Vibrational spectra of some oxide halides of the transition elements with particular reference to gas-phase and single-crystal Raman spectroscopy

Abstract

The following species have been examined in the Raman effect: (s = solid, l = melt, g = gas): VOCl₃(g), VOF₃(s), NbOCl₃(g,s), MoO₂Cl₂(g,l,s), WO₂Cl₂(s), MoO₂F₂(s), WOCl₄(g,s), MoOF₄(l,s), WOF₄(l,s), and OsO₃F₂(s). The results for the gaseous phase in all cases support the presence of principally monomeric species. In the solid state single-crystal Raman measurements on NbOCl₃ suggest that the space group is P4₂nm not P4₂/mnm as suggested from X-ray diffraction studies. Single-crystal Raman data are also reported for WOCl₄. In the case of the solid-state data a structure based on a six-co-ordinate central atom is satisfactory for NbOCl₃(with an asymmetrical Nb–O–Nb bridge). For MoO₂Cl₂ a structure analogous to that of WO₂Cl₂ is satisfactory. MoO₂F₂ can be satisfactorily interpreted as based on eight-co-ordinate molybdenum with all atoms bridging. For MoOF₄ and WOF₄ it is suggested that the compounds are fluorine-bridged (in disagreement with results obtained from X-ray data for WOF₄). For OsO₃F₂ the data are not inconsistent with six-co-ordinate osmium with cis-fluorine bridges, a terminal Os–F link and a C_3v OsO₃ residue. In the melt, MoO₂Cl₂ appears to be molecular while with MoOF₄ and WOF₄ there is evidence for terminal oxygen atoms, with presumably cis-fluorine bridges.