Alkali Phosphides

Soon after the discovery of the alkali metals the direct combinations of these with phosphorus vapour were recorded. The combination can also be effected under petroleum, the unchanged phosphorus being removed by carbon disulphide.

When alkali metals are brought into contact with red phosphorus in liquid ammonia, ammonio-phosphides such as NaP₃.3NH₃ and KP₅.3NH₃ are formed, and can be deprived of their ammonia at 180° C., leaving the phosphides as brown solids. In liquid ammonia sodium and potassium are also capable of displacing one hydrogen from phosphine, giving crystalline monophosphides (phosphamides), NaPH₂ and KPH₂, which decompose on heating giving eventually K₃P and Na₃P.

Rubidium and caesium phosphides have been made by similar methods and by distilling phosphorus in a vacuum with the alkali metal. The formulae are given as Rb₂P₅ and Cs₂P₅; K₂P₅ is also known.