Stepwise base decomposition of 12-tungstosilicate(4–)

Abstract

The reaction of [SiW₁₂O₄₀]^4–(1.5 × 10^–5 mol dm^–3) with excess of hydroxide ion at an ionic strength of 1.0 mol dm^–3 MCl, to form [SiO₄]^4– and [WO₄]^2–, has been studied. The reaction proceeds in three distinct stages, the intermediates being [SiW₁₁O₃₉]^8– and [SiW₉O₃₄]^10–. The first stage proceeds via a base-dependent path, the second-order rate constants decreasing in the order K⁺ > Na⁺ > Li⁺. The second stage, the hydrolysis of [SiW₁₁-O₃₉]^8–, has been studied in detail only in LiCl, the reaction proceeding via both base-independent and -dependent paths directly to [SiO₄]^4– and [WO₄]^2– in this medium. Comparison of the data obtained for these first two reactions with those obtained for the base hydrolysis of other approximately spherical polyanions indicates that the enthalpy of activation increases as the charge on the polyanion increases. The third stage, the hydrolysis of [SiW₉O₃₄]^10–, shows a very large dependence on the choice of cation, and studies in mixed-cation media suggest the formation of larger ion aggregates. This final reaction also has exceptional enthalpies and entropies of activation.