Phosphorus pentabromide

Phosphorus pentabromide is a reactive yellow solid of formula PBr5, which has the structure PBr4+ Br− in the solid state but in the vapor phase is completely disociated to PBr3 and Br2. Rapid cooling of this phase to 15 K leads to formation of an ionic species [PBr4]+[Br3]-.

It can be used in organic chemistry to convert carboxylic acids to acyl bromides. It is highly corrosive and should be handled with care. It decomposes above 100 °C to give phosphorus tribromide and bromine:

PBr5 →  PBr3  +  Br2