Electrochemical Investigations of the Nickel(II)-Penicillamine System. 3. A Study of the Catalytic Hydrogen Prewave in Connection with Structure of Nickel(II)-Penicillamine Complexes

Abstract

The catalytic hydrogen evolution on the dropping mercury electrode in the presence of Ni(II) and D-penicillamine (Pen) at pH around 6 yields a catalytic hydrogen prewave (CHP) with E1/2 = -1.21 V vs SCE. This wave is similar to the CHP produced by selenocysteine and cysteine described previously. The occurrence of the CHP depends on the formation of the mono(D-penicillamine-N,S)nickel(II) complex whereas bis(D-penicillamine-N,S)nickel(II) complex is inert and has no influence on the CHP electrode process. Although the analogous bis(cysteine) complex is labile, there is strong evidence that it does not take part directly in the CHP process in the Ni(II)-cysteine system. The actual proton donor in the CHP electrode process is the hydronium ion and not the acid component of the buffer. A tentative reaction mechanism was formulated with emphasis on the state of the intermediate hydrogen species. The characteristic pH, nickel ion involvement and the structure of the ligand make the CHP process an attractive model for hydrogen redox reactions catalyzed by [NiFe] hydrogenase.