Unlike
phosphorus pentachloride, solid phosphorus pentabromide is an ionic compound. Its structural formula is (tetrabromophosphonium bromide). The cation is tetrahedral. According to
X-ray crystallography, the P-Br bond lengths are 215
picometers. In the
vapor phase, it dissociates into and . Rapid cooling of the vapor to 15
K gives a mixture of
phosphorus tribromide and the similarly ionic
phosphorus heptabromide (tetrabromophosphonium tribromide or ). PBr5 can be synthesized with high purity by bromination of a solution of PBr3 in
CS2: : ==Organic chemistry==