Chemistry » Metals, Metalloids, and Nonmetals » Occurrence, Preparation, and Properties of Phosphorus

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.

Two ball-and-stick models are shown. In the left model, an orange atom labeled, “P,” is single bonded to three green atoms labeled, “C l.” The right model shows an orange atom labeled, “P,” single bonded to five green atoms labeled, “C l.”

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].

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