Potassium Osmyl Oxynitrite, K₂(OsO₃)(NO₂)₂

Potassium Osmyl Oxynitrite, K₂(OsO₃)(NO₂)₂.3H₂O, is readily obtained as the result of the direct action of a concentrated solution of potassium nitrite on osmium tetroxide:

OsO₄ + 3KNO₂ = K₂(OsO₃)(NO₂)₂ + KNO₃.

If nitric oxide is passed through a solution of osmium tetroxide in potassium hydroxide containing exactly one molecule of the former to two of the latter, potassium osmyl oxynitrite is formed, and may be isolated on concentration of the solution. The reaction probably proceeds as follows:

OsO₄ + 2KOH + 2NO = H₂OsO₄ + 2KNO₂ 2KNO₂ + H₂OsO₄ = K₂(OsO₃)(NO₂)₂ + H₂O.

An interesting method of preparing the osmyl oxynitrite consists in acting on the osmyl derivative with potassium hydroxide. Thus:

K₂(OsO₂)(NO₂)₄ + 2KOH = K₂(OsO₃)(NO₂)₂ + H₂O + 2KNO₂.

This is one of the few cases in which it is possible to pass from an osmyl salt to an osmyl oxy salt.

Potassium osmyl oxynitrite crystallises in the form of brownish black needles, which dissolve in aqueous potash, yielding potassium osmate. The crystals are only slightly soluble in cold water. At 35° C. they begin to lose their water of crystallisation and acquire a yellowish tint. When crushed in a mortar the odours of osmium tetroxide and nitrogen peroxide become apparent. When heated in a current of hydrogen to 200° C. the salt decomposes without explosion, free osmium remaining behind.