Phosphorus(V) chloride (phosphorus pentachloride)

Structure and bonding

In the gas phase and in many nonpolar solutions PCl5 exists as a covalent, trigonal bipyramidal molecule (AX5, VSEPR geometry). In the solid state it is better described as an ionic pair, composed of the tetrahedral cation [PCl4+] and the octahedral anion [PCl6-]. This dual character reflects the ability of phosphorus to expand its coordination number beyond four.

Preparation and equilibrium

PCl5 is prepared by chlorinating phosphorus trichloride. The reaction PCl3 + Cl2 ⇌ PCl5 is reversible and temperature- and pressure-dependent; heating or reducing chlorine shifts the equilibrium toward PCl3 and Cl2. Industrial syntheses and laboratory preparations control conditions to favor formation of PCl5.

Chemical behavior and typical reactions

PCl5 is a strong chlorinating agent and reacts readily with nucleophiles. It hydrolyzes vigorously with water to give phosphorus oxychloride (POCl3) and hydrogen chloride, and further hydrolysis yields phosphoric acid. It can convert alcohols and carboxylic acids to the corresponding chlorides and forms hexachlorophosphate salts in the presence of excess chloride.

• Hydrolysis: PCl5 + H2O → POCl3 + 2 HCl (overall steps)
• Chlorination: alcohols → alkyl chlorides; acids → acyl chlorides
• Complex formation: reacts with Cl- to form [PCl6-]

Applications

PCl5 is used as an intermediate in the manufacture of organophosphorus compounds and phosphorus oxychloride, and as a reagent for introducing chlorine into organic molecules. In research laboratories it serves as a convenient chlorinating or dehydrating reagent where more selective alternatives are not required.

Safety and handling

Phosphorus pentachloride is corrosive and reacts exothermically with moisture to release toxic hydrogen chloride gas and possibly chlorine. It must be handled in a dry, well-ventilated area or inert-atmosphere glovebox, with appropriate personal protective equipment and secondary containment. Spills are neutralized carefully and wastes treated according to regulations.