hydrogen Peroxide
By A. McAuley

Part / I

61

80

Substitution and Related Reactions

Chapter 1 Non-metallic Element

By G. Stedman
1 Group I11

Boron
Boron-Hydrogen Compounds
Boron-Nitrogen Compounds
Boron-Oxygen Compounds
Aluminium
Gallium
Indium

93

93
93
93
94
95
95
96
96

2 GroupN
Carbon
Silicon
Germanium
Tin
Lead

96

96
97
99
100
102

3 GroupV
Nitrogen
Phosphorus
Oxoanions
Compounds with Nitrogen
Phosphazenes
Intramolecular Processes
Arsenic
Antimony
Bismuth

102
102
104
104
106
107
107
108
108
109


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vii

Contents
4 GroupVI
Oxygen
Sulphur
Selenium
Tellurium

109
109
109
113
113

5 GroupVII
Fluorine
Chlorine
Bromine

113
113
113
114

6 GroupVIII
Xenon

114
114

Chapter 2 Inert Metal Complexes: Co-ordination Numbers
Four and Five
By J. S. Coe
1 Square-planar Complexes

Platinum(I1)
General
Aquation and Hydrolysis
N-Donor Ligands as Entering Groups
S-Donor Ligands as Entering Groups
Steric Effects
Bidentate Entering Groups
Displacement of Bidentate Groups
Ring Closure
Isomerization
cis and trans Effects
Bridged Complexes
Miscellaneous
Palladium(Ii)
Unidentate Entering Groups
Bidentate Entering Groups
Isomerization
Miscellaneous
Nickel@)
Gold(Ir1)
Rhodium@
Copper(I1)

115

115
115
115
115
116
116
117
117
118
118
119
120
122
122
122
122
124
124
125
125
126
126
127

2 Tetrahedral Complexes

127

3 Five-co-ordinate Complexes

128


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Contents

viii

Chapter 3 Inert Metal Complexes: Co-ordination Numbers
Six and Higher
By P. Moore

129

1 Introduction

129

2 Aquation: Cobalt(n1) Complexes
Unidentate Leaving Groups
Multidentate Leaving Groups
Effects of Non-leaving Ligands
[Co(en),LX]Z+Complexes
[CoL.,XJ+ Complexes
Dioximato-complexes
Bridged Dicobalt Complexes
Solvent Variation
Catalysed Aquation
Photochemistry

133
133
136
137
139
140
146
148
151
152
158

3 Aquation: Chromium(m) Complexes
Unidentate Leaving Groups
Ammonia Loss
[Cr(H,O) 5L]n+Complexes
Other Complexes
Multidentate Leaving Groups
Effects of Non-leaving Ligands
Bridged Dichromium Complexes
Non-aqueous Soivents
Catalysis
Photochemistry

161
161
161
1 62
163

1 66
167
167
168
168
170

4 Aquation: Other Complexes
d l : Molybdenum(v)
d : Molybdenum(1v)
d : Moly bdenum(nr)
d 3: Rhenium(Iv)
d4: Ruthenium(1v)
d4: Osmium(iv)
d 6: Iron@)
d &: Ruthenium(m)
d ti : Rhodium(m)
d 6 : Iridium(1)
d ti : Iron@)
d I:Ruthenium(u)
d Platinum(rv)

171
171
172
173
173
173
173
174

1 74
175
177
178
180
181


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Contents

ix
5 Base Hydrolysis
Cobalt(@ Complexes
Chromium(m) Complexes

181
181
186

6 Formation
do: Titanium@)
d : Molybdenum(v)
d3: Chromium(m)
d3: Molybdenum(n1)
d4: Rhenium(@
d : Manganese(n)
d : Iron(n1)
d : Ruthenium(1n)
d o: Iron@)

d : Ruthenium(I1)
d 6 : Cobalt@)
d : Rhodium(m)
d : Iridium(m)
d : Platinum(Iv)
Miscellaneous Metal Ions

187
187
187
187
189
190
190
190
190
191
192
193
196
197
198
198

7 Ligand Exchange and Replacement
Solvent Exchange
Ligand Exchange and Replacement: Unidentate by Unidentate
Ligand Replacement: Multidentate by Unidentate
Ligand Replacement: Unidentate by Multidentate
Ligand Replacement: Multidentate by Multidentate

198
198
199
201
201
201

8 Metal Exchange and Displacement

202

9 Isomerization and Racemization
General
Cobalt@) Complexes
Chromium(n1) Complexes
Intramolecular Rearrangements of Tris-chelate Complexes
Miscellaneous

203
203
203
205
205
206

10 Co-ordination Numbers Greater than Six

Chapter 4 Labile Metal Complexes
By D. N. Hague

1 Complex Formation involving Unsubstituted Metal Ions:
Unidentate Ligands and Solvent Exchange
Main-group Ions
Nickel(@
Transition Metals M2+(except Ni2+)
Metals of Valency Three and Higher

208
209

209
209
210
21 1
21 1


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Contents

X

2 Complex Formation involving Unsubstituted Metal Ions:
Multidentate Ligands
Main-group Ions
Nickel(@
Transition Metals M2+ (except Ni2+)
Metals of Valency Three and Higher
3 The Effect of Bound Ligands

Reactions in Water
Reactions in Non-aqueous Solvents

Chapter 5 Solvent Effects
By J. Burgess

214
214
215
218
226
228
228
233

236

1 Pure Solvents
Substitution and Solvolysis
Isotope Effects
Non-aqueous Solvents
Formation
Exchange Reactions
Isomerization and Racemization

236
236
236
237

237
239
239

2 Mixed Solvents
Diagnosis of Mechanism
Competition
Dielectric Constant Correlations
Grunwald-Winstein Analysis
Other Methods
Reactions of Known Mechanism

240
240
240
240
242
244
245

3 SaltEffects

246

Part / / I Reactions of Biochemical Interest
By D. N. Hague
1 General

25 1

2 Metal Ion Transport

25 1

3 Metal Complex Formation: Non-redox Systems

253


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Contents

Part IV

xi
4 Reactions involving Metals in Porphyrins and Related Ring
Systems
Haemoglobin and Similar Molecules
Coenzyme B1 and Similar Molecules
Cytochrome c and other Porphyrin-containing Systems

255
255
260
264

5 Redox Reactions involving Metals in other Biological and
Made1 Systems

268

Organometallic Compounds

Chapter 1 Substitution
By R. D. W. Kemmitt and M. A. R. Smith

275

1 Exchange Reactions

275

2 Substitution in Carbonyls:Carbon Monoxide Replacement
Simple Carbonyls
Polynuclear Carbonyls
Mixed-ligand Carbonyls
Groups VI and VII
Group VIII
Iron
Cobalt
Nickel
Platinum

276
276
277
279
279
280
280

282
284
284

3 Substitution in Carbonyls: Replacement of other Ligands
Group VI
Iron

285
285
286

4 Formation of Carbonyls

287

5 Cyclopentadienyls

288

6 Phosphite Complexes

289

7 Olefin Complexes

290

8 Mine Complexes
Substitution

Format ion

291
291
291


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xii

Contents

Chapter 2 Metal-Alkyl, -Aryl, and -Ally1 Bond Formation
and Cleavage
By R. D. W. Kemmitt and M. A. R. Smith

293

1 General

293

2 GroupIV

293
293

Titanium
3 GroupVI

Chromium

Molybdenum
4 Group VII

Manganese

294
294
295

295
295

5 Group VIII
Iron
Cobalt
Iridium
Nickel
Palladium
Platinum

296
296
297
298
298
299
301

6 Group1
Copper and Silver
Gold

302
302
303

7 Actinides
Uranium

304
304

Chapter 3 Homogeneous Catalysis
By R. D. W. Kemmitt and M. A. R. Smith

305

I Isomerization
Strained Carbocyclic Systems
Alkenes

305
305
309

2 Disproportionation

310

3 Oligomerization

31 1

4 Ceaddition and Co-oligomerization

3 14


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Contents

xiii

5 Homogeneous Hydrogenation
Chromium
Ruthenium
Cobalt
Rhodium
Iridium
Palladium
Hydrogen Exchange

316
316
316
318
321
322

323
323

7 Homogeneous Oxidation

325

8 Exchange Reactions

327

9 Carbonylation and Hydroformylation

327

10 Decarbonylation

329

11 Hydrosilylation

329

Chapter 4 insertion Reactions
By R. D.W. Kemmitt and M, A. R. Smith

331

1 Alkenes and Alkynes

33 1

2 Carbon Monoxide

334

3 Sulphur Dioxide

336

4 Metals

339

Chapter 5 Reactions of Co-ordinated Ligands
By R. D. W. Kemmitt and M. A. R. Smith

340

1 Carbonyls and Nitrosyls
Carbonyls
Nitrosyls

340
340
342

2 Alkenes and Akynes
Linear Alkenes and Alkynes
Cyclic Alkenes

342
342
345

3 Other Ligands

348


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Contents

xiv
Chapter 6 Oxidative Addition and Reductive Elimination
By R. D. W. Kernmitt and M. A. R. Smith
1 GroupVI

Molybdenum
Tungsten

349

349
349
349

2 GroupVLU
Rhodium

Iridium
Palladium
Platinum

350
350
350
353
354

3 Other Elements

356
356

Gold

Chapter 7 Isomerization: Intramolecular Processes
By R. D. W , Kernmitt and M. A. R. Smith

357

1 Groups IV and V

357

2 GroupVI

359
359

360
363

Group Trends
Chromium
Molybdenum and Tungsten
3 GroupMI

365

4 Group VIII: Iron Triad

366
366
366
370
371
372
373
374
374

Iron
Carbonyl and Isocyanide Complexes
Diene Complexes
Large-ring Complexes
Five-Co-ordinate Complexes
General
Ruthenium
Osmium

5 Group VII: Cobalt Triad
ML Complexes
Cobalt
Rhodium
Cluster Compounds
Miscellaneous
Iridium

375
375
376
377
377
378
379


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xv

Contents

6 Group VIII: Nickel Triad
General
Palladium
Platinum

380
380
380

381

7 Gold

384

Errata

386

Author I ndex

387


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Abbreviations for Ligands and Solvents
Abbreviations which appear only once in the text are generally defined at their point
of use; those which appear more than once are defined below.

General

R
L
LL
LLL
LLLL
X
Specific

acac
ADP
2’-AMP
3’-AMP
5‘-AMP
AN
asp
ATP
big
biPY
bzac
Cal

CDP
cod
CP

CTP
cyclam
cydta
diars
dien
dien - H
dimetn
diphos
DMF
dmg

DMSO
DNA

dPt

alkyl, aryl
unidentate ligand
bidentate ligand
terdentate ligand
quadridentate ligand
halide (except where otherwise stated)

acetylacetonate
adenosine-5’-diphosphate
adenosine-2’-monophosphate
adenosine-3’-monophosphate
adenosine5 ’-monophosphate
acetonitrile
aspartate
adenosine-5’-triphosphate
biguanide
2,2’-bipyridyl
benzoylacetonate
calmagite [1-(1-hydroxy-4-methyl-2-phenylazo)-2-naphtho1-4sulphonate]
cytosine-5’-diphosphate
cyclo-octadiene
cyclopentadieny 1
cytosine-5’-triphosphate
1,4,8,1l-tetra-azacyclotetradecane
cyclohexane-1,Zdiaminetetra-acetate
0-phenylenebisdimethy 1arsine
diethy lenetriamine
diethylenetriamine minus one nitrogen proton

NN’-dimethylpropane-1,3-diamine
1,2-bisdiphenylphosphinoethane
dimethylfomamide
dimethylglyoximate
dimethyl sulphoxide
deoxyribonucleic acid
dipropylenetriamine


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Abbreviationsfor Ligands and Solvents
dtc
dto
dtP
dtpa
edda
eddda
edds
edma
edta
edtra
egta
en
Et ,dien
ete
gedta
glu
glY
Hb
hedta

hfac
his

hm
ida
ind
ma1
Me,dien - H
Me ma1
Me tren
mida
mnt
mq
NADH
niox
nta
ox
pada
pdta
phen
Pn
PY
salen
tar
tcne
tea
terPY
2,3,2-tet
tet-a, tet-b

xvii

dit hiocarbamate
dithio-oxalate
diethyldithiophosphate
diethy lenetriaminepenta-acetat e
ethylenediaminediacetate
ethylenediaminediacetatedipropionate
NN '-e thylenediaminedisuccinate
ethylenediaminemonoacetate
ethylenediaminetetra-acetate
ethylenediaminetriacetatoacetate(i.e. quinquedentate edta)
ethylene glycol bis-(2-aminoethylether)-tetra-acetate
ethylenediamine
NNN"N"-tetraethyldiethylenetriamine

4,8-dithia-l,ll-diazaundecane
2,2'-et hylenedioxybis(ethyleneiminodiacet ate)
glutamate
glycinate
Haemoglobin
N-(2-hy droxyet hyl)et hylenediaminet riacetate
hexafluoroacetylacetonate
histidine
histamine
iminodiacetate
indenyl
malonate
NNN"N"-tetramethyldiethylenetriamine minus the nitrogen proton
methylmalonate anion

2,2',2"- tris-(NN-dimethylamino)triethylamine
N-methyliminodiacetate
maleonitriledithiolate
2-methyl-8-quinolinate
nicotinamide adenine dinucleotide (reduced form)
nioxime
nitrilotriacetate
oxalate
pyridine-Zazo-p-dimethylaniline

propylenediaminetetra-acetate
1,lO-phenanthroline
propylenediamine
pyridine
NN'- bis(salicyla1dehydo)et hylenediamine
tartrate
tet racyanoethylene
triethanolamine
2,2',2"- terpyridyl
1,4,8,11-tetra-azaundecane
5,7,7,12,14,1&hexamethyl-1,4,8,11-cyclotetra-azatetradecane


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xviii
tetren
t fac
thiox
tmd
tmeda

Abbreviationsfor Ligands and Solvents

tetraethylenepentamine
1 , l ,l -trifluoroacetylacetone
monothio-oxalate
trimethylenediamine
NNN’N’-tetramethylethylenediamine
TP
t ripolyphosphate
1,4,8,11-tetra-azacyclotetradeca-4,11trans-14-diene 5,7,7,12,14,14-hexamethyIdiene
triaminotriethylamine
tren
triethylenetetramine
trien
trig1ycine
trigly
triethylenetetraminehexa-acetate
ttha
tu
thiourea
xylenol orange
XY 1


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Part I
ELECTRON TRANSFER PROCESSES
BY

A. McAULEY
K. L. SCOTT


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introduction
BY A. McAULEY AND K. L. SCOTT

The format of this Part of the Report follows closely that of previous volumes.
Attempts have been made to cover as comprehensively as possible all the areas
involving electron-transfer processes in solution. As in previous Reports, compilations have been made of rate and thermodynamic parameters to allow for ready
comparisons of the data.
Several articles have been published dealing with differing areas of the subject
together with technical aspects. Developments in instrumentation have been brought
up to date,l and the range of rapid-reaction techniques now available for investigations of reactions in solution has been reviewed.2An empirical approach to ligand
effects on the kinetics of substitution and redox reactions3 has been shown to be
applicable to a wide variety of reaction systems and the changes effected on the redox
properties of metal ions on ligand co-ordination have been discu~sed.~
Especially
important in this respect are the stabilization of both cations and anions and backbonding effects. A short review of inorganic reaction mechanisms has been publisheds and mechanistic assignments have been made using terms in empirical rate
laws for both complexation and redox reactions of metal ions in aqueous solution.'
The importance of hydrogen-ion dependences has also been discussed. Reactions of
aquocobalt(m) ions have been described using a phenomenological model for redox
reactions.'
Metal-ion reduction of both mononuclears and dinuclears carboxylatocobalt(rr1)
complexes have been reviewed. There continues to be substantial interest in this

subject and in particular the adjacent-attack mechanism for complexes with simple
monocarboxylate ligands now appears to be well understood. The importance of the
inner-sphere mechanism in reductions by Eu2+ has been amply demonstrated.l0
Gouldll has also illustrated the usefulness of the comparison of rate data from the
reactions of a common oxidant by several reductants. Many workers are currently
involved in attempts to measure first-order rates of electron transfer within precursor complexes. In the search for likely systems, the one chosen by Taube12
M. H. Davies, B. H. Robinson, and J. R. Keefe, Ann. Reports (A), 1973, 70, 123.
'Investigation of Rates and Mechanisms of Reaction, Part II', ed. G. G. Hammes, Vol. 6 in the
series 'Techniques of Chemistry', ed. A. Weissberger, Wiley, New York, 1974.
* V. Gutmann and R. Schmid, Coordination Chem. Rev., 1974, 12,263.
V. Gutmann, Sfructure and Bonding, 1973, 15, 141.
6 A. McAuley, Ann. Reports ( A ) , 1973, 70,487.
6 G. Davies, Coordination Chem. Rev., 1974, 14, 287.
7 I. Bodek and G. Davies, Coordination Chem. Rev., 1974, 14, 269.
8 H. Taube, Ber. Bunsengesellschafr phys. Chem., 1972, 76, 964.
0 A. G. Sykes, Chem. in Britain, 1974, 10, 170.
10 F. R. F. Fan and E. S. Gould, Inorg. Chem., 1974,13, 2639.
11 F. R. F. Fan and E. S. Gould, Inorg. Chem., 1974, 13, 2647.
l a S. S. Isied and H. Taube, J . Amer. Chem. Soc., 1973, 95, 8198.
1

a

3


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4

Introduction

appears to be yielding the most promising results. A long-lived intermediate has,
however, been identified in a cobalt(~~~)-iron(rr)
electron-transfer reaction.13 The
TlI-TIIII exchange reaction has been shown to be a two-electron transfer without
intermediacy of T.111, and redox potentials have been substantially r e v i ~ e d . ~ ~ , ~ ~
Redox processes have been described in a review of platinum chemistry with
monoammine ligands.la Electron-transfer mechanisms involving organometallic
intermediates have been discussed.17 The increasing use of pulse radiolysis has
resulted in better characterization of the species produced and the acid-base properties of free radicals in solution have been reviewed.18 The correlation between
redox potentials and pKa values of the radicals emphasizes the role and importance
of acid-base equilibria of these species in electron-transfer reactions. Two volumes
on homogeneous catalysis involving metal complexes have been published,’@in
which attempts have been made to systematize the chemistry of reactions of metal
ions with small molecules both inorganic (e.g. 02,N2,CO, etc.) and organic (alkenes
and alkynes).
The increasing interest in the biological aspects of inorganic mechanisms is
reflected in several reviews. Redox reactions of metalloporphyrin complexes have
been discussed20and the principles of catalysis by metallo-enzymes described with
particular reference to proteins interacting with oxygen.21Among several interesting
papers in an excellent two-volume work edited by Eichhorn is one by Sutin on redox
reactions in co-ordination compounds.22

R. D. Cannon and J. Gardiner, Inorg. Chem., 1974, 13, 390.
B. Falcinella, P. D. Felgate, and G . S . Laurence, J.C.S. Dalton, 1974, 1367.
H. A. Schwartz, D. Comstock, J. K. Yandell, and R. W. Dodson, J. Phys. Chem., 1974,78,488.
l 6 K. P. Beaumont and C. A. McAuliffe, Inorg. Chim. ACIU,1974, 8, 105.
l7 J. K. Kochi, Accounts Chem. Res., 1974, 7, 351.
E. Hayon and M. Simic, Accounts Chem. Res., 1974, 7, 114.
l9 M. M. Taqui-Khan and A. E. Martell, ‘Homogeneous Catalysis by Metal Complexes’, Vols. I

and 11, Academic Press, New York, 1974.
2o J. H. Furhop, Structure and Bonding, 1974, 18, 1.
21 J. M. Pratt, in ‘Techniques and Topics in Bio-inorganic Chemistry’, ed. C. A. McAuliffe,
Macmillan, London, 1975.
2 2 N. Sutin, in ‘inorganic Biochemistry’, ed. G . L. Eichhorn, Elsevier, Amsterdam, 1973.

1s
l4
l5


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1
Reactions Between Two Metal Complexes
BY K. L. SCOTT

1 Reducing Agents

Chrornium(II).-Rate constants and activation parameters for reactions of chromium(r1) and of other reductants are given in Table 1 on p. 17.
One of the major interests with CrII is the determination of the mediating role of
organic 1igands.l The simplest of the inner-sphere mechanisms, designated adjacent
attack, involves an activated complex in which both metal ions are co-ordinated to a
simple ligand, the most common group being carboxylate. Proof that remote attack
could occur with bifunctional organic ligands was eventually obtained in the reduction of the isonicotinamidopenta-amminecobalt(111)complex (1).2 The use of complexes with remote carbonyl groups has enabled Gould3 to add considerably to the
list of reactions proceeding by remote attack. Evidence for this mechanism with the
p-formylcinnamato-complex(2)3comes not only from the very high rate and from

(2)

the form of the rate law [equation (l)] but also from the detection of a short-lived

-d[Co1I1l/dt =

(kl+ kz[H+l)[C0I~~][Cr2+]

(1)

chromium(rr1) carbonyl complex. Complex (2) is particularly interesting in that
mediating action by ten ligand atoms is involved. The intermediate is similar both in
its rate of dissociation (k = 7.6 s-l at 25 "C)and spectrum to the one identified in
the Cr2+reduction of the p-formylbenzoato-complex.4Reduction of the o-formyl-

4

H. Taube, Ber. Bunsengesellschaft phys. Chem., 1972, 76,964.
F. Nordmeyer and H. Taube, J. Amer. Chem. SOC.,1968,90, 1162.
E. S. Gould, J. Amer. Chem. SOC.,1974, 96, 2313.
A. Zanella and H. Taube, J. Amer. Chem. SOC.,1972,94, 6403.

5