Phosphorus halide

In chemistry, there are three series of binary phosphorus halides, containing phosphorus in the oxidation states +5, +3 and +2. All compounds have been described, in varying degrees of detail, although serious doubts have been cast on the existence of PI₅.^[1] Mixed chalcogen halides also exist.

Oxidation state +5 (PX₅)

Chemical formula	CAS number	Melting point	Boiling point	P–Xax bond length	P–Xeq bond length	Xeq–P–Xeq bond angle	Xax–P–Xeq bond angle
PF5 Phosphorus pentafluoride	[7647-19-0]	-93.7°C	-84.5°C	153 pm	158 pm	120°	90°
PCl5 Phosphorus pentachloride	[10026-13-8]	160°C	167°C	214 pm	202 pm	120°	90°
PBr5 Phosphorus pentabromide	[7789-69-7]	~106°C d
PBr7 Phosphorus heptabromide	[14337-11-2]
PI5 Phosphorus pentaiodide		See Note 1.

In the gas phase the phosphorus pentahalides have trigonal bipyramidal molecular geometry as explained by VSEPR theory.

Phosphorus pentafluoride is a relatively inert gas, notable as a mild Lewis acid and a fluoride ion acceptor. It is a fluxional molecule in which the axial (ax) and equatorial (eq) fluorine atoms interchange positions by the Berry pseudorotation mechanism.

Phosphorus pentachloride, phosphorus pentabromide, and phosphorus heptabromide are ionic in the solid and liquid states; PCl₅ is formulated as PCl₄⁺PCl₆^–, but in contrast, PBr₅ is formulated as PBr₄⁺ Br⁻, and PBr₇ is formulated as PBr₄⁺ Br₃⁻. 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 Phosphorus trifluoride	[7783-55-3]	-151.5°C	-101.8°C	156 pm	96.3°	1.03 D
PCl3 Phosphorus trichloride	[7719-12-2]	-93.6°C	76.1°C	204 pm	100°	0.56 D
PBr3 Phosphorus tribromide	[7789-60-8]	-41.5°C	173.2°C	222 pm	101°
PI3 Phosphorus triiodide	[13455-01-1]	61.2°C	227°C	243 pm	102°
P(CN)3 Phosphorus tricyanide[2]		203°C		179 pm

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-Volhard-Zelinsky reaction). Phosphorus triiodide also finds use in organic chemistry, as a mild oxygen acceptor.

The trihalides are fairly readily oxidized by chalcogens to give the corresponding oxyhalides or equivalents.

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 Diphosphorus tetrafluoride	[13824-74-3]	-86.5°C	-6.2°C	159 pm	228 pm	99.1°	98.4°
P2Cl4 Diphosphorus tetrachloride	[13467-91-1]	-28°C	~180°C d
P2Br4 Diphosphorus tetrabromide	[24856-99-3]	poorly characterized
P2I4 Diphosphorus tetraiodide	[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.^{[citation needed]} The relatively stable P₂I₄ is known to have a trans, bent configuration similar to hydrazine and finds some uses in organic syntheses, the others are of purely academic interest at the present time. Diphosphorus tetrabromide is particularly poorly described. They are subhalides of phosphorus.

Oxyhalides,thiohalides and selehalides

Chemical formula	CAS number	EINECS number	Melting point	Boiling point	Density	Refractive index	Dipole moment
POF3 Phosphorus oxyfluoride	[13478-20-1]	?	−39.1°C	−39.7°C	0,003596 g/cm^3	?	?
POCl3 Phosphorus oxychloride	[10025-87-3]	233-046-7	1.2°C	105.1°C	1.675 g/cm^3	1.461	2.54 D
POBr3 Phosphorus oxybromide	[7785-59-5]	232-177-7	56°C	192°C	2.82 g/cm^3	?	?
POI3 Phosphorus oxyiodide	[13455-04-4]	?	53°C	–	?	?	?
PSF3 Thiophosphoryl fluoride	[2404-52-6]	?	−148.8°C	−52.2°C	1.56 g/cm^3(l)	1.353	?
PSCl3 Thiophosphoryl chloride	[3982-91-0]	223-622-6	-35°C	125°C	1.668 g/cm^3	1.555	?
PSBr3 Thiophosphoryl bromide	[3931-89-3]	?	37.8°C	212°C decomp	?	?	?
PSI3 Thiophosphoryl iodide	[63972-04-3]	?	48°C	decomp	?	?	?
PSeF3 Selenophosphoryl fluoride Phosphoroselenoic trifluoride	[26083-30-7]	?	?	?	?	?	?
PSeCl3 Selenophosphoryl bromide Phosphoroselenoic trichloride	[63944-06-9]	?	?	?	?	?	?
PSeBr3 Selenophosphoryl bromide	[62565-69-9]	?	?	?	?	?	?
PSeI3 Selenophosphoryl iodide	[39383-15-8]	?	?	?	?	?	?

The oxyhalides may be prepared from the corresponding trihalides by reaction with organic peroxides 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 electrophilic than the corresponding phosphorus(III) species, and present a significant toxic hazard.

References

1. I. Tornieporth-Getting & T. Klapötke, J. Chem. Soc., Chem. Commun. 1990, 132.doi:10.1039/C39900000132
2. It is a pseudohalide.

External links

• WebElements
• NIST Standard Reference Database