Electrochemical synthesis and stability of the brown six-electron reduced β- and γ-12-tungstosilicates

The electrochemical synthesis in acid aqueous solution of the brown six-electron reduced beta- and gamma-12-tungstosilicates, abbreviated VI', is described. Their mechanism of formation has been deduced from controlled potential electrolyses and polarographic analysis of the transformations. They are obtained by successive reversible electrochemical steps and irreversible disproportionation reactions. Stability of the VI' anions in acid solution depends on the nature of the isomer and decreases in the order alpha > beta > gamma. This variation can be correlated with the structural modifications induced by rotation of the tri-oxotungstic groups and their reduction/protonation by 6e/6H+.