Article

Abstract

The oxidation reactions of Pd(ttcn)22+, Pt(ttcn)22+, and Au(ttcn)22+ (ttcn = 1,4,7-trithiacyclononane) complexes by a typical outer-sphere oxidizing reagent, Ni(tacn)23+ (tacn = 1,4,7-triazacyclononane), were studied in aqueous acidic solution and in acetonitrile. The rate constants corresponding to the oxidation and reduction reactions were isolated for Pd(ttcn)23+/2+ and Au(ttcn)23+/2+ couples in both media. The electron exchange rate constants (kex) for Pd(ttcn)23+/2+, Pt(ttcn)23+/2+, and Au(ttcn)23+/2+ couples at 25°C were estimated as 1.7 × 103, 59, 3.9 × 104 mol-1 dm3 s-1 in aqueous solution and 6.7 × 103, 1.3 × 102, and 2.7 × 105 mol-1 kg s-1 in acetonitrile, respectively. An attempt to observe the reduction reaction of Pt(ttcn)23+ was not successful because of the instability of Pt(ttcn)23+ in aqueous solution. A comparison of the electron exchange rate constants of Pd(tacn)23+/2+, Pd(ttcn)23+/2+, Pt(ttcn)23+/2+, and Au(ttcn)23+/2+ couples in aqueous solution and in acetonitrile revealed that the inner-sphere contribution to the activation free energy for each reaction is identical in both solvents and that the self-exchange rate constants of these complexes directly reflect the structural differences of these complexes.Key words: electron transfer, kinetics, Marcus theory, inner-sphere contribution.