[XeOXeOXe]2+, the Missing Oxide of Xenon(II); Synthesis, Raman Spectrum, and X-ray Crystal Structure of [XeOXeOXe][μ-F(ReO2F3)2]2
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The [XeOXeOXe](2+) cation provides an unprecedented example of a xenon(II) oxide and a noble-gas oxocation as well as a rare example of a noble-gas dication. The [XeOXeOXe](2+) cation was synthesized as its [μ-F(ReO2F3)2](-) salt by reaction of ReO3F with XeF2 in anhydrous HF at -30 °C. Red-orange [XeOXeOXe][μ-F(ReO2F3)2]2 rapidly decomposes to XeF2, ReO2F3, Xe, and O2 when the solid or its HF solutions are warmed above -20 °C. The crystal structure of [XeOXeOXe][μ-F(ReO2F3)2]2 consists of a planar, zigzag-shaped [XeOXeOXe](2+) cation (C2h symmetry) that is fluorine bridged through its terminal xenon atoms to two [μ-F(ReO2F3)2](-) anions. The Raman spectra of the natural abundance and (18)O-enriched [XeOXeOXe](2+) salts are consistent with a centrosymmetric (C2h) cation geometry. A proposed reaction pathway leading to [XeOXeOXe][μ-F(ReO2F3)2]2 consists of a series of oxygen/fluorine metathesis reactions that are presumably mediated by the transient HOXeF molecule. Quantum-chemical calculations were used to aid in the vibrational assignments of [Xe(16/18)OXe(16/18)OXe][μ-F(Re(16/18)O2F3)2]2 and to assess the bonding in [XeOXeOXe](2+) by NBO, QTAIM, ELF, and MEPS analyses. Ion pair interactions occur through Re-Fμ---Xe bridges, which are predominantly electrostatic in nature and result from polarization of the Fμ-atom electron densities by the exposed core charges of the terminal xenon atoms. Each xenon(II) atom is surrounded by a torus of xenon valence electron density comprised of the three valence electron lone pairs. The positive regions of the terminal xenon atoms and associated fluorine bridge bonds correspond to the positive σ-holes and donor interactions that are associated with "halogen bonding".
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