Phosphorus halide

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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.

Contents

Oxidation state +5 (PX5)

Chemical formula CAS number Melting point Boiling point PXax bond length PXeq bond lengthXeqPXeq bond angle XaxPXeq bond angle
PF5 Phosphorus pentafluoride [7647-19-0]-93.7°C-84.5°C153 pm158 pm120°90°
PCl5 Phosphorus pentachloride [10026-13-8]160°C167°C214 pm202 pm120°90°
PBr5 Phosphorus pentabromide [7789-69-7]~106°C d     
PBr7 Phosphorus heptabromide [14337-11-2]      
PI5 Phosphorus pentaiodide  See Tornieporth-Getting & Klapötke 1990

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 PX bond lengthXPX bond angle Dipole moment
PF3 Phosphorus trifluoride [7783-55-3]-151.5°C-101.8°C156 pm96.3°1.03 D
PCl3 Phosphorus trichloride [7719-12-2]-93.6°C76.1°C204 pm100°0.56 D
PBr3 Phosphorus tribromide [7789-60-8]-41.5°C173.2°C222 pm101° 
PI3 Phosphorus triiodide [13455-01-1]61.2°C227°C243 pm102° 
P(CN)3 Phosphorus tricyanide [2] 203°C179 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 PX bond lengthPP bond lengthXPX bond angleXPP bond angle
P2F4 Diphosphorus tetrafluoride [13824-74-3]-86.5°C-6.2°C159 pm228 pm99.1°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°Cd248 pm221 pm102.3°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 selenohalides

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°C0,003596 g/cm3 ? ?
POCl3 Phosphorus oxychloride [10025-87-3]233-046-71.2°C105.1°C1.675 g/cm31.4612.54 D
POBr3 Phosphorus oxybromide [7785-59-5]232-177-756°C192°C2.82 g/cm3 ? ?
P2O3Cl4 Pyrophosphoryl chloride [13498-14-1] ?66–68 °C (0.01 Torr)1.74 g/cm3 ? ?
PSF3 Thiophosphoryl fluoride [2404-52-6] ?−148.8°C−52.2°C1.56 g/cm3(l)1.353 ?
PSCl3 Thiophosphoryl chloride [3982-91-0]223-622-6-35°C125°C1.668 g/cm31.555 ?
PSBr3 Thiophosphoryl bromide [3931-89-3] ?37.8°C212°C decomp ? ? ?
PSI3 Thiophosphoryl iodide [63972-04-3] ?48°Cdecomp ? ? ?

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.

Selenophosphoryl fluoride is unstable at room temperature, decomposing to PF3 and Se. [3]

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Iodine compounds are compounds containing the element iodine. Iodine can form compounds using multiple oxidation states. Iodine is quite reactive, but it is much less reactive than the other halogens. For example, while chlorine gas will halogenate carbon monoxide, nitric oxide, and sulfur dioxide, iodine will not do so. Furthermore, iodination of metals tends to result in lower oxidation states than chlorination or bromination; for example, rhenium metal reacts with chlorine to form rhenium hexachloride, but with bromine it forms only rhenium pentabromide and iodine can achieve only rhenium tetraiodide. By the same token, however, since iodine has the lowest ionisation energy among the halogens and is the most easily oxidised of them, it has a more significant cationic chemistry and its higher oxidation states are rather more stable than those of bromine and chlorine, for example in iodine heptafluoride.

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Einsteinium compounds are compounds that contain the element einsteinium (Es). These compounds largely have einsteinium in the +3 oxidation state, or in some cases in the +2 and +4 oxidation states. Although einsteinium is relatively stable, with half-lives ranging from 20 days upwards, these compounds have not been studied in great detail.

Protactinium compounds are compounds containing the element protactinium. These compounds usually have protactinium in the +5 oxidation state, although these compounds can also exist in the +2, +3 and +4 oxidation states.

References

  1. I. Tornieporth-Getting & T. Klapötke, J. Chem. Soc., Chem. Commun.1990, 132. doi:10.1039/C39900000132
  2. It is a pseudohalide.
  3. Hagen, Arnulf P.; Elphingstone, Eugene A. (February 1973). "High pressure interaction of phosphorus trifluoride with oxygen, sulfur, selenium, and tellurium". Inorganic Chemistry. 12 (2): 478–480. doi:10.1021/ic50120a050.