On tetrafluorobromates(III): Crystal structures of the dibromate CsBr₂F₇ and the monobromate CsBrF₄

Alkali and alkaline earth metal tetrafluorobromates M^IBrF₄, M^II(BrF₄)₂ (M^I = Na-Cs, M^II = Sr, Ba) can be synthesized from the respective fluorides and BrF₃. By the reaction of CsF with an excess of BrF₃, the compound cesium tetrafluoridobromate(III) bromine trifluoride (1/1), CsBr₂F₇ (1), was obtained in the form of a colorless powder and as single crystals. Shown by X-ray diffraction on single crystals, its Br₂F₇^- 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), CsBrF₄ (2), was obtained as a colorless powder. Compound 2 can also be obtained from compound 1 by evacuation. The crystal structure of CsBrF₄ (2) was elucidated by Rietveld refinement on its X-ray powder pattern. Its structure is not isotypic to MBrF₄ (M = Na, K, Rb, NO) but isotypic to CsAuF₄. CsBr₂F₇ is, however, not isostructural to CsAu₂F₇. (Copyright copyright 2013 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)