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 dissociated to PBr₃ and Br₂. Rapid cooling of this phase to 15 K leads to formation of the ionic species [PBr₄]⁺[Br₃]⁻.

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

Reversing this equilibrium to generate PBr₅ by addition of Br₂ to PBr₃ is difficult in practice because the product is susceptible to further addition to yield PBr₇.