The problem explains behaviour of phosphorus pentachloride (PCl5) at various temperatures from the point of bonding and structure.

PCl5 is a white solid, which volatilises at 160oC and melts at 148oC under pressure. Molten PCl5 conducts an electric current readily. At 180oC PCl5 vapour has a density of 9.3 g/L corrected to the standard conditions. At 250oC a sample of PCl5 exerts a pressure, which is twice the expected value. Discuss these observations and comment on the species present in the melt and at 180o and 250o in the vapour.