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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.
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