Solvent effects on the kinetics of simple electrochemical reactions: Part I. Comparison of the behavior of Co(III)/(II) trisethylenediamine and ammine couples with the predictions of dielectric conitnuum theory

Abstract

The electroreduction kinetics of Co(en) 3 3+ (en-ethylenediamine), Co(NH 3) 6 3+, and Co(NH 3) 5F 2+ have been investigated atmercury electrodes in aqueous solution and six nonaqueous solvents in order to explore the influence of the solvent upon the electrode kinetics of such simple one-electron outer-sphere reactions where the composition of the reactant's coordination sphere remains fixed. Substantial variations in the experimental rate parameters were observed as the solvent was altered; these reflect the influence of the outer-shell solvent upon the reorganization barrier to electron transfer. A simple phenomenological treatment of solvent effects upon the electrode kinetics of such simple redox reactions is given. It is pointed out that double-layer corrected rate constants k corr can be evaluated at the same Galvani potential in different solvents with useful accuracy; these quantities can provide particular insight into the chemical influence of the solvent. The utility of separating such solvent effects into “intrinsic” and “thermodynamic” contributions is noted. The substantial decreases in k corr , seen when substituting several non-aqueous solvents for water, were traced to increases in the outer-shell component of the intrinsic free-energy barrier (Δ G i # ) os. In contrast, small decreases in (Δ G i # ) os are predicted by the dielectric continuum model under these circumstances. These discrepancies between theory and experiment are ascribed to contributions to (Δ G i # ) os from extensive short-range reorientation of solvent molecules. Variations in the efficiency of electron tunneling within the transition state may also provide a contribution to the observed solvent effects.