ELECTRON-TRANSFER REACTIONS OF COBALT(3) . RELATIVE RATE COMPARISONS AND FREE ENERGY RELATIONS

Data are presented for several Ru(NH3)62+ reductions of cobalt(III) complexes. The results reported demonstrate (1) that “nonbridging” ligand effects are of the same significance in outer-sphere as in inner-sphere reductions of cobalt(III), (2) that the reduction of Co(NH3)5F2+ exhibits an acid dependence similar to that of Co(NH3)5OH2+, and (3) that a unit change in charge of the cobalt complex does not alter the reactivity at high ionic strengths. The observations imply that the specific second-order rate constant, k12, for electron-transfer reactions of cobalt(III) can be represented as the product of a contribution due to the cobalt complex and a contribution due to the reducing agent. It is shown that many of the different reactivity patterns for Ru-(NH3)62+ (outer sphere) and Cr2+ (inner sphere) reductions of cobalt(III) complexes arise largely from the detailed differences in the free energies of reaction. It is postulated that the cobalt(III) center in the activated complex is very similar in outer-sphere and inner-sphere reactions, and this postulate provides a simple means of accounting for the acid-dependent reactivities of Co(NH3)5OH2+ and Co(NH3)5F2+. © 1968, American Chemical Society. All rights reserved.