Boron sesquisulphide, B₂S₃

Boron sesquisulphide (boron trisulphide), B₂S₃, is produced when boron is heated to bright redness in sulphur vapour, or when boron is heated to redness in a current of hydrogen sulphide. It may also be prepared by heating to redness in a current of carbon disulphide vapour an intimate mixture of boron sesqui-oxide and carbon. Other reactions which lead to the formation of the sulphide are the action of sulphur upon boron tri-iodide and the action of carbon disulphide and various other sulphides on boron (Moissan). The pure sesquisulphide is best prepared by heating pure boron in a stream of hydrogen sulphide largely diluted with hydrogen, and a cheap method for its preparation consists in heating commercial iron boride or manganese boride to 300°-400° in hydrogen sulphide and washing away admixed sulphur from the product with carbon disulphide. Only part of the boron in these borides can be removed as the sulphide.

The properties of boron sesquisulphide have been studied mainly by Moissan. It forms fine, white needles, of density 1.55, which melt at 310° and volatilise without decomposition when heated in a current of hydrogen. It is unacted upon at a red heat by hydrogen, nitrogen, phosphorus, carbon, and silicon. It inflames in chlorine in the cold, in bromine vapour when warmed, and in oxygen when heated to dull redness; in each case both the boron and sulphur are converted into chloride, bromide, or oxide. At a dull red heat, boron sesquisulphide is violently decomposed by potassium, sodium, magnesium, and aluminium, but iron, zinc, copper, mercury, and silver are without action upon it. It is decomposed by carbon dioxide above 300° as follows

B₂S₃ + 3CO₂ = B₂O₃ + 3CO + 3S.

It is violently decomposed by water at ordinary temperatures, boric acid and hydrogen sulphide being produced and much heat evolved: -

[B₂S₃] + 6H₂O + aq. = 2H₃BO₃aq. + 3(H₂S) + 57.8 Cals.

Boron sesquisulphide is slightly soluble in phosphorus trichloride, from which it crystallises in needles. It forms the addition-compounds B₂S₃.BCl₃ and B₂S₃.BBr₃, and according to Moissan, combines with the chlorides of phosphorus, arsenic, and antimony. When dissolved in hot carbon disulphide, saturated with hydrogen sulphide, the solvent evaporated in vacuo and the residue recrystallised from either carbon disulphide or benzene, white crystals of B₂S₃.H₂S or thiometaboric acid are obtained. This substance, which smells of hydrogen sulphide and is rapidly decomposed by water, has a molecular weight corresponding to the formula H₂B₂S₄ in benzene solutions; unlike boron sesquisulphide, it is extremely soluble in both benzene and carbon disulphide. Heated in a sealed tube, it begins to melt at 120° and forms a clear, transparent liquid at 140°; when heated in air, it readily dissociates into boron sesquisulphide and hydrogen sulphide. Thiometaboric acid dissolves in liquid ammonia; when excess of solvent is removed at ordinary temperatures, yellow crystals of B₂S₃.6NH₃ separate out.