[H(OXeF2)n][AsF6] and [FXeII(OXeIVF2)n][AsF6] (n=1, 2): Examples of Xenon(IV) Hydroxide Fluoride and Oxide Fluoride Cations and the Crystal Structures of [F3Xe---FH][Sb2F11] and [H5F4][SbF6]•2[F3Xe---FH][Sb2F11]

The xenon(IV) hydroxide fluoride and oxide fluoride salts, [H(OXeF2)(n)] [AsF6] and [FXeII((OXeF2)-F-IV)(n)][AsF6] (n = 1, 2), have been synthesized as the natural abundance and the O-18- and H-2-enriched salts and structurally characterized by low-temperature Raman spectroscopy. Quantum-chemical calculations have been used to arrive at vibrational assignments. The experimental vibrational frequencies and isotopic shift trends are reproduced by the calculated gas-phase frequencies at several levels of theory. The cation chain is limited to one or two OXeF2 subunits, which are oxygen-bridged and strongly ion-paired with the AsF6- anion. The reaction of XeOF2 center dot xHF with a superacidic HF/SbF5 solvent mixture yielded crystals of [H5F4][SbF6]center dot 2[XeF3 center dot HF][Sb2F11], [XeF3 center dot H-17][Sb2F11], and [XeF3][SbF6]. The XeF3+ cations of each salt are very similar, displaying T-shaped primary coordination of xenon to three fluorine atoms (AX(3)E(2) VSEPR arrangement) and secondary contact to the fluorine atom of HF in [H5F4][SbF6]center dot 2[XeF3 center dot HF][Sb2F11]] and [XeF3 center dot HF][Sb2F11] or to a fluorine atom of SbF6- in [XeF3][SbF6]. The secondary coordination spheres of xenon in [H5F4][SbF6]center dot 2[XeF3 center dot HF][Sb2F11] and [XeF3 center dot HF][Sb2F11] are similar; however, the F3Xe---FH+ cation of the latter salt is disordered. An additional contact between the XeF3+ cation and the SbF6- anion in [H5F4][SbF6]center dot 2[XeF3 center dot HF][Sb2F11] presumably alters the crystal packing sufficiently to give an ordered F3Xe---FH+ cation, a rare example in which HF is coordinated to a noble gas. The latter compound also provides the first documentation of the H5F4+ acidium ion, which forms a zigzag F-4-chain terminated by SbF6- anions. Enthalpies and Gibbs free energies of reaction obtained from Born-Fajans-Haber thermochemical cycles support the proposed decomposition pathways.