Intramolecular Electron Transfer at Metal Surfaces. II. Comparisons of Ligand-Bridged CR(III) Reduction Kinetics at Silver Versus Mercury Electrodes

摘要

Rate parameters have been gathered for the electroreduction of a number of Cr(III) complexes attached to silver electrodes via one or more thiocyanate, azide, chloride, or bromide bridging ligands. Unimolecular rate constants k sub et (per sec) for electron transfer within the adsorbed states as well as the surface concentration of the adsorbates were obtained directly using rapid linear sweep voltammetry. These data are also compared with corresponding kinetics and adsorption measurements at mercury electrodes in order to deduce the structure factors responsible for the substantially greater overall reactivities (i.e., larger apparent rate contents k sub app) seen at silver. Marked differences in k sub et, as well as k sub app, were found between silver and mercury surfaces these differences being sensitive to the nature of the nonbridging as well as the bridging ligands. This indicates that the metal surface exerts a significant influence upon the energetics of the elementary electron-transfer step as well as upon the stability of the precursor intermediate. Explanations for the observed structural influences upon k sub et are sought in terms of 'electronic communication' effects between the redox center and the metal surface.