Chromium Oxide Tetrafluoride and Its Reactions with Xenon Hexafluoride; the [XeF5]+ and [Xe2F11]+ Salts of the [CrVIOF5]−, [CrVOF5]2−, [CrV2O2F8]2−, and [CrIVF6]2− Anions

Molten mixtures of XeF₆ and Cr^VI OF₄ react by means of F₂ elimination to form [XeF₅ ][Xe₂ F₁₁ ][Cr^V OF₅ ]⋅2 Cr^VI OF₄ , [XeF₅ ]₂ [Cr^IV F₆ ]⋅2 Cr^VI OF₄ , [Xe₂ F₁₁ ]₂ [Cr^IV F₆ ], and [XeF₅ ]₂ [Cr^V ₂ O₂ F₈ ], whereas their reactions in anhydrous hydrogen fluoride (aHF) and CFCl₃ /aHF yield [XeF₅ ]₂ [Cr^V ₂ O₂ F₈ ]⋅2 HF and [XeF₅ ]₂ [Cr^V ₂ O₂ F₈ ]⋅2 XeOF₄ . Other than [Xe₂ F₁₁ ][M^VI OF₅ ] and [XeF₅ ][M^VI ₂ O₂ F₉ ] (M=Mo or W), these salts are the only Group 6 oxyfluoro-anions known to stabilize noble-gas cations. Their reaction pathways involve redox transformations that give [XeF₅ ]⁺ and/or [Xe₂ F₁₁ ]⁺ salts of the known [Cr^V OF₅ ]^2- and [Cr^IV F₆ ]^2- anions, and the novel [Cr^V ₂ O₂ F₈ ]^2- anion. A low-temperature Raman spectroscopic study of an equimolar mixture of solid XeF₆ and CrOF₄ revealed that [Xe₂ F₁₁ ][Cr^VI OF₅ ] is formed as a reaction intermediate. The salts were structurally characterized by LT single-crystal X-ray diffraction and LT Raman spectroscopy, and provide the first structural characterizations of the [Cr^V OF₅ ]^2- and [Cr^V ₂ O₂ F₈ ]^2- anions, where [Cr^V ₂ O₂ F₈ ]^2- represents a new structural motif among the known oxyfluoro-anions of Group 6. The X-ray structures show that [XeF₅ ]⁺ and [Xe₂ F₁₁ ]⁺ form ion pairs with their respective anions by means of Xe- - -F-Cr bridges. Quantum-chemical calculations were carried out to obtain the energy-minimized, gas-phase geometries and the vibrational frequencies of the anions and their ion pairs and to aid in the assignments of their Raman spectra.