Osmyl Oxy Derivatives

Osmyl oxy derivatives differ from osmyl salts in that they contain an additional atom of oxygen, but have two monovalent negative radicles the fewer. The general formula for these substances is thus:

M₂(OsO₃)X₂,

where M and X have the same significance as hitherto.

Osmyl oxy salts have been made the subject of careful study by Wintrebert. They can frequently be prepared by the action of an oxidisable salt upon osmium tetroxide. Thus, potassium nitrite reacts with the tetroxide as follows:

OsO₄ + 3KNO₂ = K₂(OsO₃)(NO₂)₂ + KNO₃.

Potassium oxalate reacts in a similar manner, but more slowly, whilst ammonium oxalate yields an ammino salt, namely:

OsO₂(NH₃)4C₂O₄.

Having once obtained a suitable osmyl oxy salt such as potassium or sodium osmyl oxynitrite by a method similar to the above, it is not difficult by suitably choosing the reagents to prepare many other derivatives. Thus, the ammonium salt may readily be obtained by double decomposition of ammonium chloride and the sodium salt. Thus:

Na₂(OsO₃)(NO₂)₂ + 2NH₄Cl = (NH₄)₂OsO₃(NO₂)₂ + 2NaCl.

Again, by acting on this new salt with hydrochloric acid in the cold, ammonium osmyl oxy chloride is produced:

(NH₄)₂OsO₃(NO₂)₂ + 2HCl = (NH₄)₂OsO₃Cl₂ + 2HNO₂.

When, therefore, any particular derivative is difficult to prepare by the direct method, it is frequently possible to apply indirect methods with success.

Osmyl oxy salts exhibit practically the same properties as osmyl derivatives. For example, water decomposes them with formation of osmic acid:

M₂(OsO₃)X₂ + H₂O = H₂OsO₄ + 2MX.

The reaction, which proceeds slowly in the cold, is rapid on warming. Potassium hydroxide reacts similarly. Thus:

M₂(OsO₃)X₂ + 2KOH = K₂OsO₄ + 2MX + H₂O,

whilst the halogen acids yield halogen osmates. Thus, in the case of hydrochloric acid:

M₂(OsO₃)Cl₂ + 6HCl = M₂OsCl₆ + Cl₂ + 3H₂O.

Osmyl oxy salts may frequently be converted into osmyl derivatives by the action of a reducing acid. For example, ammonium osmyl oxy-oxalate is reduced by oxalic acid to ammonium osmyl oxalate as follows:

(NH₄)₂OsO₃.C₂O₄ + H₂C₂O₄ = (NH₄)₂OsO₂(C₂O₄)₂ + H₂O.

In a similar manner it is possible to convert an osmyl oxy derivative containing a certain acid radicle into an osmyl salt with another acid radicle. Thus potassium osmyl oxynitrite with hydrochloric acid yields potassium osmyl chloride:

K₂OsO₃(NO₂)₂ + 4HCl = K₂(OsO₂)Cl₄ + 2HNO₂ + H₂O.

Transformation in the opposite direction, namely, from osmyl to osmyl oxy derivatives, is more difficult to effect, although it has been accomplished in the preparation of potassium osmyl oxynitrite.