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UBC Theses and Dissertations

The fluorides of vanadium Cavell, Ronald George

Abstract

The density, surface tension and viscosity of liquid vanadium pentafluoride have been measured. The high value of the viscosity indicates that the liquid is probably associated in a somewhat similar manner to antimony pentafluoride, thus supporting recent evidence which has suggested that inter-molecular association is also an important process in these associated fluorides The infrared spectrum of vanadium pentafluoride vapor has been measured in the region 250 to 3500 cm- ¹ and the results have been interpreted in terms of a monomeric trigonal bipyramid molecular structure in view of the normal vapor density Vanadium pentafluoride formed a 1:1 complex with selenium tetrafluoride The apparently similar sulphur tetrafluoride complex was extremely unstable Vanadium tetrafluoride is best prepared by fluorinatmg vanadium tetrachloride with anhydrous hydrogen fluoride in trichlorofluoromethane solution Vanadium tetrafluoride disproportionates readily at 100° in vacuum into the trifluoride and the pentafluoride Solid vanadium tetrafluoride also sublimes slowly at 100-120° in vacuum The available structural information suggests that in vanadium tetrafluoride the vanadium atom is surrounded by six fluorine atoms to form an octahedral VF₆ unit Four fluorines are shared with adjacent vanadium atoms thus forming a polymeric fluorine -bridge-bonded structure The infrared spectrum of solid VF₄ has been interpreted in terms of this model Bromine trifluoride and gaseous fluorine readily fluorinated vanadium tetrafluoride to the penta-fluoride In the presence of iodine pentafluoride, nitryl fluoride oxidised vanadium tetrafluoride and formed the nitryl salt NO₂VF₆. Ammonia, pyridine and selenium tetrafluoride formed 1.1 complexes with VF₄ Vanadium tetrafluoride did not react with sulphur tetrafluoride, sulphur trioxide or sulphur dioxide Potassium hexafluorovanadate (IV) was prepared frpm potassium fluoride and vanadium tetrafluoride in selenium tetrafluoride solution. The trigonal form was obtained with lattice constants of a = 5 68, c = 4 66 A. Alkaline earth fluorides did not form hexafluorovanadate salts with vanadium tetrafluoride in iodine pentafluoride KVF₅ could not be prepared from equimolar proportions of potassium fluoride and vanadium tetrafluoride in iodine pentafluoride solution All of the tetravalent vanadium fluoride compounds which have been studied obeyed the Curie-Weiss law, with very high values of the Weiss Constant Separation of antiferromagnetism and spm-orbit interaction is not possible as both effects are likely to arise from the proposed fluorine bridging The heat of hydrolysis of vanadium tetrafluoride in water has been found to be -27 5 kcal /mole, and this value was used in a Hess law calculation to obtain -332 kcal /mole for the heat of formation of vanadium tetrafluoride Vanadium pentafluoride was hydrolysed under similar conditions in a dilute alkali solution and the resultant heat of hydrolysis, -141 kcal /mole, was used to calculate the heat of formation of - 352 kcal /mole for vanadium pentafluoride. Lattice energies were estimated from a simple formula given by Kapustmskii and used in a Born-Haber cycle to calculate heats of formation Using the calculated heat of formation of vanadium trifluoride and the experimental values for vanadium tetrafluoride and vanadium pentafluoride, the spontaneity of the disproportionation of vanadium tetrafluoride was confirmed. The heat of hydrolysis of vanadium tetrachloride in water is -68 8 kcal /mole With this value, the heat of formation of the aqueous vanadyl ion was calculated as -113 kcal /mole

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