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 MoO₂F₂.2NaF.½H₂O 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.