Phosphorus halide

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	[7647-19-0]	-93.7°C	-84.5°C	153 pm	158 pm	120°	90°
PCl5	[10026-13-8]	167°C	160°C	202 pm	214 pm	120°	90°
PBr5	[7789-69-7]	~106°C d
PBr7	[14337-11-2]
PI5		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	[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
PBr3	[7789-60-8]	-41.5°C	173.2°C	222 pm	101°
PI3	[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 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	[13824-74-3]	-86.5°C	-6.2°C	159 pm	228 pm	99.1°	98.4°
P2Cl4	[13467-91-1]	-28°C	~180°C d
P2Br4	[24896-99-3]	poorly characterized
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. 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.

Oxyhalides and thiohalides

Chemical formula	CAS number	EINECS number	Melting point	Boiling point	Density	Refractive index	Dipole moment
POF3	[7783-55-3]	?	−39.1°C	−39.7°C	0,003596 g/cm^3	?	?
POCl3	[10025-87-3]	233-046-7	1.2°C	105.1°C	1.675 g/cm^3	1.461	2.54 D
POBr3	[7785-59-5]	232-177-7	56°C	192°C	2.82 g/cm^3	?	?
POI3	?	?	53°C	–	?	?	?
PSF3	[2404-52-6]	?	−148.8°C	−52.2°C	1.56 g/cm^3(l)	1.353	?
PSCl3	[3982-91-0]	223-622-6	-35°C	125°C	1.668 g/cm^3	1.555	?
PSBr3	[3931-89-3]	?	37.8°C	212°C decomp	?	?	?
PSI3	[63972-04-3]	?	48°C	decomp	?	?	?
PSeF3 Phosphoroselenoic trifluoride	[26083-30-7]	?	?	?	?	?	?
PSeCl3 Phosphoroselenoic trichloride	[63944-06-9]	?	?	?	?	?	?
PSeBr3	[62565-69-9]	?	?	?	?	?	?
PSeI3	[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

External links

• WebElements
• NIST Standard Reference Database