There are three series of binary phosphorus halides, containing phosphorus in the oxidation states +5, +3 and +2. All twelve compounds have been described, in varying degrees of detail, although serious doubts have been cast on the existence of PI5. Template:Fn
Oxidation state +5 (PX5)
|Chemical formula||CAS number||Melting point||Boiling point||P–Xax bond length||P–Xeq bond length||Xax–P–Xeq bond angle||Xax–P–Xax bond angle|
|PF5||[7647-19-0]||-93.7°C||-84.5°C||153 pm||158 pm||120°||90°|
|PI5||See Note 1.|
Phosphorus pentafluoride is a relatively inert gas, notable as a mild Lewis acid and a fluoride ion acceptor. It is fluxional molecule. Phosphorus pentachloride and phosphorus pentabromide are ionic in the solid and liquid states, and are best described as PX4+PX6–. They are widely used as chlorinating and brominating agents in organic chemistry.
Oxidation state +3 (PX3)
|Chemical formula||CAS number||Melting point||Boiling point||P–X bond length||X–P–X bond angle||Dipole moment|
|PF3||[7783-55-3]||-151.5°C||-101.8°C||156 pm||96.3°||1.03 D|
|PCl3||[7719-12-2]||-93.6°C||76.1°C||204 pm||100°||0.56 D|
The phosphorus(III) halides are the best known of the three series. They are usually prepared by direct reaction of the elements, or by transhalogenation.
Phosphorus trifluoride is used as a ligand in coordination chemistry, where it resembles carbon monoxide. Phosphorus trichloride is a major industrial chemical and widely-used starting material for phosphorus chemistry. Phosphorus tribromide is used in organic chemistry to convert alcohols to alkyl bromides and carboxylic acids to acyl bromides (e.g. in the Hell-Vollhard-Zelinsky reaction). Phosphorus triiodide also finds use in organic chemistry, as a mild oxygen acceptor.
Oxidation state +2 (P2X4)
|Chemical formula||CAS number||Melting point||Boiling point||P–X bond length||P–P bond length||X–P–X bond angle||X–P–P bond angle|
|P2F4||[13824-74-3]||-86.5°C||-6.2°C||159 pm||228 pm||99.1°||98.4°|
|P2I4||[13455-00-0]||125.5°C||d||248 pm||221 pm||102.3°||94.0°|
Phosphorus(II) halides may be prepared by passing an electric discharge through a mixture of the trihalide vapour and hydrogen gas. They are of purely academic interest at the present time. Diphosphorus tetrabromide is particularly poorly described.
Oxyhalides and thiohalides
|Chemical formula||CAS number||EINECS number||Melting point||Boiling point||Density||Refractive index||Dipole moment|
|POCl3||[10025-87-3]||233-046-7||1.2°C||105.1°C||1.675 g/cm3||1.461||2.54 D|
The thiohalides, also known as thiophosphoryl halides may be prepared from the trihalides by reaction with elemental sulfur in an inert solvent. The corresponding selenohalides are also known.
- NIST Standard Reference Database
- N. N. Greenwood & A. Earnshaw, Chemistry of the Elements (2nd ed.), Butterworth-Heinemann, Oxford, 1997.
- I. Tournieporth & T. Klapötke, J. Chem. Soc., Chem. Commun. 1990, 132.