The Unprecedented Tetrakis-(disulfato)-silicate Anion [Si(S2O7)4]4-, its Germanium Congener [Ge(S2O7)4]4-, and the Tris-(disulfato)-metallates [M(S2O7)3]2- (M=Si, Ge, Ti), Stabilized by Divalent Counter Cations B2+ (B=Sr, Ba, Pb)

The reaction of oleum (65?% SO3) with the tetrachlorides of silicon, germanium, and titanium, respectively, led to the complex disulfates Sr2[M(S2O7)4] (M=Si, Ge), Ba[M(S2O7)3] (M=Si, Ge, Ti) and Pb[M(S2O7)3] (M=Ge, Ti) if strontium, barium, and lead were used as divalent counter cations. The strontium compounds exhibit the unique tetrakis-(disulfato)-metallate anions [M(S2O7)4]4- with the silicon and germanium atoms in octahedral coordination of two chelating and two monodentate disulfate groups. All of the other compounds display tris-(disulfato)-metallate anions [M(S2O7)3]2- with three chelating disulfate groups surrounding the M atoms. Thermoanalytical investigations on the germanium compounds Sr2[Ge(S2O7)4] and Ba[Ge(S2O7)3] revealed their decomposition in multi-step processes leading to a mixture of BSO4 and BGe4O9 (B=Sr, Ba), while the thermal degradation of Pb[Ti(S2O7)3] yields PbTiO3. For selected examples, IR data are additionally presented.