Phosphorus halides
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/* See also */ Phosphorus Chlordies goes straight back this page
There are three series of binary '''phosphorus halides''', containing [[phosphorus]] in the [[oxidation state]]s +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 PI<sub>5</sub>. {{fn|(1)}}
== Oxidation state +5 (PX<sub>5</sub>) ==
{| class="wikitable"
|-
! [[Chemical formula]]
! [[CAS number]]
! [[Melting point]]
! [[Boiling point]]
! P–X<sub>''ax''</sub> [[bond length]]
! P–X<sub>''eq''</sub> bond length
! X<sub>''ax''</sub>–P–X<sub>''eq''</sub> [[bond angle]]
! X<sub>''ax''</sub>–P–X<sub>''ax''</sub> bond angle
|-
| [[Phosphorus pentafluoride|PF<sub>5</sub>]]
| [7647-19-0]
| -93.7°C
| -84.5°C
| 153 pm
| 158 pm
| 120°
| 90°
|-
| [[Phosphorus pentachloride|PCl<sub>5</sub>]]
| [10026-13-8]
| 167°C
| 160°C
|
|
|
|
|-
| [[Phosphorus pentabromide|PBr<sub>5</sub>]]
| [7789-69-7]
| ~106°C ''d''
|
|
|
|
|
|-
| PI<sub>5</sub>
|
| colspan=6| ''See'' [[#Notes|''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 PX<sub>4</sub><sup>+</sup>PX<sub>6</sub><sup>–</sup>. They are widely used as chlorinating and brominating agents in organic chemistry.
== Oxidation state +3 (PX<sub>3</sub>) ==
{| class="wikitable"
|-
! [[Chemical formula]]
! [[CAS number]]
! [[Melting point]]
! [[Boiling point]]
! P–X bond length
! X–P–X bond angle
! [[Dipole moment]]
|-
| [[Phosphorus trifluoride|PF<sub>3</sub>]]
| [7783-55-3]
| -151.5°C
| -101.8°C
| 156 pm
| 96.3°
| 1.03 D
|-
| [[Phosphorus trichloride|PCl<sub>3</sub>]]
| [7719-12-2]
| -93.6°C
| 76.1°C
| 204 pm
| 100°
| 0.56 D
|-
| [[Phosphorus tribromide|PBr<sub>3</sub>]]
| [7789-60-8]
| -41.5°C
| 173.2°C
| 222 pm
| 101°
|
|-
| [[Phosphorus triiodide|PI<sub>3</sub>]]
| [13455-01-1]
| 61.2°C
| 227°C
| 243 pm
| 102°
|
|}
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 [[alcohol]]s to [[Alkyl halide|alkyl bromides]] and [[carboxylic acid]]s to [[Acyl halide|acyl bromides]] (''e.g.'' in the [[Hell-Volhard-Zelinsky reaction]]). [[Phosphorus triiodide]] also finds use in organic chemistry, as a mild oxygen acceptor.
The trihalides are fairly readily oxidized by [[chalcogen]]s to give the corresponding [[#Oxyhalides and thiohalides|oxyhalides]] or equivalents.
== Oxidation state +2 (P<sub>2</sub>X<sub>4</sub>) ==
{| class="wikitable"
|-
! [[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
|-
| [[Diphosphorus tetrafluoride|P<sub>2</sub>F<sub>4</sub>]]
| [13824-74-3]
| -86.5°C
| -6.2°C
| 159 pm
| 228 pm
| 99.1°
| 98.4°
|-
| [[Diphosphorus tetrachloride|P<sub>2</sub>Cl<sub>4</sub>]]
| [13467-91-1]
| -28°C
| ~180°C ''d''
|
|
|
|
|-
| [[Diphosphorus tetrabromide|P<sub>2</sub>Br<sub>4</sub>]]
| [24896-99-3]
| colspan=6| ''poorly characterized''
|-
| [[Diphosphorus tetraiodide|P<sub>2</sub>I<sub>4</sub>]]
| [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 ==
{| class="wikitable"
|-
! [[Chemical formula]]
! [[CAS number]]
! [[EINECS number]]
! [[Melting point]]
! [[Boiling point]]
! [[Density]]
! [[Refractive index]]
! [[Dipole moment]]
|-
| [[Phosphorus oxyfluoride|POF<sub>3</sub>]]
| ?
| ?
| −39.1°C
| −39.7°C
| ?
| ?
| ?
|-
| [[Phosphorus oxychloride|POCl<sub>3</sub>]]
| [10025-87-3]
| 233-046-7
| 1.2°C
| 105.1°C
| 1.675 g/cm<sup>3</sup>
| 1.461
| 2.54 D
|-
| [[Phosphorus oxybromide|POBr<sub>3</sub>]]
| [7785-59-5]
| 232-177-7
| 56°C
| 192°C
| 2.82 g/cm<sup>3</sup>
| ?
| ?
|-
| [[Phosphorus oxyiodide|POI<sub>3</sub>]]
| ?
| ?
| 53°C
| –
| ?
| ?
| ?
|-
| [[Thiophosphoryl fluoride|PSF<sub>3</sub>]]
| ?
| ?
| −148.8°C
| −52.2°C
| ?
| ?
| ?
|-
| [[Thiophosphoryl chloride|PSCl<sub>3</sub>]]
| [3982-91-0]
| 223-622-6
| -35°C
| 125°C
| 1.668 g/cm<sup>3</sup>
| 1.555
| ?
|-
| [[Thiophosphoryl bromide|PSBr<sub>3</sub>]]
| ?
| ?
| 37.8°C
| 212°C ''decomp''
| ?
| ?
| ?
|-
| [[Thiophosphoryl iodide|PSI<sub>3</sub>]]
| ?
| ?
| 48°C
| ''decomp''
| ?
| ?
| ?
|-
|}
The '''oxyhalides''' may be prepared from the corresponding trihalides by reaction with [[organic peroxide]]s or [[ozone]]: they are sometimes referred to as '''phosphoryl halides'''.
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.
The oxyhalides and thiohalides are significantly more [[Electrophile|electrophilic]] than the corresponding phosphorus(III) species, and present a significant [[toxic]] hazard.
== See also ==
== References ==
* [http://www.webelements.com/ WebElements]
* [http://webbook.nist.gov/chemistry/ NIST Standard Reference Database]
* N. N. Greenwood & A. Earnshaw, ''Chemistry of the Elements'' (2nd ed.), Butterworth-Heinemann, Oxford, '''1997'''.
== Notes ==
#I. Tournieporth & T. Klapötke, ''J. Chem. Soc.'', ''Chem. Commun.'' '''1990''', 132.
[[Category:Phosphorus compounds]]
[[Category:Nonmetal halides]]