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Cyanogen Diselenide or Selenocyanogen, Se2(CN)2

By heating together selenious anhydride, anhydrous hydrocyanic acid and acetic anhydride to 100° C. under pressure in a sealed tube until solution is complete, there are formed on evaporation of the product, yellowish crystals which have been regarded as impure cyanogen diselenide, Se2(CN)2. On examination under the microscope these yellowish crystals appear as a mixture of brownish plates and needle-shaped crystals.

The action of iodine on silver selenocyanate yields selenocyanogen. When potassium selenocyanate dissolved in anhydrous acetone reacts with a solution of lead tetra-acetate in chloroform, lead selenocyanate and selenocyanogen result.

The diselenide has an intensely disagreeable smell and its vapours are very poisonous. It is hydrolysed by water thus:

2Se2(CN)2 + 3H2O = H2SeO3 + HCN + 3HCNSe;

the selenocyanic acid is only stable in neutral or alkaline solution. The diselenide is also readily decomposed by warm alcohol, and by dilute acids or alkalis. It undergoes polymerisation when heated in carbon disulphide solution, forming cyanogen mono- and tri-selenides:

2Se2(CN)2 = Se(CN)2 + Se3(CN)2.

Selenocyanogen may be identified by conversion into diantipyryl selenide, C22H22O2N4Se, m.pt. 236° C.
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