Thermochemical and related properties of halides of refractory metals.

The basis of this thermo chemical study has been the determination of enthalpies of formation from measured enthalpies of hydrolysis. The enthalpies of formation of the gaseous hexafluorides of tungsten, molybdenum and rhenium, the solid pentafluorides of tungsten and molybdenum and the solid oxytetrafluorides of tungsten and molybdenum have been determined as -411.85 1.4, -374.8 2.5, -322.6 3.2, -349.9 3.5, -331.4 1.3, -358.5 2.2, and -329.8 1.2 kcal mol-1 respectively. Bond strengths and stabilities have been discussed. The enthalpies of formation of the salts KWVF6, KMoVF6 and K2WIVCl6 have been determined -532.0 2.6, -493.25 1.9 and -329.9 1.8 kcal mol-1 respectively. From the enthalpies of formation, the first electron affinities of tungsten and molybdenum hexafluorides and the second electron affinity of tungsten hexachloride have been estimated as 117.9 4, 115.4 4 and -52.0 kcal mol-1 respectively (at 0 K). The fluoride ion affinities of solid tungsten and molybdenum pentafluorides have also been estimated as 119.1 6 and 98.1 6 kcal mol-1 respectively (at 298 K). The use of fluorides in tungsten halogen incandescent lamps has been investigated. Using infra-red spectroscopy, it has been found that all the covalent fluorides which were investigated formed silicon tetrafluoride in small quartz envelope in candlescent lamps. Radiochemical tracer experiments in twin filament lamps have shown that tungsten is transferred from a cold (non-running) filament to a hot (running) filament in the presence of a suitable fluoride. The distribution of the transferred tungsten has been measured and compared with transfer in the presence of a bromide. The hypothesis that the presence of fluorine in a lamp leads to the setting up of a regenerative cycle which minimizes the rate of formation of "hot spots" and hence extends filament life is supported. Preliminary attempts to prepare mixed bromide fluorides of tungsten (VI) have shown N.M.R. evidence for tungsten (VI) bromide pentafluoride.