Mechanistic Aspects of Inorganic Reactions

475 Pages·1982·5.92 MB·English

by  David B. Rorabacher and John F. Endicott (Eds.)

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In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Mechanistic Aspects
of Inorganic Reactions
David B. Rorabacher, EDITOR
Wayne State University
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198
ACS SYMPOSIUM SERIES
AMERICAN CHEMICAL SOCIETY
WASHINGTON, D. C. 1982
In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Library of Congress Cataloging in Publication Data
Mechanistic aspects of inorganic reactions.
1
00 (ACS symposium series, ISSN 0097-6156; 198)
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98.f Me"cBhaasneidsm os na tt Whea yCnoen fSetraetnec Ue niovner sIintyor, gDaentirco iRt, eMacitcihoin
01 gan, June 10-12, 1981."
2-
8 Includes bibliographies and index.
9
1
k- 1. Chemical reactions—Congresses. 2. Chemistry,
1/b Inorganic—Congresses.
2 I. Rorabacher, David, 1935- . II. Endicott, John,
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1 1932- . III. Conference on Inorganic Reaction
10. Mechanisms (1981: Wayne State University). IV.
oi: Series.
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82 | QISDB5N0 10.-M84431 2-07341-988 2 541A.C3'S9M C8 18928- 113-841876
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Dat Copyright © 1982
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In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
ACS Symposium Series
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98.f M. Joan Comstock, Series Editor
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7, 1 Robert Baker Robert Ory
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b Donald D. Dollberg Leon Petrakis
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James D. Idol, Jr. Davis L. Temple, Jr.
Herbert D. Kaesz Gunter Zweig
In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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8.f FOREWORD
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98 The ACS SYMPOSIUM SERIES was founded in 1974 to provide
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0.1 IN CHEMISTRY SERIES except that in order to save time the
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2 | d mitted by the authors in camera-ready form. Papers are re
98 viewed under the supervision of the Editors with the assistance
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7, of the Series Advisory Board and are selected to maintain the
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In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
PREFACE
^^[OST INORGANIC MECHANISTIC INFORMATION is obtained from kinetic
studies, and, partly as a consequence of increased utilization of fast reaction
techniques, major progress in this area has been made during the past
25 years.
1 Although experimental kinetic parameters generally reflect the most
0
pr0 energetically favorable pathway, other reaction pathways are always avail
98. able, and a change in reaction conditions (temperature, pressure, reaction
1
2-0 matrix, structural modifications of the reactants, etc.) may alter the rela
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9 tive significance of competing pathways. As the theory of reaction
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bk- energetics has become increasingly sophisticated, so each available reaction
1/
2 pathway can be considered to represent a specific cross-section of a
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0. continuous multidimensional potential energy hypersurf ace connecting the
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oi: ground-state reactants and products. Even with the advent of large,
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2 | high-speed computers, the theoretical models must be simplified in some
98 manner if the calculations are to become manageable. The approach
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7, taken in the selection of appropriate simplifications remains an area of
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er active controversy.
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e Theoretical models generally evolve from an effort to explain
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e experimental results; however, the ultimate test of any theoretical model
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ati ing the current status of our understanding of the principles underlying
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solution undergoing either inner-sphere ligand substitution or electron
transfer to and from the metal center. Such studies relate to the important
selective role of metal catalysts in many areas of enzymatic, commercial,
and modern synthetic chemistry. Clearly, this field has now matured to
the point where basic theoretical considerations, although incomplete,
can provide a logical framework for understanding the chemical reactivity
of such systems and stimulate the investigation of (1) new and unique
reaction pathways, (2) modified reagents, and (3) unorthodox matrices.
Reactions involving nonmetallic species or nontraditional reactions
of metal complexes (unusual oxidation states, reactions with different
reaction partners, etc.) are less commonly studied but are becoming of
increased interest as mechanistic inorganic chemistry has come of age. A
ix
In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
small sampling of such areas is included in this volume. Some of these
studies have been stimulated by the emphasis on solar energy conversion,
environmental contamination, nitrogen fixation, biological processes, or
theoretical curiosity; but all such studies have added to our understanding
of the relative energetics of competing pathways.
The articles and discussion comments contained in this volume
derive from a Conference on Inorganic Reaction Mechanisms. The organi
zation of the conference was stimulated both by an interest of the National
Science Foundation in surveying the current status of this area and by a
need expressed by some 100 researchers active in mechanistic investiga
tions whose opinions were canvassed by the organizers.
1 The major goals of this conference were to provide a forum for the
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pr assessment of the current state of the various areas of mechanistic inorganic
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9 chemistry and to generate extensive dialogue among the participants that
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2- might facilitate the articulation of existing problem areas. To this end,
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19 nearly half the conference was devoted to open discussion. Many of the
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02 were expressed in the General Discussions following the papers or in
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0. informal discussions. This volume includes revised material from these
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oi: oral discussions and short oral presentations. In addition, chapter nine
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9 one of the poster presentations. Therefore, there is no discussion follow
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7, ing this chapter.
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er In organizing this conference, the editors were generously and actively
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Se conference participants. We are particularly grateful for the assistance
ate: rendered by the other members of the organizing committee: Ralph
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n Wilkins, Norman Sutin, Richard Lindvedt, and Stanley Kirshner. Because
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we acknowledge the financial support of the National Science Foundation,
Dow Chemical Company, and Wayne State University.
DAVID B. RORABACHER
JOHN F. ENDICOTT
Wayne State University
Department of Chemistry
Detroit, Michigan
March 1982
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In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
1
Substitution Reactions of Chelates
DALE W. MARGERUM
Purdue University, Department of Chemistry, West Lafayette, IN 47907
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h The kinetics and mechanisms of substitution
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8. reactions of metal complexes are discussed with
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01 emphasis on factors affecting the reactions of
82- chelates and multidentate ligands. Evidence for
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1 associative mechanisms is reviewed. The substitu­
k-
b tion behavior of copper(III) and nickel(III) com­
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2 plexes is presented. Factors affecting the forma­
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0.1 tion and dissociation rates of chelates are con­
doi: 1 pshidileicr ed saulbosntgi tuwtiiothn prreoatcotnio-ntrsa nsoffe r meatnadl npuecplteidoe-
2 | complexes. The rate constants for the replacement
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19 of tripeptides from copper(II) by triethylene-
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ber 2 hteytdraromgiennes dine ctrheea ses ecboyn d1 0a8 nads tmhiertdh yalm ginroou pasc idr erpelasci­e
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e dues of the tripeptides. This is attributed to
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e: reactions.
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ati In recent years there has been a tendency to assume that
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about substitution reactions which remain to be answered and
many aspects which have not been explored. The question of
associative versus dissociative mechanisms is still unresolved
and is important both for a fundamental understanding and for
the predicted behavior of the reactions. The type of experi­
ments planned can be affected by the expectation that reactions
are predominantly dissociative or associative. The substitution
behavior of newly characterized oxidation states such as copper-
(III) and nickel(III) are just beginning to be available. Acid
catalysis of metal complex dissociation provides important
pathways for substitution reactions. Proton-transfer reactions
to coordinated groups can accelerate substitutions. The main
0097-6156/82/0198-0003$ 10.00/0
© 1982 American Chemical Society
In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
4 MECHANISTIC ASPECTS OF INORGANIC REACTIONS
topic of this lecture concerns the mechanisms of chelate and
multidentate ligand substitutions, an area replete with fasci­
nating, important, and often unexpected results.
Dissociative versus Associative Mechanisms
The success of the Eigen-Wilkins mechanism (1-4) in corre­
lating the characteristic metal ion water exchange rate constant
with ligand substitution rate constants has been impressive.
The concepts are now incorporated in many textbooks and the pre­
dictions have been very useful, but the emphasis has been on the
dissociative nature of these reactions. In many cases a disso­
ciative interchange (1^) mechanism (5) has been assumed for met­
01 al ion substitutions, with a consequent tendency to neglect the
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98. dence that entering ligands can have very pronounced effects on
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0 the rates of aquo metal ion substitution reactions. A com­
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k-1 reactions of trivalent aquo metal ions shows a strong dependence
1/b on the nature of the entering ligand and suggests associative
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0 3+ 3+ 3+ 3+
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0. mechanisms (6). This is the case for Ti , V , Cr , Fe ,
1
oi: 3+ 3+
d Mo and In and indicates that an associative interchange (I )
82 | a
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1 mechanism is likely. In recent years there has been a growing
27, recognition of the importance of associative mechanisms, in part
er due to the studies of activation volumes.
b
m Stranks and Swaddle (7) found a sizeable negative value
epte (-9.3 cm3 m-o1l ) for the activation volume (AΦV ) of H0 exchange
e: S 3+ 9
at with CriH^O)^ . A negative activation volume is characteristic
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n of an associative or associative interchange reaction rather
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ati than a dissociative reaction, because the volume of water in the
blic bulk solvent is greater than the volume of a coordinated water
u molecule. Although the AV* value is slightly positive (8) for
P
3+
the water exchange with CoiNH^),.!^ , the values are negative
for the corresponding reactions of the pentaammine complexes of
Cr(III) (9), Rh(III) (9), and Ir(III) (10) as shown in Table I.
This suggests an associative interchange (I ) mechanism for
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these reactions. On the other hand an I, mechanism is con-
Φ d 3 -1
sistent with the positive AV value of +7.1 cm mol found for
2+
Ni(H0)^ (1_1). Recently Merbach and coworkers (12) have cited
2
evidence for a gradual changeover of mechanism from 1^ to 1^ for
the water exchange reactions of divalent metal ions in going
In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D., el al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1982.