Some Inorganic Reaction Mechanisms

The articles and discussion comments contained in this volume derive from a Conference on Inorganic Reaction Mechanisms. The organization 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 investigations whose opinions were canvassed by the organizers. 

We have already touched on some aspects of inorganic reaction mechanisms kinetically inert metal centres such as Co(III) (Section 21.10) and organometallic reaction types (Section 23.7). Now, we discuss in more detail the mechanisms of ligand substitution and electron-transfer reactions in coordination complexes for the substitution reactions, we confine our attention to square planar and octahedral complexes, for which kinetic data are plentiful. 

The arrangement of Chapters 1 to 5 of this Part of this Report by type of compound results in a dispersal of references dealing with the role of the solvent. To remedy this, some of these references have been collected together here, both to illustrate current approaches to determining the role of solvents in inorganic reaction mechanisms and the use of rate-constant variation with solvent composition trends as diagnostic tools. 

The bulk of the information about inorganic reaction mechanisms has been accumulated over the past thirty years. Initially a somewhat undue emphasis was placed on the reactions of cobalt(III) and platinum(II) complexes, but in recent years the chemistry of other transition elements whose reactions are slow has been extensively studied. The development of readily available techniques for the study of rapid reactions has allowed kinetic studies to be extended to the rapidly reacting systems, and few areas of inorganic chemistry remain which are not amenable to some form of kinetic study. 

In this volume, the format, coverage, and approach in dealing with various aspects of Inorganic Reaction Mechanisms are similar to those adopted for the previous volumes of the series. Once again, rather than make an attempt at cataloguing all the articles in which mention is made 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. It is our hope, however, that any resultant oversimplification or misrepresentation is minimal. Review articles cited are those which should be accessible to the great majority of readers. 

The format, coverage, and approach of this third volume dealing with Inorganic Reaction Mechanisms are similar to those for the previous volumes. In view of the appearance of the Specialist Periodical Reports on Organo-metallic Chemistry, we have been much more selective in our choice of references in this area. We have concentrated on references in which kinetics or reaction mechanisms form the principal interest, rather than attempting, as in the first two volumes, to cite all references with some mention of kinetics or mechanism. Solvent effects on reactivities and mechanisms of inorganic reactions, which were somewhat neglected in Volume 2, are considered in a separate chapter in this volume. 

Some of the lUPAC recommendations presented apply primarily to physical organic chemistry, but the recommendations will certainly influence the development of naming inorganic reaction mechanisms, as well. The idea is to remove mechanistic ambiguities of the traditional system. 

It appears that the electron transfer in some biological systems is temperature independent at temperatures close to absolute zero. It is therefore inferred that such reactions proceed through a quantum mechanical tunneling mechanism, sometimes involving distances up to 30 A (3 nm). Such electron transfers seem to be involved in biological redox reactions of chloroplasts and mitochondria. It appears that the electron-transfer reactions will be one of the central subjects of inorganic reaction mechanism studies in the future. 

E. Basolo and R. G. Pearson, Mechanisms of Inorganic Reactions, 2nd ed., John Wiley Sons, Inc., New York, 1967. An excellent volume that stresses the reactions of complexes ia solution a background and a detailed theory section is iacluded that is largely crystal field theory, but some advantages and disadvantages of molecular orbital theory are iacluded. 

This account of the kinetics of reactions between (inorganic) solids commences with a consideration of the reactant mixture (Sect. 1), since composition, particle sizes, method of mixing and other pretreatments exert important influences on rate characteristics. Some comments on experimental methods are included here. Section 2 is concerned with reaction mechanisms formulated to account for observed behaviour, including references to rate processes which involve diffusion across a barrier layer. This section also includes a consideration of the application of mechanistic criteria to the classification of the kinetic characteristics of solid-solid reactions. Section 3 surveys rate processes identified as the decomposition of a solid catalyzed by a solid. Section 4 reviews other types of solid + solid reactions, which may be conveniently subdivided further into the classes... 

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