Phosphorane



A phosphorane is a functional group in chemistry with pentavalent phosphorus. It has the general structure PR5. The parent compound is the non-stable phosphoran PH5 or λ5-Phosphan (lambda5phosphan) according to IUPAC nomenclature. In the same vein Pentaphenylphosphoran Ph5P is properly named Pentaphenyl-λ5-Phosphan.

Phosphoranes adopt a trigonal bipyramidal molecular geometry with the two apical bonds longer than the three equatorial bonds. The accepted view for the chemical bonding taking place is that of a rehybridization in phosphorus of sp3 orbitals with a dz2 to a new sp3p orbital. The dz2 is considered very energetic and alternative theories attempt to explain this type of bonding better. In a three-center four-electron bond model two resonance structures along the apical bonds are X-...P+-X and X-P+...X-.

Phosphoranes of the type R3P=CR2 with a phosphorus carbon double bond or ylide are reagents in the Wittig reaction, for instance methylenetriphenylphosphorane or Ph3P=CH2.

Other phosphorus compounds are trivalent phosphines and tetravalent phosphonium salts.