Phosphorus halides

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Phosphorus halides are compounds of phosphorus with halogens. They come in three main oxidation-state series: +5, +3, and +2. PI5 is disputed, but other members exist. Mixed chalcogen halides also occur.

In the gas phase, phosphorus pentahalides like PF5 have a trigonal bipyramidal shape. PF5 is a relatively inert gas that acts as a mild Lewis acid and can accept fluoride. It is fluxional, with axial and equatorial fluorines swapping positions by Berry pseudorotation.

The +5 halides PCl5, PBr5, and related species are often ionic in solid and liquid form. They are commonly described as salts such as PCl4+ with PCl6−, PBr4+ with Br−, and PBr4+ with Br3− (tribromide). These compounds are widely used to chlorinate and brominate organic molecules.

The +3 halides are the best known. They are usually made directly from the elements or by transhalogenation. PF3 (phosphorus trifluoride) is a useful ligand in coordination chemistry, resembling carbon monoxide. PCl3 is a major industrial chemical and a common starting material. PBr3 converts alcohols to alkyl bromides and carboxylic acids to acyl bromides (as in the Hell–Volhard–Zelinsky reaction). PI3 is a mild oxygen acceptor. The trihalides can be oxidized by chalcogens to form oxyhalides or related species.

Phosphorus(II) halides can be made by passing an electric discharge through a mixture of a trihalide vapor and hydrogen gas. The stable diphosphorus tetraiodide, P2I4, has a trans, bent structure like hydrazine and has some uses in synthesis; other diphosphorus halides are mostly of academic interest. They are subhalides of phosphorus.

Oxyhalides (phosphoryl halides) and thio-/seleno-phosphoryl halides can be prepared from the trihalides by reacting with organic peroxides or ozone (to oxyhalides) or with sulfur or selenium in inert solvent. These species are usually more electrophilic—and more toxic—than the phosphorus(III) halides. Selenophosphoryl fluoride is unstable at room temperature, decomposing to PF3 and selenium.