Chromium(VI) Oxyfluoride Dianions, [Cr₂O₄F₆]²⁻ and [CrO₂F₄]²⁻; Syntheses and Structures of [XeF₅]₂[Cr₂O₄F₆], [XeF₅]₂[Cr₂O₄F₆] ⋅ nX (X=HF, n=4; X=XeOF₄, n=2), and [XeF₅][Xe₂F₁₁][CrO₂F₄]

Abstract

The fluorobasic character of the strong oxidative fluorinator, XeF6, and the oxidative resistance of the [XeF5]+ and [Xe2F11]+ cations have been exploited for the syntheses of several novel Cr(VI) dianions. The reactions of XeF6 and CrO2F2 in anhydrous HF and in melts by direct fusion of the reactants yielded the first dinuclear Cr(VI) oxyfluoro‐dianion salts, [XeF5]2[Cr2O4F6], [XeF5]2[Cr2O4F6]∙4HF, [XeF5]2[Cr2O4F6]∙2XeOF4, and mononuclear Cr(VI) oxyfluoro‐dianion salt, [XeF5][Xe2F11][CrO2F4]. The salts were structurally characterized by low‐temperature (LT) single‐crystal X‐ray diffraction (SCXRD) and LT Raman spectroscopy. The [CrO2F4]2− and [Cr2O4F6]2− dianions have distorted octahedral cis‐dioxo Cr(VI) coordination spheres in which two F‐atoms are trans to one another and two F‐atoms are trans to O‐atoms. The [Cr2O4F6]2− dianion is the fluorine‐bridged dimer of the [CrO2F3]− anion. Quantum‐chemical (QC) calculations were used to obtain the energy‐minimized, gas‐phase geometries, and the calculated vibrational spectra of the gas‐phase dianions and their ion‐pairs which were used to aid in the vibrational frequency assignments of the crystalline salts. NBO and MEPS analyses and SCXRD show these salts are intimate ion‐pairs in which their cations and anions interact through primarily electrostatic Xe‐‐‐F σ‐hole bonds.