Structure of oxides and oxoacids of phosphorus

Phosphorus forms an important series of oxides — principally phosphorus trioxide (P4O6) and phosphorus pentoxide (P4O10) — built on the same P4 tetrahedral framework as elemental white phosphorus. In P4O6, the four phosphorus atoms occupy the corners of a tetrahedron exactly as in P4, but each of the six edges of the tetrahedron is now bridged by an oxygen atom, so each phosphorus is bonded to three bridging oxygens (and retains a lone pair, since phosphorus is formally +3 here). The P-O bond distance in P4O6 (165.6 pm) is noticeably shorter than a typical single P-O bond (184 pm), which is attributed to pπ-dπ back-bonding between filled oxygen 2p orbitals and empty phosphorus 3d orbitals, imparting partial double-bond character. In P4O10, each phosphorus atom additionally forms one further, terminal P=O bond using its now-available lone pair as a donor into an empty oxygen-centred acceptor orbital (a coordinate/dative bond), so that every phosphorus is bonded to three bridging oxygens plus one terminal oxygen, giving the +5 oxidation state. The terminal P-O bond length in P4O10 (143 pm) is even shorter than the bridging P-O bonds, again reflecting significant π-bond character from lateral overlap between a filled oxygen 2p orbital and an empty phosphorus 3d orbital; the bridging P-O-P angle is about 123° and the O-P-O angle about 102°. …