Classification of Redox Reactions

The most important single development in the understanding of the mechanisms of redox reactions has probably been the recognition and establishment of outer-sphere and inner-sphere processes. Outer-sphere electron transfer involves intact (although not completely undisturbed) coordination shells of the reactants. In inner-sphere redox reactions, there are marked changes in the coordination spheres of the reactants in the formation of the activated complex. 

Reaction 2 in Table 5.1 must qualify for an outer-sphere redox category since the bipyridine could not become detached, even by just one end of the bidentate ligand, from the inert iron(II) or iron(III) centers during the course of the rapid redox reaction. There is thus no bond breaking or making during the electron transfer, a situation making them ideal for treatment by the theoretical chemist (Sec. 5.4). 

Reaction 4 in Table 5.1, on the other hand, was one of the first-established examples of an inner-sphere redox reaction. The rapid reaction gives CrCl as a product, characterized spectrally after separation by ion exchange from the remainder of the species in solution. It is clear that since CrCl could not possibly be produced from Cr and Cl ions during the brief time for reaction and ion-exchanger manipulation, it must arise from the redox pro- 

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