X-ray Crystal Structures of [XeF][MF₆] (M = As, Sb, Bi), [XeF][M₂F₁₁] (M = Sb, Bi) and Estimated Thermochemical Data and Predicted Stabilities for Noble-Gas Fluorocation Salts using Volume-Based Thermodynamics

The crystal structures of the xenon(II) salts, [XeF][SbF6], [XeF][BiF6], and [XeF][Bi2F11], have been determined for the first time, and those of XeF2, [XeF][AsF6], [XeF][Sb2F11], and [XeF3][Sb2F11] have been redetermined with greater precision at −173 °C. The Bi2F11− anion, which has a structure analogous to those of the As2F11− and Sb2F11− anions, has been structurally characterized by single crystal X-ray diffraction for the first time as its XeF+ salt. The fluorine bridge between the bismuth atoms is asymmetric with Bi---Fb bond lengths of 2.092(6) and 2.195(6) Å and a Bi---Fb′---Bi bridge bond angle of 145.3(3)o. The XeF+ cations interact with their anions by means of Xe---Fb---M bridges. Consequently, the solid-state Raman spectra of [XeF][MF6] (M = As, Sb, Bi) were modeled as the gas-phase ion pairs and assigned with the aid of quantum-chemical calculations. Relationships among the terminal Xe−Ft and bridge Xe---Fb bond lengths and stretching frequencies and the gas-phase fluoride ion affinities of the parent Lewis acid that the anion is derived from are considered. The analogous krypton ion pairs, [KrF][MF6] (M = As, Sb, Bi) were also calculated and compared with their previously published X-ray crystal structures. The calculated cation−anion charge separations indicate that the [XeF][MF6] salts are more ionic than their krypton analogues and that XeF2 is a stronger fluoride ion donor than KrF2. The lattice energies, standard enthalpies, and free energies of formation for salts containing the NgF+, Ng2F3+, XeF3+, XeF5+, Xe2F11+, and XeOF3+ (Ng = Ar, Kr, Xe) cations were estimated using volume-based thermodynamics (VBT) based on crystallographic and estimated ion volumes. These estimated parameters were then used to predict the stabilities of noble-gas salts. VBT is used to examine and predict the stabilities of, inter alia, the salts [XeFm][SbnF5n+1] and [XeFm][AsnF5n+1] (m = 1, 3; n = 1, 2). VBT also confirms that XeF+ salts are stable toward redox decomposition to Ng, F2, and MF5 (M = As, Sb), whereas the isolable krypton compounds and the unknown ArF+ salts are predicted to be unstable by VBT with the ArF+ salts being the least stable.