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Chromium tetrafluoride and related compounds. Sadana, Yoginder Nath

Abstract

The present investigation was concerned with a study of the preparation and properties of chromium tetrafluoride and its complexes; magnetic properties and X-ray structures were also studied. Attempts to prepare the pentafluoride of chromium using fluorinating agents other than fluorine were unsuccessful. Chromium tetrafluoride was prepared by the action of fluorine on the heated metal, the products being involatile chromium trifluoride, volatile chromium tetrafluoride, and more volatile fluorides which were condensed at -78°C. The best yields of chromium tetrafluoride were obtained at 350°C and with a moderate flow-rate of fluorine. Chromium tetrafluoride is a glassy solid which forms blue vapours when heated. It does not dissolve in the usual organic solvents and is hydrolysed readily in moist air. It is paramagnetic, the moment corresponding to two unpaired electrons, and has a Weiss Constant of -70°. Chemically, chromium tetrafluoride is surprisingly inert and at room temperature does not react with ammonia, pyridine, sulphur dioxide, sulphur trioxide, iodine pentafluoride, bromine trifluoride, bromine pentafluoride, chlorine trifluoride, or selenium tetrafluoride. However, it reacts instantaneously with water, giving Cr⁺³ and CrO₄¯² ions in solution. When heated with bromine trifluoride for longer periods, reaction occurs to give CrF₃.0.5BrF₃. On heating with a mixture of bromine trifluoride and bromine pentafluoride, a wine-coloured solution was obtained, and the residue after distillation of volatile materials was CrOF₃.0.25BrF₃. The reaction of selenium tetraf luoride with chromium tetrafluoride at 120°C yielded two compounds, CrF₃SeF₄ and CrF₂.SeF₄. Complexes of the type ACrF₅, where A = K, Rb, or Cs, were prepared for the first time, by heating the respective alkali metal chloride and chromium tetrafluoride in a 1:1 molar ratio in bromine trifluoride. The resulting complex compounds contained half a molecule of bromine trifluoride per molecule of the complex, if the removal of excess bromine trifluoride, after the reaction, was carried out at 100°C. However, if removal was carried out at 160°C, the pure complexes were obtained. Complexes of the type A₂CrF₆, where A = K or Cs, were prepared in bromine trifluoride solutions. The presence of extra bromine trifluoride in the molecule of the complex compound affected the structure seriously in potassium complexes but had no effect on cesium complexes. Thus K₂CrF₆0.5BrF₃ was tetragonal and on heating at 160°C in, vacuo yielded the cubic modification of pure K₂CrF₆; Cs₂CrF₆.0.5BrF₃and Cs₂CrF₆ were both cubic. Reactions of chromium trioxide with bromine trifluoride, bromine pentafluoride, and chlorine trifluoride were also investigated. With bromine fluorides, the products obtained were the corresponding adducts of CrOF₃. These are very reactive and sensitive to moist air. The compounds are paramagnetic. The reaction with chlorine trifluoride was very interesting because the appearance of the product depended on/ the experimental conditions. If the reaction was carried out at ordinary temperature, the product was a buff-coloured powdery mass, but if the reaction was performed by passing chlorine trifluoride vapour over heated chromium trioxide (100—120°C), the product was a brick-red substance. Chemical analyses and magnetic measurements indicated that both these products were identical and had the composition CrOF₃.0.25ClF₃. The brick-red product could be converted into the light buff-coloured powdery mass by heating at 70°C in vacuo. Reactions of potassium dichromate with bromine trifluoride, bromine pentafluoride, and chlorine trifluoride were also investigated. Potassium dichromate reacted with these smoothly at room temperature and the product of reaction in each case was KCrOF₄. The compounds investigated in the present research are tabulated below.[ Table omitted ]

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