Syntheses, Structures, and Bonding of NgF ⋅CrOF , NgF ⋅2CrOF (Ng=Kr, Xe), and (CrOF ).

The noble-gas difluoride adducts, NgF ⋅CrOF and NgF ⋅2CrOF (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 CrOF renders it incapable of inducing fluoride ion transfer from NgF (Ng=Kr and Xe) to form ion-paired salts of the [NgF] cations having either the [CrOF ] or [Cr O F ] anions. The crystal structures show the NgF ⋅CrOF adducts are comprised of F -Ng-F - - -Cr(O)F structural units in which NgF is weakly coordinated to CrOF by means of a fluorine bridge, F , in which Ng-F is elongated relative to the terminal Ng-F bond. In contrast with XeF ⋅2MOF (M=Mo or W) and KrF ⋅2MoOF , in which the Lewis acidic, F (O)M- - -F - - -M(O)F moiety coordinates to Ng through a single M- - -F -Ng bridge, both fluorine ligands of NgF coordinate to CrOF molecules to form F (O)Cr- - -F -Ng-F - - -Cr(O)F adducts in which both Ng-F bonds are only marginally elongated relative to the Ng-F bonds of free NgF . Quantum-chemical calculations show that the Cr-F bonds of NgF ⋅CrOF and NgF ⋅2CrOF are predominantly electrostatic with a small degree of covalent character that accounts for their nonlinear Cr- - -F -Ng bridge angles and staggered O-Cr- - -F -Ng-F dihedral angles. The crystal structures and Raman spectra of two CrOF polymorphs have also been obtained. Both are comprised of fluorine-bridged chains that are cis- and trans-fluorine-bridged with respect to oxygen.