Rhodium Sesquisulphide, Rh₂S₃

Two varieties of Rhodium Sesquisulphide, Rh₂S₃, are known, differing somewhat in their properties, yet apparently possessing the same chemical composition:

This variety results from "dry" methods of preparation, such as heating rhodium trichloride, prepared at 440° C., in a current of hydrogen sulphide at 360° C. In carrying out the operation it is essential that all the air should be expelled from the apparatus before heating, and that the hydrogen sulphide be entirely free from water and hydrogen chloride. The temperature during the course of the reaction should not exceed about 360° C., since at 400° C. the hydrogen sulphide partially dissociates, the hydrogen produced effecting the reduction of a portion of the chloride to metallic rhodium, which contaminates the final product. For the same reason hydrogen sulphide free from intermixed hydrogen is essential, and is best obtained by the action of hydrochloric acid on antimony sulphide. The reaction proceeds only slowly with rhodium trichloride prepared by the action of chlorine on the metal at dull redness; whilst ammonium chlor-rhodite gives unsatisfactory results, probably owing to the formation of alkali poly sulphides that are volatilisable only with difficulty.

As obtained in this way rhodium sesquisulphide is a black, unctuous powder conserving the same crystalline form as the chloride from which it was prepared. It is not acted upon by acids, even aqua regia having no effect. Bromine and alkali sulphides are likewise without action on it.

When heated in an inert gas the sesquisulphide is stable up to above 500° C. At higher temperatures it loses sulphur, the resulting product varying in composition, but approaching that required for the monosulphide, RhS. There is good reason to believe, however, that the product is really a mixture and not a definite monosulphide, although it is extremely difficult, if not indeed impossible, to remove the whole of the sulphur by heat. When heated strongly in air or oxygen, oxides of sulphur and rhodium are produced.

β-variety, produced by "wet" methods. To this end a rhodium salt, such as the trichloride, is decomposed by passing a current of hydrogen sulphide through its solution, which is then raised to and maintained at 100° C. Insufficient hydrogen sulphide should be added to precipitate the whole of the metal in the first case, since a sulphydrate, Rh₂S₃.3H₂S, is formed, and this gradually decomposes at the higher temperature, the excess of metal in solution being precipitated by the hydrogen sulphide thereby liberated.

Prepared in this manner, rhodium sesquisulphide is a black powder, insoluble in alkali sulphides, as also in nitric and hydrochloric acids, and even aqua regia at 100° C. Moist air is without action on it, as also is bromine.