Phosphorus pentachloride is the chemical compound with the formula PCl5. It is one of the most important phosphorus chlorides, others being PCl3 and POCl3. PCl5 finds use as a chlorinating reagent. It is a colourless, water-sensitive and moisture-sensitive solid, although commercial samples can be yellowish and contaminated with hydrogen chloride. Structure The structures for the phosphorus chlorides are invariably consistent with VSEPR theory. The structure of PCl5 depends on its environment. Gaseous and molten PCl5 is a neutral molecule with trigonal bipyramidal geometry and (D3h) symmetry. The hypervalent nature of this species (as well as of , see below) can be explained with the inclusion of non-bonding molecular orbitals (molecular orbital theory) or resonance (valence bond theory). This trigonal bipyramidal structure persists in nonpolar solvents, such as CS2 and CCl4. In the solid state PCl5 is an ionic compound, formulated . In solutions of polar solvents, PCl5 undergoes self-ionization. Dilute solutions dissociate according to the following equilibrium: PCl5 + Cl− At higher concentrations, a second equilibrium becomes more prevalent: 2 PCl5 + The cation and the anion are tetrahedral and octahedral, respectively. At one time, PCl5 in solution was thought to form a dimeric structure, P2Cl10, but this suggestion is not supported by Raman spectroscopic measurements. Related pentachlorides AsCl5 and SbCl5 also adopt trigonal bipyramidal structures. The relevant bond distances are 211 pm (As−Cleq), 221 pm (As−Clax), 227 pm (Sb−Cleq), and 233.3 pm (Sb−Clax). At low temperatures, SbCl5 converts to the dimer, dioctahedral Sb2Cl10, structurally related to niobium pentachloride. Preparation PCl5 is prepared by the chlorination of PCl3. This reaction is used to produce around 10,000 tonnes of PCl5 per year (as of 2000). PCl3 + Cl2 PCl5 (ΔH = −124 kJ/mol) PCl5 exists in equilibrium with PCl3 and chlorine, and at 180 °C the degree of dissociation is about 40%. Because of this equilibrium, samples of PCl5 often contain chlorine, which imparts a greenish coloration. Reactions Hydrolysis In its most characteristic reaction, PCl5 reacts upon contact with water to release hydrogen chloride and give phosphorus oxides. The first hydrolysis product is phosphorus oxychloride: PCl5 + H2O → POCl3 + 2 HCl In hot water, hydrolysis proceeds completely to orthophosphoric acid: PCl5 + 4 H2O → H3PO4 + 5 HCl Lewis acidity Phosphorus pentachloride is a Lewis acid. This property underpins many of its characteristic reactions, autoionization, chlorinations, hydrolysis. A well studied adduct is PCl5(pyridine). Chlorination of organic compounds In