Molybdenum Pentachloride, MoCl₅

Molybdenum Pentachloride, MoCl₅, is best prepared by gently heating molybdenum in chlorine, when the former glows with production of a reddish vapour which somewhat readily dissociates. On cooling, the vapour yields black deliquescent crystals which melt at 194° C. and the resulting liquid boils at 268° C. The compound is also produced by the interaction of molybdenum trioxide with phosphorus pentachloride.

The pentachloride is a little unstable in air; when heated to about 1330° C. or less it leaves a residue of molybdenum; when heated in hydrogen at 250° C. it is reduced to amorphous molybdenum trichloride. Its aqueous solution is unstable in air, especially on warming, when hydrogen chloride is more rapidly evolved and the blue oxide remains. Decomposition of its solution in hydrochloric acid also readily takes place. In alcohol and ether it dissolves to a green solution; in sulphuric acid its solution is bluish green and in nitric acid colourless; alkalies dissolve it with production of the hydrated dioxide and a molybdate.