Potassium Osmyl Chloride, K₂(OsO₂)Cl₄

Potassium Osmyl Chloride, K₂(OsO₂)Cl₄ is readily prepared by the action of hydrochloric acid upon potassium osnivl nitrite, or upon the oxy-osmyl salt, K₂(OsO₂)O(NO₂)₂. Thus:

K₂(OsO₂)O(NO₂)₂ + 4HCl = K₂(OsO₂)Cl₄ + 2HNO₂ + H₂O.

If a very slight excess of acid is taken, the reaction is quantitative. The action must not be allowed to proceed too energetically, otherwise the chlorosmate is formed (vide supra).

The salt is also obtained by acting on potassium osmate with hydrochloric acid. At first there is precipitated a black powder, usually regarded as osmic acid. This dissolves entirely in excess of the acid on warming, yielding the osmyl chloride. Thus:

K₂OsO₄ + 2HCl = 2KCl + H₂OsO₄.
H₂OsO₄ + 2KCl + 2HCl = K₂(OsO₂)Cl₄ + 2H₂O.

Although the neutral aqueous solution is not very stable, the contrary is true in the presence of a small quantity of acid.

Cooling of a concentrated, warm solution yields the anhydrous salt, whilst slow cooling causes the dihydrate, K₂(OsO₂)Cl₄.2H₂O, to crystallise out as red octahedra. The last named rapidly lose their water on warming in a current of hydrogen gas.

In the anhydrous condition potassium osmyl chloride also crystallises in red octahedra, which are readily soluble in water. From the solution a black precipitate of osmic acid separates as explained above. When heated in a current of hydrogen the salt is decomposed at 200° C., metallic osmium being obtained.