ELECTRON-TRANSFER IN THE ELECTROCHEMISTRY OF METALS, METAL-COMPOUNDS AND METAL-COMPLEXES

Because electron-transfer occurs at the electrode/solution interface, the oxidation of metal electrodes involves electron removal (within the interface) from a solvent molecule or basic constituent (ligand) rather than from the valence-electron shell of the metal (e.g. Ag+Cl- [GRAPHICS] Ag-Cl, E(Ag/AgCl)degrees, = +0.22 V versus NHE; Cl- [GRAPHICS] Cl-., E(Cl-/Cl.)degrees = +2.41 V). The difference in oxidation potential for the free ligand in the absence of the metal electrode and in its presence is a measure of the metal-ligand differential bond energy (e.g. for Ag-Cl, -Delta G(BF) = -Delta E degrees x 23.1 kcal (eV)(-1) = 50.6 kcal mol(-1)). Likewise, oxidation of reduced transition-metal complexes and metalloporphyrins are ligand-centered (or solvent-centered) (e.g. (FeCl3-)-Cl-II [GRAPHICS] (FeCl3)-Cl-III), as is the reduction of the oxidized state ((FeCl3)-Cl-III, [GRAPHICS] (FeCl3-)-Cl-III). In each case the potential for the oxidation of free ligand (or solvent) is decreased in the presence of metal or reduced-metal complex by an amount that is proportional to the metal-ligand bond energy (-Delta G(BF)).