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Osmyl Derivatives

Salts containing the group OsO2 in the acid radicle are known as osmyl derivatives, and have the general formula:

M2(OsO2)X4,

where M stands for a monovalent metal and X for a monovalent acid radicle. These substances have been carefully studied by Wintrebert.

Of the various osmyl derivatives the potassium salts may be regarded as the most important, since from them it is easy to prepare the corresponding salts of silver, barium, etc., by simple double decomposition.

The potassium salts may be obtained by the action of a suitable reducing agent upon osmium tetroxide in the presence of a simple potassium salt or its hydroxide. Thus, for example, potassium osmyl nitrite results when the tetroxide is reduced by nitric oxide in the presence of potassium nitrite solution:

OsO4 + 2NO + 2KNO2 = K2(OsO2)(NO2)4.

On the other hand, the corresponding oxalate may be produced by treating a solution of the tetroxide in potassium hydroxide with excess of oxalic acid. Thus:

OsO4 + 2KOH + 3H2C2O4 = K2(OsO2)(C2O4)2 + 2CO2 + 4H2O.

Similarly, the action of hydrochloric acid on the tetroxide in the presence of potassium chloride solution leads to the formation of potassium osmyl chloride:

OsO4 + 4HCl + 2KCl = K2(OsO2)Cl4 + Cl2 + 2H2O.

In this case the hydrochloric acid itself acts as the reducing agent, free chlorine being liberated.

It is interesting to note, in passing, that a similar reaction applied to ruthenium tetroxide led Howe to discover the oxychlor-ruthenates of caesium and rubidium.

Osmyl sulphites readily lend themselves to preparation in this manner. Rosenheim and Sasserath first prepared sodium osmyl sulphite by passing sulphur dioxide through a solution of osmium tetroxide in caustic soda.

Now in the foregoing reactions the osmium is used in the form of its tetroxide, in which its valency is 8. The first action is that of the reducing agent, which lowers its valency to six. Then comes the formation of the osmyl derivative. For example, the preparation of potassium osmyl oxalate takes place in two stages, namely, (1) the reduction of OsO4 to OsO3, which, in the presence of the potassium hydroxide, yields potassium osmate, K2OsO4; and (2) interaction of the last named with 2 molecules of oxalic acid to form the osmyl derivative. These stages may be expressed as follows:
  1. OsO4 + H2C2O4 + 2KOH = K2OsO4 + 2CO2 + 2H2O
  2. K2OsO4 + 2H2C2O4 = K2(OsO2)(C2O4)2 + 2H2O.
By starting, therefore, with potassium osmate, in which the osmium is already in the hexavalent condition, the osmyl derivatives may usually be prepared with greater ease, the reaction proceeding according to equation (2) above, or, in general, as follows:

K2OsO4 + 4HX = K2(OsO2)X4 + 2H2O.

Osmyl derivatives may change their acid radicles in contact with free acids in the same manner as ordinary salts. For example, just as hydrochloric acid decomposes potassium nitrite with the formation of potassium chloride:

KNO2 + HCl = KCl + HNO2,

so potassium osmyl nitrite, in the same way, yields potassium osmyl chloride. Thus:

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

Interchange of the negative radicles can also take place between osmyl and neutral salts. Thus the addition of neutral potassium oxalate to a concentrated solution of potassium osmyl chloride yields a precipitate of potassium osmyl oxalate:

K2(OsO2)Cl4 + 2K2C2O4 = K2(OsO2)(C2O4)2 + 4KCl.

Potassium nitrite, under similar conditions, causes crystals of potassium osmyl nitrite to form.

Osmyl derivatives are not stable in neutral aqueous solution, although a small quantity of acid suffices to prevent decomposition. The neutral water decomposes them with the formation of a black precipitate of what has usually been described as osmic acid. Thus, in the case of the chloride:

K2(OsO2)Cl4 + 2H2OH2OsO4 + 2KCl + 2HCl.

The reaction, however, does not continue to completion, since the free hydrochloric acid formed during the initial stages preserves the remainder from decomposition.

With aqueous potassium hydroxide solution, potassium osmate results. Thus:

K2(OsO2)X4 + 4KOH = K2OsO4 + 4KX + 2H2O.

Boiling with an excess of hydrochloric acid results in the complete displacement of oxygen by chlorine, a chlorosmate or osmichloride being formed:

K2(OsO2)X4 + 8HCl = K2OsCl6 + 4HX + Cl2 + 2H2O.

Addition of ammonia to an aqueous solution of an osmyl salt causes the formation of an ammine derivative. Thus:

K2(OsO2)X4 + 4NH3 = (OsO2)(NH3)4X2 + 2KX.

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