XeOF3−, an Example of an AX3YE2Valence Shell Electron Pair Repulsion Arrangement; Syntheses and Structural Characterizations of [M][XeOF3] (M = Cs, N(CH3)4)
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The XeOF(3)(-) anion has been synthesized as its Cs(+) and N(CH(3))(4)(+) salts and structurally characterized in the solid state by low-temperature Raman spectroscopy and quantum-chemical calculations. Vibrational frequency assignments for [Cs][XeOF(3)] and [N(CH(3))(4)][XeOF(3)] were aided by (18)O enrichment. The calculated anion geometry is based on a square planar AX(3)YE(2) valence-shell electron-pair repulsion arrangement with the longest Xe-F bond trans to the oxygen atom. The F-Xe-F angle is bent away from the oxygen atom to accommodate the greater spatial requirement of the oxygen double bond domain. The experimental vibrational frequencies and trends in their isotopic shifts are reproduced by the calculated gas-phase frequencies at several levels of theory. The XeOF(3)(-) anion of the Cs(+) salt is fluorine-bridged in the solid state, whereas the anion of the N(CH(3))(4)(+) salt has been shown to best approximate the gas-phase anion. Although [Cs][XeOF(3)] and [N(CH(3))(4)][XeOF(3)] are shock-sensitive explosives, the decomposition pathways for the anions have been inferred from their decomposition products at 20 degrees C. The latter consist of XeF(2), [Cs][XeO(2)F(3)], and [N(CH(3))(4)][F]. Enthalpies and Gibbs free energies of reaction obtained from Born-Fajans-Haber thermochemical cycles support the proposed decomposition pathways and show that both disproportionation to XeF(2), [Cs][XeO(2)F(3)], and CsF and reduction to XeF(2), CsF, and O(2) are favorable for [Cs][XeOF(3)], while only reduction to XeF(2) accompanied by [N(CH(3))(4)][F] and O(2) formation are favorable for [N(CH(3))(4)][XeOF(3)]. In all cases, the decomposition pathways are dominated by the lattice enthalpies of the products.
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