Phosphorus Thiotribromide, PSBr₃

Phosphorus Thiotribromide, PSBr₃, has been prepared in a number of different ways:—

1. By the action of phosphorus pentabromide on hydrogen sulphide, according to the equation
   
   PBr₅ + H₂S = PSBr₃ + 2HBr
2. By distillation of the tribromide with sulphur.
3. By combination of the pentabromide with the pentasulphide:—
   
   P₂S₅ +3PBr₅ = 5PSBr₃
   
   The product may be freed from pentabromide by washing with warm water and dried by dissolving in CS₂ and standing over CaCl₂.
4. By a reaction between ammonium trithiophosphate and hydrogen bromide.

The compound crystallised from carbon disulphide in yellow octahedra, having a density of 2.7 to 2.87 and a melting-point of +36.4° to +39° C. It gives off a pungent vapour, and distils at about 212° C. with some decomposition at first into PBr₃, then PSBr₃ follows, and finally there is a residue of sulphur and phosphorus. It is only slightly hydrolysed by water even at 100° C., the products being phosphoric acid, hydrogen bromide and some sulphide, together with some sulphur and phosphorous acid. Alcoholysis is said to yield ethyl thiophosphate, (C₂H₅)₃PSO₃.

Thiophosphoryl bromide dissolves sulphur and mixes with other halides such as phosphorus trichloride, also with ether, chloroform and carbon disulphide.

Several other thiobromides of phosphorus have been prepared, which can be regarded as derivatives of pyrophosphoric acid. Diphosphorus monothiohexabromide, P₂SBr₆, is prepared by distilling PSBr₃, and appears as a yellow oil which can be solidified to crystals melting at -5° C. The boiling-point is 205° C. When mixed with water it gives a crystalline hydrate melting at +35° C. P₂S₃Br₄ (diphosphorus trithiotetrabromide) is prepared by the action of bromine on P₄S₆ dissolved in carbon disulphide. In appearance and properties it is very similar to the other thiobromides.