Some extant anomalies in the kinetics of simple inorganic outer-sphere electrode reactions

Some experimental characteristics of selected simple inorganic outer-sphere electrode reactions which deviate from the expectations of conventional electron-transfer models are discussed so as to identify some extant issues deserving further theoretical and/or experimental examination. Attention is focused on the one-electron reduction of Co(III) ammine complexes in view of their chemical irreversibility, enabling very wide ranges of rate-potential behavior to be evaluated straightforwardly in different reaction environments. Kinetic behavior in homogeneous-phase and interfacial systems is intercompared directly by converting the former to ‘equivalent-electrochemical’ rate units. Unexpectedly large (up to ca. 106-fold) variations in the work-corrected rate constants for ostensibly outer-sphere Co(III) ammine reductions are observed at a fixed driving force as the reducing environment is altered. The lowest reactivities, observed with inorganic reductants and CO-coated Pt(111), probably reflect in part non-adiabatic pathways. The marked-by (103 to 104-fold) faster rates observed on Au(111) (and other Au and Pt electrodes) relative to mercury | aqueous interfaces, however, reflect chiefly lower solvation Gibbs energy barriers at the former interface. These differences apparently reflect variations in the reaction site, as deduced partly by examining unimolecular outer-sphere rates and transfer coefficients. The latter measurements also indicate the presence of asymmetric barriers, associated with electron transfer-induced changes in inner-shell force constants. The interpretation of unexpectedly large D/H isotope effects in terms of solvent reorganization is also discussed in relation to recent molecular-dynamics predictions.