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

A kinetic study of the electron transfer reaction between [Ru(NH3)(5)pz](2) (pz=pyrazine) and [Co(C2O4)(3)](3-) has been carried out in several water -cosolvent mixtures (the cosolvents being methanol, ethylene glycol, tert-b utyl alcohol and glucose). The results obtained disagree with the predictio n of the classical continuum model of solvent effects, which is that there is a linear variation of ln k(obs)(') vs. 1/epsilon (s). However, they can be rationalized considering that the activation process depends on the natu re of the solvent, according to the Marcus-Hush treatment of electron trans fer processes. The differences in the rate constants for several water-coso lvent mixtures, as reaction media, can be explained taking into account the preferential solvation of the reactants. Monte Carlo simulations of a simp le model yield results which give qualitative support to this idea. It is a lso concluded that solvent effects, and in particular solvent mixtures, are asymmetric in relation to their effects on electron transfer reactions. Na mely, a decrease of the dielectric constant of the solvent produces opposit e effects on the driving force for cation-anion reactions depending on whic h reactant acts as oxidant or reductant.