Study of polyoxotungstates with the Keggin structure by cyclic voltammetry

Cyclic voltammetry was used to study, in aqueous solution (pH 2-6), polyoxotungstates with the Keggin structure with the general formula alpha-[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 ease of reduction of the several species studied and on the stability of the reduced species. The transition metal substituted heteropolytungstates proved to be, in general, more difficult to reduce than the corresponding lacunary species, under the same conditions. For all species, the peak potentials shift to more negative values as the pH increases. 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. For some of the complexes, in acidic solutions (pH 2.2), namely [PW11Ni(H2O)O-39](5-), a new species, resulting from the two-electron reduction, was observed. This was attributed to an isomerization alpha-->beta of the reduced anions. (C) 1998 Elsevier Science S.A. All rights reserved.