Syntheses, Structures, and Bonding of NgF
(Ng=Kr, Xe), and (CrOF
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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 Ft -Ng-Fb - - -Cr(O)F4 structural units in which NgF2 is weakly coordinated to CrOF4 by means of a fluorine bridge, Fb , in which Ng-Fb is elongated relative to the terminal Ng-Ft bond. In contrast with XeF2 ⋅2MOF4 (M=Mo or W) and KrF2 ⋅2MoOF4 , in which the Lewis acidic, F4 (O)M- - -Fb - - -M(O)F3 moiety coordinates to Ng through a single M- - -Fb -Ng bridge, both fluorine ligands of NgF2 coordinate to CrOF4 molecules to form F4 (O)Cr- - -Fb -Ng-Fb - - -Cr(O)F4 adducts in which both Ng-Fb bonds are only marginally elongated relative to the Ng-F bonds of free NgF2 . Quantum-chemical calculations show that the Cr-Fb bonds of NgF2 ⋅CrOF4 and NgF2 ⋅2CrOF4 are predominantly electrostatic with a small degree of covalent character that accounts for their nonlinear Cr- - -Fb -Ng bridge angles and staggered O-Cr- - -Fb -Ng-Ft 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.
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