Complex Osmisulphites

On passing sulphur dioxide through an alkaline solution of osmium tetroxide in potassium hydroxide until it becomes almost neutral, a brown crystalline precipitate is obtained, namely, the complex sodium osmisulphite, 3Na₂O.OsO₃.4SO₂ + 5H₂O, or:

+ 5H₂O

The salt dissolves easily in warm water, yielding a reddish brown solution. On treatment with caustic soda, a violet solution of sodium osmate is obtained, whilst barium chloride and silver nitrate precipitate respectively a light brown barium salt and a yellowish brown silver salt. By treatment of osmium tetroxide in potassium hydroxide under similar conditions with sulphur dioxide the complex 7K₂0.4OsO₃.10SO₂ has been obtained, both in the anhydrous condition and with 3 and 7 molecules of water respectively.

In concentrated alkaline solution, prolonged action of sulphur dioxide yields the complex 11K₂O.4OsO₃.14SO₂ + 7H₂O.

Both of these salts, on treatment with caustic potash, decompose quantitatively into potassium osmate and sulphite.

By the action of sodium hydrogen sulphite solution on one of sodium chlorosmate at the boiling-point, a whitish brown crystalline compound is obtained, to which the formula

[Os(SO₃)₆]Na₈.8H₂O

is given.

If, however, an excess of sulphite solution is used, and the mixture allowed to stand for several weeks, needle-shaped crystals of sodium chlorosmisulphite separate out, ranging in colour from dark yellow to bright brown, and containing two atoms of chlorine, thus:



The same salt is also formed by mixing solutions of normal sodium sulphite and chlorosmate in the cold. If, however, the solution is gently warmed, the monochlor derivative,



is produced.

Potassium Hydrogen Chlorosmisulphite, OsCl₄(SO₃)₄K₆H₂, obtained by the prolonged action of potassium hydrogen sulphite on potassium osmichloride, crystallises in dark red, monoclinic prisms. When heated with hydrochloric acid it evolves sulphur dioxide, and is converted into potassium osmichloride.

Other complex derivatives have been prepared.