Reactions with Oleum under Harsh Conditions: Characterization of the Unique [M(S 2 O 7 ) 3 ] 2− Ions (M=Si, Ge, Sn) in A 2 [M(S 2 O 7 ) 3 ] (A=NH 4 , Ag)

The reactions of group 14 tetrachlorides MCl 4 (M=Si, Ge, Sn) with oleum (65 % SO 3 ) at elevated temperatures lead to the unique complex ions [M(S 2 O 7 ) 3 ] 2− , which show the central M atoms in coordination with three chelating S 2 O 7 2− groups. The mean distances MO within the anions increase from 175.6(2)–177.5(2) pm (M=Si) to 186.4(4)–187.7(4) pm (M=Ge) to 201.9(2)–203.5(2) pm (M=Sn). These distances are reproduced well by DFT calculations. The same calculations show an increasing positive charge for the central M atom in the row Si, Ge, Sn, which can be interpreted as the decreasing covalency of the MO bonds. For the silicon compound (NH 4 ) 2 [Si(S 2 O 7 ) 3 ], 29 Si solid‐state NMR measurements have been performed, with the results showing a signal at −215.5 ppm for (NH 4 ) 2 [Si(S 2 O 7 ) 3 ], which is in very good agreement with theoretical estimations. In addition, the vibrational modes within the [MO 6 ] skeleton have been monitored by Raman spectroscopy for selected examples, and are well reproduced by theory. The charge balance for the [M(S 2 O 7 ) 3 ] 2− ions is achieved by monovalent A + counter ions (A=NH 4 , Ag), which are implemented in the syntheses in the form of their sulfates. The sizes of the A + ions, that is, their coordination requirements, cause the crystallographic differences in the crystal structures, although the complex [M(S 2 O 7 ) 3 ] 2− ions remain essentially unaffected with the different A + ions. Furthermore, the nature of the A + ions influences the thermal behavior of the compounds, which has been monitored for selected examples by thermogravimetric differential thermal analysis (DTA/TG) and XRD measurements.