Electrochemical investigations of the nickel(II)-penicillamine system. 2. Direct identification of complex species involved in catalytic nickel reduction on the dropping mercury electrode

The catalytic polarographic nickel prewave was investigated by making appropriate correlations between prewave current and complex species concentrations as calculated by means of available formation constants. It was concluded that the active species is (D-penicillaminato-N,S)nickel(II) [NIL], whereas the bis-ligand complex, [NiL2](2-), is inert and does not play any role in the electrode process. The catalytic character of the electrode process originates from the regeneration of [NiL] by the reaction of adsorbed ligand molecules with free nickel ions available in the bulk of the solution. Conversely, all the complex species in the Ni2+-cysteine system are labile. Consequently, the reaction mechanism in this case may include the dissociation of the complex [NiL2](2-) as an alternative path for the generation of the active species, [NiL]. The bell-shaped form of the prewave was interpreted in terms of potential-dependent catalyst adsorption.