Phosphorus(III) oxide
Ball-and-stick model of the P4O6 molecule
Phosphorus in orange, oxygen in red
Packing of P4O6 molecules in the crystal structure
Other names
Phosphorus sesquioxide
Phosphorous anhydride
Tetraphosphorous hexoxide
3D model (JSmol)
ECHA InfoCard 100.032.414 Edit this at Wikidata
EC Number
  • 235-670-5
  • InChI=1S/O6P4/c1-7-2-9-4-8(1)5-10(3-7)6-9 checkY
  • InChI=1/O6P4/c1-7-2-9-4-8(1)5-10(3-7)6-9
  • O1P3OP2OP(OP1O2)O3
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)
Acidity (pKa) 9.4
See Text
GHS labelling:
GHS06: Toxic
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
Related compounds
Other anions
Phosphorus trisulfide
Other cations
Dinitrogen trioxide
Arsenic trioxide
Antimony trioxide
Related compounds
Phosphorus pentoxide
Phosphorous acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Phosphorus trioxide is the chemical compound with the molecular formula P4O6. Although the molecular formula suggests the name 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. A white solid that melts at room temperature, it is waxy, crystalline and highly toxic, with garlic odor.[1]


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

P4 + 3 O2 → P4O6

By-products include red phosphorus suboxide.[1]

Chemical properties

Phosphorus trioxide reacts with water to form phosphorous acid, reflecting the fact that it is the anhydride of that acid.[2]

P4O6 + 6 H2O → 4 H3PO3

It 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 form diphosphorus tetraiodide.[1]

P4O6 reacts with ozone at 195 K to give the unstable compound P4O18.[3]

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

In a disproportionation reaction, 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.[3]

As a ligand

Structure of P4O6·Fe(CO)4.

P4O6 is a ligand for transition metals, comparable to phosphite. An illustrative complex is P4O6·Fe(CO)4.[4] With BH3, a dimeric adduct is produced:[3]

Liquid and solid phosphorus trioxide at its melting point
Liquid and solid phosphorus trioxide at its melting point
Structure of P8O12(BH3)2.


  1. ^ a b c A. F. Holleman; Wiberg, Egon; Wiberg, Nils (2001). Inorganic Chemistry. Boston: Academic Press. ISBN 0-12-352651-5.
  2. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  3. ^ 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.
  4. ^ M. Jansen & J. Clade (November 1996). "Tetracarbonyl(tetraphosphorus hexoxide)iron". Acta Crystallogr. C. 52 (11): 2650–2652. doi:10.1107/S0108270196004398.