A Raman spectroscopic study of the XeOF4/XeF2 system and the X-ray crystal structure of α-XeOF4·XeF2

The mixed oxidation state complexes, α-XeOF4·XeF2 and β-XeOF4·XeF2, result from the interaction of XeF2 with excess XeOF4. The X-ray crystal structure of the more stable α-phase shows that the XeF2 molecules are symmetrically coordinated through their fluorine ligands to the Xe(VI) atoms of the XeOF4 molecules which are, in turn, coordinated to four XeF2 molecules. The high-temperature phase, β-XeOF4·XeF2, was identified by low-temperature Raman spectroscopy in admixture with α-XeOF4·XeF2; however, the instability of the β-phase precluded its isolation and characterization by single-crystal X-ray diffraction. The Raman spectrum of β-XeOF4·XeF2 indicates that the oxygen atom of XeOF4 interacts less strongly with the XeF2 molecules in its crystal lattice than in α-XeOF4·XeF2. The 19F and 129Xe NMR spectra of XeF2 in liquid XeOF4 at −35°C indicate that any intermolecular interactions that exist between XeF2 and XeOF4 are weak and labile on the NMR time scale. Quantum-chemical calculations at the B3LYP and PBE1PBE levels of theory were used to obtain the gas-phase geometries and vibrational frequencies as well as the NBO bond orders, valencies, and NPA charges for the model compounds, 2XeOF4·XeF2, and XeOF4·4XeF2, which provide approximations of the local XeF2 and XeOF4 environments in the crystal structure of α-XeOF4·XeF2. The assignments of the Raman spectra (−150°C) of α- and β-XeOF4·XeF2 have been aided by the calculated vibrational frequencies for the model compounds. The fluorine bridge interactions in α- and β-XeOF4·XeF2 are among the weakest for known compounds in which XeF2 functions as a ligand, whereas such fluorine bridge interactions are considerably weaker in β-XeOF4·XeF2.