Molybdenum Trichloride, MoCl₃

Molybdenum Trichloride, MoCl₃, is obtained by reduction of the pentachloride by hydrogen at 250° C.; by passing the vapour of the pentachloride over the heated metal; or by passing a mixture of the pentachloride vapour and carbon dioxide through a heated tube. It is a reddish-brown amorphous substance, stable in air at ordinary temperatures, but on heating in air decomposes, leaving an impure residue of the dichloride.

The trichloride is insoluble in water and alcohol; by boiling water it is decomposed with hydrolysis. A solution may, however, be obtained by electrolysis either of a solution of molybdic anhydride in hydrochloric acid, or of a solution of ammonium molybdate in dilute sulphuric acid, a mercury cathode being employed. The substance is dissolved by sulphuric and by nitric acids, but not by hydrochloric acid. With gaseous ammonia, at 340° C. the compound Mo₂(NH₂)₃Cl₃ is obtained, while at 760° C. the nitride Mo₃N₂ is produced. A solution of ammonia gives a black, easily oxidisable compound MoNH₄O₄, while with liquid ammonia the products are Mo₂(NH₂)₃Cl₃.10NH₃ and Mo₂(NH₂)₃Cl₃.

A hydrate of molybdenum trichloride has been stated to exist. By the action of potassium amalgam on a solution of molybdic acid in excess of hydrochloric acid, garnet-red, soluble prisms of the double chloride 3KCl.MoCl₃.2H₂O are obtained. Similar double salts, of composition K₃MoCl₆ and 2(NH₄)₃MoCl₆.5H₂O, have been prepared by electrolysing strongly acid solutions of molybdic acid, using a mercury cathode, adding the alkali chloride to the resulting purple-red solution, and evaporating. The compounds Rb₂MoCl₅.H₂O, (NH₄)₂MoCl₅.H₂O, and Cs₂MoCl₅.H₂O have been prepared, and the last named has been shown to exist in three different crystalline modifications.