Silicothio-urea, SiS(NH₂)₂

Silicothio-urea, SiS(NH₂)₂, is formed by the action of ammonia on silicon thiobromide by a reaction analogous to that by which the corresponding carbon compound is formed:

SiSBr₂ + 4NH₃ = SiS(NH₂)₂ + 2NH₄Br.

Silico-urea is not known, for no simple silicon oxyhalide, SiOX₂, exists from which it could be prepared.

Silicothio-urea differs from its carbon analogue in being decomposed by water; hence the reaction for its preparation is carried out in anhydrous solution, as follows: Silicon thiobromide is dissolved in benzene, and dry ammonia is passed into the solution at low temperature, when silicothio-urea separates, together with ammonium bromide. Liquid ammonia is employed to dissolve the latter, and the silicothio-urea then remains as a white powder, which slowly decomposes at atmospheric temperature, evolving ammonia and hydrogen sulphide.

Water hydrolyses this compound into silicic acid, ammonia, and hydrogen sulphide. It possesses feebly basic properties, and forms salts with the halogen hydracids.