Phosphorus pentabromide

Phosphorus pentabromide is a reactive, yellow solid of formula, which has the structure in the solid state but in the vapor phase is completely dissociated to and. Rapid cooling of this phase to 15 K leads to formation of the ionic species phosphorus heptabromide.
It can be used in organic chemistry to convert carboxylic acids to acyl bromides. It is highly corrosive. It strongly irritates skin, eyes and causes severe skin burns. It decomposes above 100 °C to give phosphorus tribromide and bromine:
Reversing this equilibrium to generate by addition of to is difficult in practice because the product is susceptible to further addition to yield phosphorus heptabromide.