Influence of Charge Distribution on the Properties of Keggin- and Dawson-Type Heteropolyanions

Abstract

The cyclic voltammetric behavior of selected Keggin- and Dawson-type heteropolyanions has been studied as a function of electrolyte composition. The buffer capacity of the electrolyte is shown to be an important parameter that can be used to reveal the symmetry or dissymetry in charge distribution within each oxometalate. A fully symmetrical heteropolyanion like [P2W18O62]6− exhibits a uniform protonability, while its α1 and α2 lacunary derivatives present a donor set of hard oxide ions with stepwise acid-base equilibria. The behavior of these sites is revealed readily in unbuffered or poorly buffered media at pH = 3, and is ascribed to the change in symmetry of charge distribution in the molecules. It is shown that cations other than the proton, even in very high excess, fail to give rise to the classical voltammograms of lacunary heteropolyanions. The same behavior also appears in α2[Mo2P2W15O61]10−, α[PW11O39]7−, α[SiW11O39]8−, [H2P2W12O48]12−, and, to a lesser extent, in α2[Ni(H2O)P2W17O61]8− and α2[Zn(H2O)P2W17O61]8−. The analogy in the cyclic voltammograms of most dication-substituted heteropolyanions with those of the precursor lacunary species could thus be explained.