Scope and pattern of this chapter

The vast number of thermodynamically possible reactions obtained by permuting oxidants and reductants within the scope of this review present major problems of classification and selection. To only a limited extent is the modernity or detail of a paper indicative of its relevance, some of the definitive papers having been published before 1950. Discussion has been concentrated, therefore, at points where a kinetic investigation of a reaction has resulted in a real advance in our understanding both of its mechanism and of those of related reactions, and work which has been more of a confirmatory nature will not receive comparable consideration. Detailed reference to products, spectra, etc. will be made only when the kinetics produce real ambiguities. 

Classification exclusively in terms of a few basic mechanisms is the ideal approach, but in a comprehensive review of this kind, one is presented with all reactions, and not merely the well-documented (and well-behaved) ones which are readily denoted as inner- or outer-sphere electron transfer, hydrogen atom transfer from coordinated solvent, ligand transfer, concerted electron transfer, etc. Such an approach has been made on a more limited scale. Turney has considered reactions in terms of the charges and complexing of oxidant and reductant but this approach leaves a large number to be coped with under further categories. 

As regards oxidation by metal ions, we have chosen to select groups of from two to eleven metal ions, the members of which are known to display similar 

Reductions by metal ions are covered in Section 6 in terms of (/) electron-acceptance and ii) electron-acceptance concerted with homolytic fission. One group of reactions, which includes oxidations and reductions by metal ions, is that between a metal ion and a neutral free radical. These form a self-contained class which is treated separately in Section 7. 

This procedure emphasises the overall division of oxidants into one-equivalent and two-equivalent types defined in terms of how many equivalents of reducing species, e.g. electrons or hydrogen atoms, are taken up by the oxidant in the primary act, as deduced from the behaviour of the oxidant towards either hydrazine , sulphite ion or captive ligand according to the schemes (a)-(c) respectively. 

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