Thermochemistry of the gaseous tungsten fluorides

Equilibrium and electron impact techniques, both employing mass spectrometry, were used to study the gaseous tungsten fluorides WF2, WF3, WF4, and WF5. The molecular species were generated by the reaction of both WF6 and SF6 with tungsten in an effusion cell at 1000 to 2200 K under demonstrable equilibrium conditions. Reaction enthalpies were derived from second-law analysis of various gaseous equilibria and from electron impact threshold measurements, yielding the standard heats of formation, ΔHf298, of WF2 (−20.6±3.2 kcal/mol); WF3 (−121.2±2.8 kcal/mol); WF4 (−222.0±2.5 kcal/mol); and WF5 (−309.1±2.0 kcal/mol). A lower limit of 92 kcal/mol was obtained for the standard heat of formation of WF. Information about molecular ionization potentials is available from the threshold appearance potentials. Thermochemical data were also obtained for gaseous WSF3, WSF2, and WS2F2. Various aspects of the chemical bonding are discussed.