Pentachlorides

Pentachlorides are chemical compounds containing five chlorine atoms bonded to a central element, typically with the formula MCl5. They occur for several elements, especially nonmetals and early transition metals. In many cases they are molecular species in the gas phase, but in the solid state they frequently polymerize and adopt extended networks.

The best-known pentachloride is phosphorus pentachloride, PCl5. In the gas phase it has a trigonal bipyramidal structure, but solid PCl5 is polymeric. PCl5 is a strong Lewis acid and reacts with bases to form adducts; it hydrolyzes readily in water to phosphoric acid and hydrochloric acid: PCl5 + 4 H2O → H3PO4 + 5 HCl. It is used to chlorinate alcohols and carboxylic acids and to convert alcohols to alkyl chlorides, among other applications.

Other known pentachlorides include vanadium pentachloride (VCl5), niobium pentachloride (NbCl5), tantalum pentachloride (TaCl5) and antimony pentachloride (SbCl5). These compounds are typically less volatile and often exist as polymeric solids; some are strong Lewis acids and form adducts with chloride or other ligands. For example, SbCl5 can form SbCl6− in chloride-rich media. The chemistry of metal pentachlorides often reflects high oxidation states and a tendency to polymerize in the solid state.

Safety and handling: pentachlorides are moisture sensitive; hydrolysis produces corrosive acids and chlorine-containing gases. They are typically handled under strictly controlled, anhydrous conditions.