Kinetics of Reduction of Aqueous Hexaammineruthenium(III) Ion at Pt and Au Microelectrodes: Electrolyte, Temperature, and Pressure Effects

Rate constants K-el obtained by impedance spectroscopy for the reduction of Ru(NH3)(6)(3+) at polycrystalline Pt and Au ultramicroelectrodes depend strongly on the identity and concentration of the anion present in the order CF3SO3- < Cl- < ClO4-, but not on the cation of the supporting electrolyte (Na+, K+, H+). For Cl- as the sole anion present, K-el is directly proportional to the total [Cl-], such that k(el) would be zero if Cl- were hypothetically absent, indicating that Cl- is directly involved in mediation of the Ru(NH3)(6)(3+/2+) electron transfer. For CF3SO3- as the sole counterion, the dependence of k(el) on the total [CF3SO3-] is not linear, possibly because blocking of the available electrode surface becomes dominant at high triflate concentrations. Volumes of activation Delta V-el double dagger for reduction of Ru(NH3)(6)(3+) at an electrode in presence of Cl- or CF3SO3- are much more negative than predictions based on theory (Swaddle, T. W. Chem. Rev. 2005, 105, 2573) that has been successful with other electron transfer reactions but which does not take into account the involvement of the anions in the activation process. The strongly negative Delta V-el double dagger values probably reflect solvation increases peculiar to activation processes of Ru-III/II am(m)ine complexes, possibly together with promotion of desorption of surface-blocking Cl- or CF3SO3- from electrodes by applied pressure. Frumkin corrections for Ru(NH3)(6)(3+) within the diffuse double layer would make Delta V-el double dagger even more negative than is observed, although the corrections would be small. The strongly negative Delta V-el double dagger values are inconsistent with reduction of Ru(NH3)(6)(3+) in direct contact with the metallic electrode surface, which would entail substantial dehydration of both the electrode and Ru(NH3)(6)(3+). Reduction of Ru(NH3)(6)(3+) can be regarded as taking place in hard contact with adsorbed water at the outer Helmholtz plane.