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 [Cr2O2F9]− 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.

The reactions of the fluoride‐ion donor, XeF6, with the fluoride‐ion acceptors, M′OF4 (M′=Cr, Mo, W), yield [XeF5]+ and [Xe2F11]+ salts of [M′OF5]− and [M2O2F9]− (M=Mo, W). Xenon hexafluoride and MOF4 react in anhydrous hydrogen fluoride (aHF) to give equilibrium mixtures of [Xe2F11]+, [XeF5]+, [(HF)nF]−, [MOF5]−, and [M2O2F9]− from which the title salts were crystallized. The [XeF5][CrOF5] and [Xe2F11][CrOF5] salts could not be formed from mixtures of CrOF4 and XeF6 in aHF at low temperature (LT) owing to the low fluoride‐ion affinity of CrOF4, but yielded [XeF5][HF2]⋅CrOF4 instead. In contrast, MoOF4 and WOF4 are sufficiently Lewis acidic to abstract F− ion from [(HF)nF]− in aHF to give the [MOF5]− and [M2O2F9]− salts of [XeF5]+ and [Xe2F11]+. To circumvent [(HF)nF]− formation, [Xe2F11][CrOF5] was synthesized at LT in CF2ClCF2Cl solvent. The salts were characterized by LT Raman spectroscopy and LT single‐crystal X‐ray diffraction, which provided the first X‐ray crystal structure of the [CrOF5]− anion and high‐precision geometric parameters for [MOF5]− and [M2O2F9]−. Hydrolysis of [Xe2F11][WOF5] by water contaminant in HF solvent yielded [XeF5][WOF5]⋅XeOF4. Quantum‐chemical calculations were carried out for M′OF4, [M′OF5]−, [M′2O2F9]−, {[Xe2F11][CrOF5]}2, [Xe2F11][MOF5], and {[XeF5][M2O2F9]}2 to obtain their gas‐phase geometries and vibrational frequencies to aid in their vibrational mode assignments and to assess chemical bonding.

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trans-Dihydride complexes are important in many homogeneous catalytic processes. Here vibrational spectroscopy and density functional theory (DFT) methods are used for the first time to reveal that 4d and 5d metals transmit more effectively than the 3d metals influence of the ligand trans to the hydride and also couple the motions of the trans-hydrides more effectively. This property of the metal is linked to higher hydride reactivity. The IR and Raman spectra of trans-FeH2(dppm)2, trans-RuH2(PPh­(OEt)2)4, and mer-IrH3(PiPr2CH2pyCH2PiPr2) provide M–H force constants and H–M–H interaction force constants that increase as FeII < RuII < IrIII. DFT methods are used to determine, for the first time, the effect of the metal ion (MnI, ReI, FeII, RuII, OsII, CoIII, RhIII, IrIII, PtIV) and ligands on the gap in wavenumbers between the symmetric νsym H–M–H and antisymmetric νasym H–M–H vibrational modes of hydrides that are mutually trans in d6 octahedral complexes. The magnitude of this gap reflects the degree of coupling of, or interaction between, these modes, and this is shown to be a distinctive property of the metal ion. The more polarizable 4d and 5d metal ions are found to have an average gap of 246 cm–1, while the 3d metals have only 90 cm–1. This has been verified experimentally for 3d, 4d, and 5d transition-metal trans-dihydrides, where both the IR and Raman spectra have been measured: trans-RuH2(PPh­(OEt)2)4 (from the literature) and trans-FeH2(PPh2CH2PPh2)2 and mer-IrH3(PiPr2CH2pyCH2PiPr2) (this work). Because the 4d and 5d metal ions tend to be better catalysts for the hydrogenation of substrates with polar bonds, this gap may be a fundamental determinant of the kinetic hydricity of the catalyst. Finding the magnitude of this gap and a new estimate of the large hydride trans-effect (Δνt −235 cm–1) allows us to improve the simple equation reported previously, which allows a better estimate of νM–H.

Molten mixtures of XeF6 and CrVIOF4 react by means of F2 elimination to form [XeF5][Xe2F11][CrVOF5]⋅2 CrVIOF4, [XeF5]2[CrIVF6]⋅2 CrVIOF4, [Xe2F11]2[CrIVF6], and [XeF5]2[CrV2O2F8], whereas their reactions in anhydrous hydrogen fluoride (aHF) and CFCl3/aHF yield [XeF5]2[CrV2O2F8]⋅2 HF and [XeF5]2[CrV2O2F8]⋅2 XeOF4. Other than [Xe2F11][MVIOF5] and [XeF5][MVI2O2F9] (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 [Xe2F11]+ salts of the known [CrVOF5]2− and [CrIVF6]2− anions, and the novel [CrV2O2F8]2− anion. A low‐temperature Raman spectroscopic study of an equimolar mixture of solid XeF6 and CrOF4 revealed that [Xe2F11][CrVIOF5] 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 [CrVOF5]2− and [CrV2O2F8]2− anions, where [CrV2O2F8]2− represents a new structural motif among the known oxyfluoro‐anions of Group 6. The X‐ray structures show that [XeF5]+ and [Xe2F11]+ 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.