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Molybdenum Hexafluoride, MoF6

Molybdenum Hexafluoride, MoF6, the only fluoride known with certainty to exist, is best prepared by the action of fluorine on the finely divided metal at 60° to 70° C., the product being collected in a vessel cooled by a mixture of solid carbon dioxide and alcohol. It forms white crystals which melt at 17° C., the boiling-point of the liquid being 35° C. It is decomposed by water, yielding the blue oxide, but does not react with chlorine, sulphur dioxide, or dry air; it is absorbed by alkali or ammonium hydroxides, and forms double salts with alkali fluorides. It also reacts with ammonia, with production of a brown powder.

Interaction of molybdenum pentachloride and anhydrous hydrogen fluoride yields a fluoride of molybdenum, while the existence of fluorides of the formulae MoF3 and MoF4 has been affirmed. Double salts of the formulae KMoF4.H2O, (NH4)MoF4.H2O, K3Mo2F9.2H2O, and (NH4)3Mo2F9.2H2O have been described.

Two oxyfluorides, respectively MoOF4 and MoO2F2, have been prepared by the action of anhydrous hydrogen fluoride upon the corresponding oxychlorides in a platinum vessel surrounded by a freezing mixture. The former compound, MoOF4, which distils at 230° C., is a white hygroscopic solid, of melting-point 97° C. and boiling-point 180° C.; in air it decomposes, simultaneously becoming blue in colour. The latter compound, MoO2F2, which sublimes at 265° to 270° C., forms white hygroscopic crystals of density 3.494; with a little water it turns blue in colour, but with much water the mixture is colourless. Heated in air, it decomposes, evolving hydrogen fluoride and leaving a residue of molybdic anhydride. An alternative method of preparation of the oxyfluoride MoO2F2 consists in heating together molybdic anhydride and lead fluoride, cryolite, or other metallic fluoride in a stream of carbon dioxide or oxygen. Compounds of this oxyfluoride with metallic fluorides ("fluoroxymolybdates") are known.

With sodium, the compound MoO2F2.2NaFH2O is formed as a crystalline salt by treating the normal molybdate with a slight excess of hydrofluoric acid. The potassium salts which have been prepared are MoO2F2.2KF.H2O and MoO2F2.KF.H2O, while to other compounds the following formulae have been assigned: MoOF3.2KF.H2O and 3MoOF3.5KF.H2O. With rubidium, the compound MoO2F2.RbF.2H2O has been described, as also have the following compounds of ammonium: MoO2F2.2NH4F; MoO2F2.NH4F; MoO2F2.NH4F.H2O; 3MoO2F2.5NH4F.H2O; MoO2F2.3NH4F; MoO2F2.4NH4F.(NH4)2MoO4; MoO3.2NH4F; MoOF3.2NH4F; 3MoOF3.5NH4F.H2O.
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