Cyanogen Monosulphide, (CN)₂S

Cyanogen Monosulphide or Thiocyanic Thioanhydride (CN)₂S, is formed by the interaction of silver thiocyanate and cyanogen iodide in ether solution, silver iodide separating out:

CNSAg + CNI = AgI + (CN)₂S.

It is also formed when mercuric cyanide is treated with sulphur chloride, and together with cyanogen trisulphide, (CN)₂S₃, when silver cyanide interacts with sulphur chloride.

Cyanogen monosulphide crystallises in rhomboidal crystals which partly sublime at 30° to 40° C. and melt at 60° C. It is soluble in water, alcohol, ether, carbon disulphide, chloroform, benzene and sulphuric acid. It is decomposed by hydrochloric acid. It precipitates iodine from potassium iodide. Alcoholic potash decomposes it into potassium cyanate and potassium thiocyanate:

(CN)₂S + 2KOH = KCNS + KCNO + H₂O.

This is in accordance with its character as a thio-anhydride. It is transformed into hydrogen cyanide and thiocyanic acid by the action of hydrogen, hydrogen sulphide or potassium sulphide.