Rates of electron transfer within outer-sphere precursor complexes in the system penta-ammine(dimethyl sulphoxide)cobalt(III) and substituted pentacyanoferrates(II)

Abstract

The kinetics of outer-sphere electron-transfer reactions between [Co(NH₃)₅(dmso)]³⁺(dmso = dimethyl sulphoxide) and a series of [Fe(CN)₅L]^3– complexes (L = imidazole, ammonia, pyridine, pyrazine, isonicotinamide, or pyrazine-2-carboxamide) have been investigated as a function of the ligand L. The formation constants of the precursor [Co(NH₃)₅(dmso)]³⁺∥[Fe(CN)₅L]^3– complexes are nearly constant in the series (400–550 dm³ mol^–1) and practically temperature independent in the range 10–35 °C (ΔH ca. 0, ΔS= 50–55 J K^–1 mol^–1). The rates of electron transfer in the precursor complexes show a systematic increase from the pyrazinamide to the imidazole complex, with a linear dependence of ln(k_et) against ΔE^⊖, as predicted theoretically. This increase in rate is paralleled by a decrease in the activation enthalpies from 109 to 92 kJ mol^–1, the activation entropies remaining nearly constant at 71–75 J K^–1 mol^–1.