New oxonium bromochalcogenates(IV) - Synthesis, structure, and properties of [H3O][TeBr5]center dot 3 C4H8O2 and [H3O](2)[SeBr6]

Dark red crystals of the composition [H3O][TeBr5] . 3 C4H8O2 (1) were isolated from a saturated solution of TeBr4 in 1,4-dioxane containing a small amount of water. In this compound (space group P2(1)/m, a = 8.922(4) Angstrom, b = 13.204(7) Angstrom, c = 9.853(5) Angstrom, beta = 91.82(4)degrees at 150 K) a square pyramidal [TeBr5](-) anion has been isolated for the first time. The coordination sphere of the anion is completed to a distorted octahedron by weak interaction with a dioxane molecule of the cationic system. The [(HO)-O-3](+) cations are connected to chains by dioxane molecules. At room temperature the compound is stable only in its mother liquor. Crystalline[H3O](2)[SeBr6] (2) (space group Fm3m, a = 10.421(1) Angstrom at 170 K) is a bromoselenous acid of high symmetry. The [H3O](+) ion is only weakly coordinated by Br atoms of the anion. The anions are isolated octahedral [SeBr6](2-) units. The structure is isotypic to the K-2[PtCl6] structure. Despite being a halogeno-chalcogen(IV) acid, 2 exhibits a remarkable thermal stability. Both oxonium compounds were characterized by single-crystal X-ray structure analyses. Vibrational spectra of 2 are reported.