Potassium Osmyl Nitrite, K₂(OsO₂)(NO₂)₄

Potassium Osmyl Nitrite, K₂(OsO₂)(NO₂)₄, results when osmium tetroxide is reduced with nitric oxide in the presence of potassium nitrite:

OsO₄ + 2NO + 2KNO₂ = K₂(OsO₂)(NO₂)₄.

It may also be prepared by cooling a warm solution of potassium osmyl chloride, containing a trace of acid, after addition of potassium nitrite in the requisite quantity. Thus:

K₂(OsO₂)Cl₄ + 4KNO₂ = K₂(OsO₂)(NO₂)₄ + 4KCl.

The crystals of potassium osmyl nitrite obtained by cooling of the warm, concentrated solution readily decompose; even in closed tubes, if exposed to sunlight, the decomposition is rapid, needles of osmium tetroxide and brown nitrous fumes appearing:

K₂(OsO₂)(NO₂)₄ = OsO₄ + 2NO + 2KNO₂.

The crystals, are decomposed by water, but osmic acid is not precipitated owing to oxidation of the osmium to tetroxide by the nitrous acid. With excess of potassium hydroxide potassium osmate is formed:

K₂(OsO₂)(NO₂)₄ + 4KOH = K₂OsO₄ + 4KNO₂ + 2H₂O.

If, however, potassium hydroxide is added, little by little, to a solution of the osmyl nitrite in the proportion of two molecules of hydroxide to one of nitrite, a new salt, namely, potassium osmyl oxy-nitrite, is obtained. Thus:

K₂(OsO₂)(NO₂)₄ + 2KOH = K₂(OsO₂)O(NO₂)₂ + 2KNO₂ + H₂O.

Ammonia reacts with an aqueous solution of osmyl nitrite to give an ammino derivative, OsO₂(NH₃)₄(NO₂)₂.