Vanadium Oxytrichloride, VOCl₃

Vanadium Oxytrichloride, VOCl₃, commonly known as "vanadyl chloride," is the easiest to prepare of all the halogen or oxyhalogen compounds of vanadium. It distils over as a yellow liquid when vanadium pentoxide is strongly heated, or when vanadiun trioxide is gently heated, in a current of chlorine. Addition of charcoal accelerates the reactions:

1. 2V₂O₅ + 6Cl₂ = 4VOCl₃ + 3O₂.
2. 3V₂O₃ + 6Cl₂ = 4VOCl₃ + V₂O₅.

Instead of using chlorine, gaseous hydrogen chloride may be employed, provided that a strong dehydrating agent, phosphorus pentoxide or zinc chloride, is present to remove the water formed and so stop the right-to-left reaction in

V₂O₅ + 6HCl2 ⇔ VOCl₃ + 3H₂O.

Halogenation in the presence of sulphur, or by means of sulphur halides, is also available. When chlorine is conducted into a mixture of vanadium pentoxide and sulphur, or when powdered vanadium pentoxide is treated with sulphur monochloride vapours, an immediate reaction sets in, with formation of vanadium oxytrichloride. A quantitative yield of this compound is also obtained when vanadium trichloride is heated in oxygen at 500° to 600° C.

Vanadium oxytrichloride is a light yellow, mobile, transparent liquid, which boils at 127.15° C at 764.5 mm. Other determinations gave the b.pt. As 126.7° C. at 767 mm. And 124.4° C. at 723 mm. The density has been found to be 1.8656 at 0° C., or 1.8362 at 15.5° C., compared with water at 4° C. The expansion on being heated from 0° to 130° C. has been measured. The oxytrichloride remains a liquid at -15° C., and its vapour density at 186° C. is 6.108 (air = 1), showing it to be undecomposed. It is not decomposed on prolonged boiling with metallic sodium, potassium, or magnesium. On exposure to damp air, or on being treated with a small quantity of water, it displays the usual unstable character of the pentavalent vanadium halides in that it undergoes rapid hydrolysis and thereby becomes coated with red flakes of vanadium pentoxide:

2VOCl₃ + 3H₂O = V₂O₅ + 6HCl.

With more water, it passes into solution; dilute solutions, which are brownish-yellow or red, according to the concentration, evolve chlorine and become blue on standing, undergoing reduction to the tetravalent state. On evaporation to dryness, all the chlorine is evolved and the residue consists of the pentoxide. Vanadium oxytrichloride is also soluble in ether and alcohol. It dissolves chlorine, bromine, iodine, yellow phosphorus, and sulphur, and is miscible with various liquid hydrocarbons and chlorinated hydrocarbons; its use as an industrial solvent has therefore been suggested.

Addition of pyridine hydrochloride to an alcoholic solution of vanadium oxytrichloride yields an addition compound, VOCl₃.C₅H₅N.HCl.C₂H₅OH. On being brought into reaction with cupric oxide, a dark green copper chlorovanadate is formed, Cu(VO₃)₂.CuCl; litharge similarly yields a brick-red lead chlorovanadate, Pb(VO₃)₂.PbCl₂. From the instability of these compounds towards water it is assumed that the chlorine atoms are directly attached to the vanadium atom. The copper salt is, therefore, formulated thus: . The reaction with magnesium oxide proceeds differently and gives magnesium hexavanadate, Mg₂V₆O₁₇.19H₂O.