STUDY OF POLYOXOTUNGSTATES WITH THE KEGGIN STRUCTURE BY CYCLIC VOLTAMMETRY

Cyclic voltammetry was used to study, in aqueous solution (pH 2-6), po lyoxotungstates with the Keggin structure with the general formula alp ha-[XW11M(H2O)O-39](n-) X=P, Si, B and M=Cr(III), Fe(III), Co(II,III), Ni(II). The results provide valuable information on the comparative e ase of reduction of the several species studied and on the stability o f the reduced species. The transition metal substituted heteropolytung states proved to be, in general, more difficult to reduce than the cor responding lacunary species, under the same conditions. For all specie s, the peak potentials shift to more negative values as the pH increas es. In the range of potentials studied (-1100 mV to 1100 mV), only the boron compounds could be reduced irreversibly. The dependence of the reduction potentials on the pH was studied for the Ni(II) complexes. F or some of the complexes, in acidic solutions (pH 2.2), namely [PW11Ni (H2O)O-39](5-), a new species, resulting from the two-electron reducti on, was observed. This was attributed to an isomerization alpha-->beta of the reduced anions. (C) 1998 Elsevier Science S.A. All rights rese rved.