ROLE OF THE BRIDGING LIGAND IN ELECTROCHEMICAL INNER-SPHERE ELECTRON-TRANSFER PROCESSES

The treatment of the relative electron-transfer rates for electrochemical inner- and outer-sphere pathways in terms of “intrinsic” and “thermodynamic” contributions is discussed in order to explore the possible roles of bridging ligands in catalyzing such processes. By combining rate measurements with adsorption data for the appropriate ligand-bridged intermediates, this treatment has been applied to some inner- and outer-sphere reactions involving Cr(III)/Cr(II) and Co(III)/Co(II) couples at the mercury-aqueous interface. For complexes containing azide and thiocyanate bridging ligands, the rate increases that result from the presence of ligand-bridged pathways appear to arise merely from the presence of more favorable thermodynamic work terms, rather than by a decrease in the reorganization energy required for electron transfer. A similar conclusion applies, although with less certainty, to chloride and bromide bridges. The physical relation between the reorganization barriers for heterogeneous and homogeneous inner-sphere processes is also considered, and the extent of catalysis of heterogeneous and homogeneous Cr(III)/Cr(II) exchange reactions by bridging anions is compared. Sizable decreases in the intrinsic reorganization barrier for ligand-bridged vs. outer-sphere pathways are seen for the homogeneous exchange reactions, but negligible or only modest decreases are seen for the corresponding electrochemical processes. The explanation for these differences is sought in terms of electronic coupling effects between the homogeneous reacting centers. © 1979, American Chemical Society. All rights reserved.