Chapter 17 - Electron-transfer reactions

Abstract

Electron transfer reactions are an integral part of many fundamental processes in chemistry, biology and materials sciences, but they are rarely described in chemical kinetics books. In particular, outer-sphere electron transfers, which proceed without concomitant bond-breaking–bond-forming processes, follow distinctly different kinetics and are the subject of this chapter. First, the theory of Marcus for electron transfer reactions is presented in this chapter. This theory shaped the current understanding of this field and predicted the observation of a free-energy relationship characterized by a decrease in rate with an increase in the exothermicity of reactions, when their driving forces are four times higher than their intrinsic barrier. The experimental observation of this prediction coined the term ‘Marcus inverted region’. Next, a critique of Marcus theory is presented and the alternative view of the Intersecting-State Model is offered. Both theories are applied to self-exchange reactions of transition-metal complexes and organic species. Electron transfers can occur at large distances between reactants and the associated non-adiabatic factors are discussed. Special attention is given to the observation of ‘inverted regions’ in some systems, but not in all systems, and to the occurrence of ‘double-inverted regions’ in ultra-exothermic reactions. Finally, electrochemical electron transfers are presented.