Phosphorus Pentafluoride, PF₅

This compound was discovered by Thorpe, who prepared it by the reaction between AsF₃ and PCl₅:—

5AsF₃ + 3PCl₅ = 3PF₅ + 5AsCl₃

The same method and precautions may be adopted as already described for the trifluoride. The first condenser is, however, kept at about -60° C. The pentafluoride can also be made by heating PCl₅ with PbF₂, or by combining PF₃ with Br₂ and warming the product:—

5PF₃Br₂ = 3PF₅ + 2PBr₅

The pentabromide being a solid is easily separated, and it only remains to free the gas from a small quantity of bromine by allowing it to stand over mercury. It can be made similarly from PF₃Cl₂. It is produced, together with some PF₃, by the action of fluorine upon phosphorus.

Phosphorus pentafluoride is a heavy colourless gas with an unpleasant smell; it strongly attacks the skin and respiratory tract. Unlike the trifluoride it fumes strongly in air and is rapidly absorbed by water. It neither burns nor supports combustion. It does not attack dry glass at room temperatures. The density is 4.5 (air = l) according to Thorpe and Moissan, and the formula is established from this and the analysis. It condenses to a colourless liquid at -75° C. and freezes to a white solid at -83° C.

This compound is by far the most stable pentahalide of phosphorus. It is not dissociated by moderate heat or weak sparks from an induction coil, but on strong sparking it gives PF₃ and F₂, the latter attacking the glass. The small extent of the dissociation is shown by the fact that the pentafluoride does not react with isoamyl alcohol at the boiling-point of the latter. It is not attacked by oxygen, fluorine or iodine. With dry ammonia it forms an ammine, PF₅.5NH₃, and with N₂O₄ an addition compound PF₅.N₂O₄. It is completely hydrolysed by water (or alkalies) giving phosphoric and hydrofluoric acids (or their salts). It is not reduced by heating with phosphorus or sulphur.