[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]
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The xenon(IV) hydroxide fluoride and oxide fluoride salts, [H(OXeF2)n][AsF6] and [FXe(II)(OXe(IV)F2)n][AsF6] (n = 1, 2), have been synthesized as the natural abundance and the (18)O- and (2)H-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·xHF with a superacidic HF/SbF5 solvent mixture yielded crystals of [H5F4][SbF6]·2[XeF3·HF][Sb2F11], [XeF3·HF][Sb2F11], and [XeF3][SbF6]. The XeF3(+) cations of each salt are very similar, displaying T-shaped primary coordination of xenon to three fluorine atoms (AX3E2 VSEPR arrangement) and a secondary contact to the fluorine atom of HF in [H5F4][SbF6]·2[XeF3·HF][Sb2F11] and [XeF3·HF][Sb2F11] or to a fluorine atom of SbF6(-) in [XeF3][SbF6]. The secondary coordination spheres of xenon in [H5F4][SbF6]·2[XeF3·HF][Sb2F11] and [XeF3·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]·2[XeF3·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 F4-chain terminated by SbF6(-) anions. Enthalpies and Gibbs free energies of reaction obtained from Born-Fajans-Haber thermochemical cycles support the proposed decomposition pathways.
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