Preparation and characterisation of adducts of bismuth pentafluoride and antimony pentafluoride by vibrational spectroscopy, X-ray powder diffraction, and single-crystal X-ray crystallography

Abstract

Adducts between BiF₅ and SbF₅ were prepared in liquid WF₆ at room temperature. Sublimation of the product gave various volatile crystalline products with analyses corresponding to BiF₅(SbF₅)₃, BiF₅(SbF₅)₂, BiF₅(SbF₅)_1.5, and (BiF₅)_nSbF₅(n= 2, 3, or 20.6). The adducts (BiF₅)_nSbF₅(n= 1 or 1.5), were also prepared. The BiF₅(SbF₅)_n(n= 1.5, 2, or 3) adducts were of low melting point and of higher volatility than (BiF₅)_nSbF₅(n 1). Single-crystal X-ray diffraction studies showed that BiF₅(SbF₅)_n(n= 2 or 3) were isomorphous with tetrameric (NbF₅)₄, and an X-ray powder diffraction photograph of BiF₅(SbF₅)_1. was very similar to that of BiF₅(SbF₅)_n(n= 2 or 3), indicating that it was also isostructural with (NbF₅)₄. The BiF₅(SbF₅)₃ structure likely consists of disordered BiF₅(SbF₅)₃ tetramers, and BiF₅(SbF₅)₂ likely consists of a mixture of disordered BiF₅(SbF₅)₃ and (BiF₅.SbF₅)₂ tetramers in the appropriate ratios. The molecular-beam mass spectra of SbF₅(BiF₅)_n(n= 2 or 3) show fragment ions attributable to BiF₅ and SbF₅ as well as various associated pentafluoride clusters, including BiF₅(SbF₅)₃ for the n= 3 adduct. The vibrational spectra of BiF₅(SbF₅)_n(n= 1.5, 2, or 3) were similar and indicative of the presence of SbF₅ and BiF₅ units joined by cis-bridged fluorine atoms. (BiF₅)_nSbF₅(n= 1, 1.5, 2, 3, or 20.6), were shown by X-ray powder diffraction photography to be isomorphous with polymeric trans-bridged BiF₅ and therefore to consist of disordered BiF₅ and SnF₅ units linked by trans-bridged fluorine atoms. The trans-bridged polymeric BiF₅-type structure of the adducts was further supported by their vibrational spectra. The behaviour of BiF₅-(SbF₅)_n(n= 2 or 3) in WF₆ solution is discussed. An estimated phase diagram for the system BiF₅–SbF₅ was constructed.