Electron-Transfer Kinetics of Redox Centers Anchored to Metal Surfaces: Weak- versus Strong-Overlap Reaction Pathways.

Unimolecular rate constants k sub et are presented for the one-electron electroreduction of various CoIII(NH3)5X complexes bound to mercury, platinum, and gold surfaces via either small inorganic or extended organic ligands X. These 'surface intramolecular' rate parameters are compared with estimates of k sub et for outer-sphere pathways obtained from homogeneous rate data in order to ascertain the consequences of reactant-surface binding upon the electron-transfer energetics. Only small or moderate decreases in the elementary reorganization barrier Delta G* sub et occur upon surface attachment, suggesting the occurrence of weak-overlap pathways. Aside from the dependence of k sub et upon the bridging ligand X at a given electrode potential due to differences in thermodynamic driving force, the observed variations in k sub et appear to be due in part to variations in the electronic transmission coefficient k sub el. Measurements of electrochemical frequency factors for reactants containing a series of thiophenecarboxylate bridges indicate that the substantial decreases in k sub et observed upon interruption of ligand conjugation arise predominantly from decreases in k sub el. (Author)