Phosphorus trioxide

Phosphorus trioxide
Other names Phosphorus(III) oxide, Phosphorus sesquioxide, Phosphorous oxide, Phosphorous anhydride
Identifiers
CAS number	1314-24-5
PubChem	123290
ChemSpider	109897 Y
ChEBI	CHEBI:37372 Y
Jmol-3D images	Image 1
SMILES O1P3OP2OP(OP1O2)O3
InChI InChI=1S/O6P4/c1-7-2-9-4-8(1)5-10(3-7)6-9 Y Key: VSAISIQCTGDGPU-UHFFFAOYSA-N Y InChI=1/O6P4/c1-7-2-9-4-8(1)5-10(3-7)6-9 Key: VSAISIQCTGDGPU-UHFFFAOYAV
Properties
Molecular formula	P4O6
Molar mass	219.88 g mol−1
Appearance	colourless monoclinic crystals or liquid
Density	2.135 g/cm3
Melting point	23.8 °C
Boiling point	173.1 °C
Solubility in water	reacts
Acidity (pKa)	9.2
Structure
Molecular shape	See Text
Dipole moment	0
Related compounds
Other anions	Phosphorus trisulfide
Other cations	Dinitrogen trioxide Arsenic trioxide Antimony trioxide
Related compounds	Phosphorus pentoxide Phosphorous acid
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Phosphorus trioxide is the chemical compound with the molecular formula P₄O₆. Although it should properly be named tetraphosphorus hexoxide, the name phosphorus trioxide preceded the knowledge of the compound's molecular structure, and its usage continues today. This colorless solid is structurally related to adamantane. It is formally the anhydride of phosphorous acid, H₃PO₃, but cannot be obtained by the dehydration of the acid. It is a white, waxy, crystalline and highly toxic solid^[1] and has an odor similar to that of garlic.^{[citation needed]}

Preparation

It is obtained by the combustion of phosphorus in a limited supply of air at low temperature.

P₄(s) + 3 O₂(g) → P₄O₆(s)

By-products include red phosphorus suboxide.^[1]

Chemical Properties

Phosphorus trioxide reacts with cold water to form phosphorous acid.

P₄O₆(s) + 6 H₂O(l) → 4 H₃PO₃(aq)

It reacts vigorously with hot water, via a complex set of reactions, to form red phosphorus, phosphines, H₃PO₃ and H₃PO₄.

P₄O₆ reacts with hydrogen chloride to form H₃PO₃ and phosphorus trichloride.

P₄O₆ + 6 HCl → 2 H₃PO₃ + 2 PCl₃

With chlorine or bromine it forms the corresponding phosphoryl halide, and it reacts with iodine in a sealed tube to from diphosphorus tetraiodide.^[1]

As a ligand

P₄O₆ is a ligand for transition metals, comparable to phosphite. Tetracarbonyl(tetraphosphorus hexaoxide)iron, P₄O₆·Fe(CO)₄, is an example of a complex containing the phosphorus trioxide cage as a ligand. Its molecular structure as determined by single-crystal X-ray diffraction^[2] is shown below:

References

1. ^ ^{a b c} A. F. Holleman; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Boston: Academic Press. ISBN 0-12-352651-5. 
2. ^ M. Jansen and J. Clade (November 1996). "Tetracarbonyl(tetraphosphorus hexaoxide)iron". Acta Cryst. C52 (11): 2650–2652. doi:10.1107/S0108270196004398.