Vanadous Chloride, VCl₃

Vanadous Chloride, vanadium trichloride, VCl₃, is obtained by the action of hydrogen chloride on finely divided vanadium at 300° to 400° C., or by heating vanadium tetrachloride to 140° C. in a current of carbon dioxide, which removes the chlorine formed at the same time. It can be conveniently made also by boiling vanadium oxy-trichloride, VOCl₃, vanadium tetrachloride, VCl₄, or a mixture of both, with sulphur, under reflux. The reactions involved are:

1. 2VCl₄ + 2S = 2VCl₃ + S₂Cl₂.
2. 2VOCl₃ + S = 2VCl₃ + SO₂.

In the former case the sulphur chloride is distilled off, and in both cases the excess of sulphur is removed by distillation in an atmosphere of carbon dioxide or nitrogen.

Anhydrous vanadium trichloride is a crystalline solid of the colour of peach blossom. It is extremely hygroscopic, deliquescing to a brown liquid. It gives green solutions in alcohol and ether. Density at 18° C., 3.00. The absorption spectrum has been studied. On being strongly heated in hydrogen it is reduced to vanadium dichloride, VCl₂, and ultimately to the metal. Tin also reduces it to the dichloride. On being heated in a current of nitrogen to a dark red heat it is decomposed into the tetrachloride, which distils off, and the dichloride, which remains behind:

2VCl₃ = VCl₄ + VCl₂.

The following change also takes place: -

2VCl₄- 2VCl₃ + Cl₂,

so that the quantity of tetrachloride produced is determined for each temperature by the partial pressure of the chlorine. If the trichloride is heated in a current of chlorine which is free from oxygen, complete conversion into the tetrachloride ensues.

The hexahydrate, VCl₃.6H₂O, is prepared by dissolving vanadous hydroxide, V(OH)₃, in hydrochloric acid in the absence of air, or by the electrolytic reduction of a solution of vanadium pentoxide, V₂O₅, in hydrochloric acid. It is a green, crystalline compound, very hygroscopic, and readily soluble in water, alcohol, or ether. On being heated it decomposes before all its water has been driven off. Its aqueous solutions are brown or yellow, according to the concentration; on addition of acid the solution assumes the green colour characteristic of solutions of trivalent vanadium salts, and as oxidation to the tetravalent state takes place the solution becomes blue. When hydrated vanadium trichloride and rubidium chloride are together dissolved in water, in the requisite proportions, and the solution is saturated with hydrochloric acid and concentrated, crystals of a double salt, VCl₃.2RbCl.H₂O, are obtained. The corresponding ammonium, potassium and caesium double salts have been prepared. The magnesium double chloride has the composition VCl₃.MgCl₂-H₂O; another double potassium salt, VCl₃.KCl, is known. Addition of silver nitrate gives a precipitate which consists of a mixture of metallic silver and silver chloride; with thallium sulphate, however, a precipitate of thallium chloride is thrown down, from which it can be inferred that vanadium trichloride undergoes normal ionisation in solution, as in the case of the corresponding violet chromium chloride, CrCl₃, and ferric chloride, FeCl₃.

The similarity between the trichlorides of iron and vanadium is further shown in their behaviour towards ammonia. At ordinary temperatures ammonia reacts with vanadium trichloride to produce vanadium nitride, VN, and ammonium chloride, but if liquid ammonia is poured over vanadium trichloride, a quantitative yield of a reddish-brown salt, [V(NH₃)₆]Cl₃, is obtained, which recalls the hexammines of cobalt and chromium. The ammonia molecules are not, however, very firmly held, and the compound is, therefore, more comparable with the corresponding hexammine of ferric chloride [Fe(NH₃)₆]Cl₃.

Hexammino-vanadium trichloride is insoluble in water, alcohol, and ether. On being exposed to dry air it decomposes slowly and becomes white:

[V(NH₃)₆]Cl₃ + O + 2H₂O = NH₄VO₃ + 3NH₄Cl + 2NH₃.

On being exposed to damp air it slowly becomes green, due to the formation of the hexa-aquo-vanadium salt, [V(H₂O)₆]Cl₃. It is decomposed by boiling water,

[V(NH₃)₆]Cl₃ + 8H₂O = V(OH)₃ + 8NH₃ + 8NH₄Cl,

and dissolves in dilute hydrochloric acid to give a solution which possesses the green colour characteristic of trivalent vanadium salts. By the action of nitric acid a quantitative yield of hexammino-vanadium nitrate, [V(NH₃)₆](NH₃)₃, is obtained.