Experimental and simulation studies of the electron transfer reaction between [Ru(NH3)5pz]2+ and [Co(C2O4)3]3−

Abstract

A kinetic study of the electron transfer reaction between [Ru(NH₃)₅pz]²⁺ (pz = pyrazine) and [Co(C₂O₄)₃]³⁻ has been carried out in several water–cosolvent mixtures (the cosolvents being methanol, ethylene glycol, tert-butyl alcohol and glucose). The results obtained disagree with the prediction of the classical continuum model of solvent effects, which is that there is a linear variation of ln k_obs^′s. 1/ε_s. However, they can be rationalized considering that the activation process depends on the nature of the solvent, according to the Marcus–Hush treatment of electron transfer processes. The differences in the rate constants for several water–cosolvent mixtures, as reaction media, can be explained taking into account the preferential solvation of the reactants. Monte Carlo simulations of a simple model yield results which give qualitative support to this idea. It is also concluded that solvent effects, and in particular solvent mixtures, are asymmetric in relation to their effects on electron transfer reactions. Namely, a decrease of the dielectric constant of the solvent produces opposite effects on the driving force for cation–anion reactions depending on which reactant acts as oxidant or reductant.