On Tetrafluorobromates(III): Crystal Structures of the Dibromate CsBr_2F_7 and the Monobromate CsBrF_4

摘要

Alkali and alkaline earth metal tetrafluorobromates M~IBrF_4, M~(II)(BrF_4)_2 (M~I = Na-Cs, M~(II) = Sr, Ba) can be synthesized from the respective fluorides and BrF_3. By the reaction of CsF with an excess of BrF_3, the compound cesium tetrafluoridobromate(III) bromine trifluoride (1/1), CsBr_2F_7 (1), was obtained in the form of a colorless powder and as single crystals. Shown by X-ray diffraction on single crystals, its Br_2F_7~- anion is neither planar, nor is its Br-μ-F-Br unit linear, as previously deduced by Raman spectroscopy. In stoichiometric reactions of CsF with BrF3, the compound cesium tetrafluoridobromate(III), CsBrF_4 (2), was obtained as a colorless powder. Compound 2 can also be obtained from compound 1 by evacuation. The crystal structure of CsBrF_4 (2) was elucidated by Rietveld refinement on its X-ray powder pattern. Its structure is not isotypic to MBrF_4 (M = Na, K, Rb, NO) but isotypic to CsAuF_4. CsBr_2F_7 is, however, not isostructural to CsAu_2F_7.