New compounds bearing [M(S₂O₇)₃]^2- anions (M = Si, Ge, Sn): Syntheses and characterization of A₂[Si(S₂O₇)₃] (A = Na, K, Rb), A₂[Ge(S₂O₇)₃] (A = Li, Na, K, Rb, Cs), A₂[Sn(S₂O₇)₃] (A = Na, K), and the unique germanate Hg₂[Ge(S₂O₇)₃]Cl₂ with cationic ¹_∞[HgCl_2/2]⁺ chains

The reaction of the group 14 tetrachlorides MCl₄ (M = Si, Ge, Sn) with oleum (65 % SO₃) at elevated temperatures led to the unique anionic complexes [M(S₂O₇)₃]^2- that show the central M atoms in coordination of three chelating S₂O₇^2- groups. The mean distances M-O within the complexes increase from 175 pm (M = Si) via 186 pm (M = Ge) up to 200 pm (M = Sn). The charge balance for the [M(S₂O₇)₃]^2- anions is achieved by alkaline metal ions A⁺ (A = Li, Na, K, Rb, Cs) which were implemented in the syntheses in form of their sulfates. The size of the A⁺ ions, i.e. their coordination requirement causes the crystallographic differences in the crystal structures, while the structure of the complex [M(S₂O₇)₃]^2- anions remains essentially unaffected. Furthermore, we were able to characterize the unique germanate Hg₂[Ge(S₂O₇)₃]Cl₂ which forms when HgCl₂ is added as a source for the counter cation. The Hg²⁺ and the Cl^- ions form infinite cationic chains according to ¹_∞[HgCl_2/2]⁺ which take care for the charge compensation. For selected examples of the compounds the thermal behavior has been monitored by means of thermal analyses and X-ray powder diffraction. For A being an alkaline metal the decomposition product is a mixture of the sulfates A₂SO₄ and the dioxides MO₂, whereas Hg₂[Ge(S₂O₇)₃]Cl₂ shows a more complicated decomposition. The tris-(disulfato)-silicate Na₂[Si(S₂O₇)₃] has additionally been examined by solid state ²⁹Si and ²³Na NMR spectroscopic measurements. (Copyright copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)