Hexcichlorosmates, M₂OsCl₆

Osmium tetrachloride yields a series of double salts known as osmichlorides or chlorosmates, M₂OsCl₆. Of these the potassium, sodium, and ammonium salts are best known.

Potassium Chlorosmate, K₂OsCl₆, is obtained by prolonged boiling of osmic acid with hydrochloric acid, and subsequent evaporation of the solution with potassium chloride. Dark brown crystals of the chlorosmate separate out, leaving osmium trichloride in solution.

Potassium chlorosmate may also be obtained by heating the finely divided metal with potassium chloride in chlorine, by double decomposition of the sodium salt with potassium chloride, or by the action of hydrochloric acid on potassium osmyl oxynitrite. It is soluble in water, yielding a yellow solution from which alcohol precipitates it as a red crystalline powder. The crystals are deep red octahedra, isomorphous with the corresponding palladium, iridium, and platinum analogues.

When ignited, the dry salt yields a residue of metallic osmium and potassium chloride.

The solution decomposes on warming, the tetroxide being produced and a black powder deposited.

From the solution alkalies and ammonia precipitate the hydrated dioxide, whilst excess of ammonia yields osmium diammine hydroxide. Ferrous sulphate and alkali formates reduce the salt, liberating metallic osmium.

Sodium Chlorosmate, Na₂OsCl₆.2H₂O, may be obtained in a similar manner to the potassium salt. Rosenheim and Sasserath described a new method, according to which sodium osmisulphite is gradually heated up in a current of dry hydrogen chloride. The mass becomes black at first, then froths up in consequence of the escape of sulphur dioxide; finally a reddish brown product remains behind, which in air immediately becomes brick-red. It dissolves in a small quantity of water to a green solution which on concentration over sulphuric acid yields long orange-coloured prisms. When boiled with a concentrated solution of sodium hydrogen sulphite, sodium chlorosmate yields the osmisulphite, Na₈Os(SO₃)₆.8H₂O.

Ammonium Chlorosmate, (NH₄)₂OsCl₆, is prepared by double decomposition of the sodium salt with ammonium chloride.1,2 Upon recrystallisation, brown octahedra are obtained.

Methyl and ethyl ammonium chlorosmates have been prepared.