Influence of charge distribution on the properties of Keggin- and Dawson-type heteropolyanions

The cyclic voltammetric behavior of selected Keggin- and Dawson-type hetero polyanions has been studied as a function of electrolyte composition. The b uffer capacity of the electrolyte is shown to be an important parameter tha t can be used to reveal the symmetry or dissymetry in charge distribution w ithin each oxometalate. A fully symmetrical heteropolyanion like [P2W18O62] (6-) exhibits a uniform protonability, while its cr, and at lacunary deriva tives present a donor set of hard oxide ions with stepwise acid-base equili bria. The behavior of these sites is revealed readily in unbuffered or poor ly buffered media at pH = 3, and is ascribed to the change in symmetry of c harge distribution in the molecules. It is shown that cations other than th e proton, even in very high excess, fail to give rise to the classical volt ammograms of lacunary heteropolyanions. The same behavior also appears in a lpha (2)[Mo2P2W15O61](10-), alpha [PW11O39](7-), alpha [SiW11O39](8-), [H2P 2W12O48](12-), and, to a lesser extent, in alpha (2)[Ni(H2O)P2W17O61](8-) a nd alpha (2)[Zn(H2O)P2W17O61](8-) The analogy in the cyclic voltammograms o f most dication-substituted heteropolyanions with those of the precursor la cunary species could thus be explained.