Phosphorus Halogen Compounds

Phosphorus Halogen Compounds

Phosphorus will react directly with the halogens, forming trihalides, PX₃, and pentahalides, PX₅. The trihalides are much more stable than the corresponding nitrogen trihalides; nitrogen pentahalides do not form because of nitrogen’s inability to form more than four bonds.

The chlorides PCl₃ and PCl₅, both shown in the figure below, are the most important halides of phosphorus. Phosphorus trichloride is a colorless liquid that is prepared by passing chlorine over molten phosphorus. Phosphorus pentachloride is an off-white solid that is prepared by oxidizing the trichloride with excess chlorine. The pentachloride sublimes when warmed and forms an equilibrium with the trichloride and chlorine when heated.

This image shows the molecular structure of PCl₃ (left) and PCl₅ (right) in the gas phase.

Like most other nonmetal halides, both phosphorus chlorides react with an excess of water and yield hydrogen chloride and an oxyacid: PCl₃ yields phosphorous acid H₃PO₃ and PCl₅ yields phosphoric acid, H₃PO₄.

The pentahalides of phosphorus are Lewis acids because of the empty valence d orbitals of phosphorus. These compounds readily react with halide ions (Lewis bases) to give the anion \({\text{PX}}_{6}{}^{\text{−}}.\) Whereas phosphorus pentafluoride is a molecular compound in all states, X-ray studies show that solid phosphorus pentachloride is an ionic compound, \({[\text{PCl}}_{4}{}^{\text{+}}][{\text{PCl}}_{6}{}^{\text{−}}],\) as are phosphorus pentabromide, \({[\text{PBr}}_{4}{}^{\text{+}}]\)[Br⁻], and phosphorus pentaiodide, \({[\text{PI}}_{4}{}^{\text{+}}]\)[I⁻].