Electron-Transfer Reactions in Non-Aqueous Media. IV. A Temperature-Dependence Study of the Reduction of CoCl(NH₃)₅²⁺ by Iron(II) in N,N-Dimethylformamide

Abstract

A detailed study has been made of the temperature dependence of the rate of reduction of CoCl-(NH₃)₅²⁺ by iron(II) in N,N-dimethylformamide. The observed rate constants (k_obs) for this reaction are complex functions of an equilibrium constant (K) for the formation of a bridged intermediate, the rate constant for electron transfer in this bridged intermediate (k), and the iron(II) concentration. From studies of the dependence of k_obs on iron(II) concentration at five temperatures the temperature dependences of both K and k have been resolved, yielding respectively ΔHº -20k±12 kJ mol^-1, ΔSº -44±40 J K^-1 mol^-1 and ΔH* 107±4 kJ mol^-1, ΔS* 57±16 J K^-1 mol^-1. The results are interpreted in terms of a bridged intermediate in which the iron atom is tetrahedrally coordinated.