Phosphorus trioxide

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.

Preparation

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.

Chemical properties

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

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.

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

File: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.

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.

As a ligand

File:Phosphorus-trioxide-iron-tetracarbonyl-complex-from-xtal-3D-balls.png

Structure of P₄O₆·Fe(CO)₄.

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

File:Adduct of phosphorus oxide with BH3.png

Structure of P₈O₁₂(BH₃)₂.