Phosphorus trioxide

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Phosphorus trioxide
Ball-and-stick model of the P4O6 molecule
Phosphorus in orange, oxygen in red
Packing of P4O6 molecules in the crystal structure
Identifiers
CAS number 1314-24-5 YesY
PubChem 123290
ChemSpider 109897 YesY
ChEBI CHEBI:37372 YesY
Jmol-3D images Image 1
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 (74.8 °F; 296.9 K)
Boiling point 173.1 °C (343.6 °F; 446.2 K)
Solubility in water reacts
Acidity (pKa) 9.4
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
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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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]

Preparation[edit]

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[edit]

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]

When P4O6 is allowed to react with ozone at 195 K in dichloromethane, the highly unstable compound P4O18 is formed.[2]

Reaction of phosphoprus trioxide with ozone.png

P4O18 decomposes above 238 K in solution with the release of O2 gas. Decomposition of dry P4O18 is explosive.

P4O6 is converted into the mixed P(III)P(V) species P4O8 when heated in a sealed tube at 710 K, with the side product being red phosphorus.[2]

As a ligand[edit]

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[3] is shown below:

Tetraphosphorus-hexaoxide-iron-tetracarbonyl-complex-from-xtal-3D-balls.png

P4O6 can also donate electrons to Lewis bases such as BH3. Adducts with one to two equivalents of BH3 complexed with one mole of P4O6 has been reported. When P4O6 is reacted with one molar equivalent of (CH3)2S•BH3, followed by slow crystallization from the toluene solution at 244K, P8O12(BH3)2 rather than an adduct of P4O6 is formed:[2]

Adduct of phosphorus oxide with BH3.png

References[edit]

  1. ^ a b c A. F. Holleman; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Boston: Academic Press. ISBN 0-12-352651-5. 
  2. ^ a b c .Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 473. ISBN 978-0-13-175553-6. 
  3. ^ M. Jansen and J. Clade (November 1996). "Tetracarbonyl(tetraphosphorus hexaoxide)iron". Acta Cryst. C52 (11): 2650–2652. doi:10.1107/S0108270196004398. 

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