Steric effects in electron transfer reactions. Part II. Rates of reduction of cobalt(III) (pentaalkylamine)(aquo) complexes at mercury and platinum electrodes

Rates of reduction for complexes with the general formula Co(III)(R-NH₂)₅(H₂O)³⁺ where R=H, CH₃, n-C₃H₇ and n-C₄H₉ have been measured in aqueous 0.2 M KPF₆ at mercury electrodes using de and normal pulse polarography and at platinum electrodes using rotating disk voltammetry. The complexes with larger amine ligands are reduced at more positive potentials at both electrodes. The reduction of each complex occurs at more positive potentials at platinum than at mercury electrodes; furthermore, the addition of KI to the electrolyte causes positive shifts in the waves at both electrodes. At mercury electrodes, the complexes derived from larger alkylamines exhibit transfer coefficients that are much greater than 0.5. Trends in the rate constants for the homogeneous reduction of the Co(III) complexes by V²⁺ and Cr²⁺ exhibit a smaller dependence on the size of the alkylamine ligands than is expected from the electrochemical data.