Phosphorus pentabromide is a reactive, yellow solid of formula, which has the structure (tetrabromophosphonium bromide) 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 (tetrabromophosphonium tribromide).[1]
It can be used in organic chemistry to convert carboxylic acids to acyl bromides. It is highly corrosive. It strongly irritates skin and eyes. 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 .[2]