Electrochemical properties of iron(III)-substituted heteropolytungstate anions

The electrochemical behavior of a series of heteropolytungstate anions in which one of the W6+ cations is replaced by Fe3+ is described. The anions investigated were FeXW11Om−39 (X = P, As, m = 4; X = Ge, Si, m = 5). All of the anions exhibit a reversible, one-electron couple associated with the Fe3+ center and a pair of two-electron waves attributed to electron addition and removal from the tungsten-oxo framework that comprises each anion's structure. The dependence of the formal potentials of the couples on pH and ionic strength is interpreted in terms of competition between protonation and ion-pairing of the reduced forms of the couples. The results are contrasted with the apparently less intricate behavior of the unsubstituted anions, SiW12O5−40 and PW12O4−40, that was described recently (B. Keita and L. Nadjo, J. Electroanal. Chem., 227 (1987) 77).