Syntheses, Structures, and Bonding of NgF2⋅CrOF4, NgF2⋅2CrOF4 (Ng=Kr, Xe), and (CrOF4)∞

The noble-gas difluoride adducts, NgF₂ ⋅CrOF₄ and NgF₂ ⋅2CrOF₄ (Ng=Kr and Xe), have been synthesized and structurally characterized at low temperatures by Raman spectroscopy and single-crystal X-ray diffraction. The low fluoride ion affinity of CrOF₄ renders it incapable of inducing fluoride ion transfer from NgF₂ (Ng=Kr and Xe) to form ion-paired salts of the [NgF]⁺ cations having either the [CrOF₅ ]^- or [Cr₂ O₂ F₉ ]^- anions. The crystal structures show the NgF₂ ⋅CrOF₄ adducts are comprised of F_t -Ng-F_b - - -Cr(O)F₄ structural units in which NgF₂ is weakly coordinated to CrOF₄ by means of a fluorine bridge, F_b , in which Ng-F_b is elongated relative to the terminal Ng-F_t bond. In contrast with XeF₂ ⋅2MOF₄ (M=Mo or W) and KrF₂ ⋅2MoOF₄ , in which the Lewis acidic, F₄ (O)M- - -F_b - - -M(O)F₃ moiety coordinates to Ng through a single M- - -F_b -Ng bridge, both fluorine ligands of NgF₂ coordinate to CrOF₄ molecules to form F₄ (O)Cr- - -F_b -Ng-F_b - - -Cr(O)F₄ adducts in which both Ng-F_b bonds are only marginally elongated relative to the Ng-F bonds of free NgF₂ . Quantum-chemical calculations show that the Cr-F_b bonds of NgF₂ ⋅CrOF₄ and NgF₂ ⋅2CrOF₄ are predominantly electrostatic with a small degree of covalent character that accounts for their nonlinear Cr- - -F_b -Ng bridge angles and staggered O-Cr- - -F_b -Ng-F_t dihedral angles. The crystal structures and Raman spectra of two CrOF₄ polymorphs have also been obtained. Both are comprised of fluorine-bridged chains that are cis- and trans-fluorine-bridged with respect to oxygen.