Intramolecular Electron Transfer at Metal Surfaces. III. Influence of Bond Conjugation on Reduction Kinetics of Cobalt(III) Anchored to Metal Surfaces via Thiophenecarboxylate Ligands.
PURDUE UNIV LAFAYETTE IN DEPT OF CHEMISTRY
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Unimolecular rate constants, K sub et, are reported for the one-electron electroreduction of pentaamminecobalt III anchored to mercury, gold, or copper electrodes via various thiophenecarboxylate ligands where the thiophene sulfur acts as the surface binding group and the carboxylate is coordinated to the cobalt reactants. At mercury, the rate parameters were found to be sensitive to the bridging ligand structure, substantially ca. 10-20 fold smaller values of K sub et being found for ligands containing one or more methylene groups between the thiophene sulfur and carboxylate oxygen than those featuring uninterrupted bond conjugation between the metal surface donor and CoIII acceptor sites. Various contributions to these reactivity differences are asse-sed on the basis of the measured variations in the stability constants of the adsorbed precursor state, the rate constants for the homogeneous reduction of these complexes by hexammineruthenium II, the unimolecular frequency factors obtained from the electrochemical activation parameters, and by comparison with ele-trochemical outer-sphere rate data. Taken together, these results provide strong evidence that the smaller values of K sub et values of the electronic transmission coefficient than are achieved with the conjugated ligands. Comparison is made with analogous results for intramolecular electron transfer via extended organic ligands in homogeneous solution. Author
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