Phosphorus Pentoxide - Structure

Structure

Phosphorus pentoxide crystallizes in at least four forms or polymorphs. The most familiar one, shown in the figure, comprises molecules of P₄O₁₀. Weak van der Waals forces hold these molecules together in a hexagonal lattice (However, in spite of the high symmetry of the molecules, the crystal packing is not a close packing). The structure of the P₄O₁₀ cage is reminiscent of adamantane with T_d symmetry point group. It is closely related to the corresponding anhydride of phosphorous acid, P₄O₆. The latter lacks terminal oxo groups. Its density is 2.30 g/cm3. It boils at 423 °C under atmospheric pressure; if heated more rapidly it can sublimate. This form can be made by condensing the vapor of phosphorus pentoxide rapidly, the result is an extremely hygroscopic solid.

The other polymorphs are polymeric, but in each case the phosphorus atoms are bound by a tetrahedron of oxygen atoms, one of which forms a terminal P+–O− bond (the common representation of a P=O bond as in the image above is incorrect as computational chemistry has found that an expanded d-orbital model is not a very stabilising interaction and therefore not an important contributor to bonding). The macromolecular form can be made by heating the compound in a sealed tube for several hours, and maintaining the melt at a high temperature before cooling the melt to the solid. The O-form (density 3.05 g/cm3, melting point 580 °C), adopts a layered structure consisting of interconnected P₆O₆ rings, not unlike the structure adopted by certain polysilicates. A lower density phase, the so-called O' form, consists of a 3-dimensional framework is also known, density 2.72 g/cm3. The remaining polymorph is a glass or amorphous form; it can be made by fusing any of the others.