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Phosphorane

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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 d_z² to a new sp³p orbital. The d_z² 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^- ^[1].

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.