Pentahalides

Pentahalides are chemical compounds in which a central atom is bonded to five halogen atoms, giving a general formula MX5. They are typically covalent species and include examples where the central atom is from group 15 or where a halogen serves as the central atom in a pentacoordinate environment. Many MX5 molecules exhibit hypervalent or expanded-octet bonding and can show distinct solid-state structures, such as dimerization in PCl5 to form P2Cl10.

In many pentahalides with a central atom from group 15, such as PF5, PCl5, AsF5, and SbF5, the five halide ligands arrange in a trigonal bipyramidal geometry in the absence of lone pairs. When the central atom itself carries a lone pair, as in IF5, the geometry becomes square pyramidal. The central atoms are typically in high oxidation states (often +5), and several MX5 species are strong Lewis acids. Solid-state PCl5 is known to exist as P2Cl10, while in solution PCl5 can accept halide ligands to form PCl6− under chloride-rich conditions.

Common examples and uses include phosphorus pentafluoride (PF5) and phosphorus pentachloride (PCl5). PF5 is used as a fluorinating agent and catalyst precursor in various chemical transformations, whereas PCl5 is a versatile chlorinating and phosphorylating reagent. SbF5 is notable as a very strong Lewis acid and, when combined with HF, forms the superacid fluoroantimonic acid HSbF6. AsF5 and IF5 also function as Lewis acids and reagents in specialty syntheses; many pentahalides are moisture sensitive and hydrolyze readily in water.