Phosphoranes
A phosphorane (IUPAC name:
Phosphoranes adopt a trigonal bipyramidal molecular geometry with the two apical bonds longer than the three equatorial bonds. Bonding is described by three-center, 4-electron bonds, as also invoked for the closely related molecule phosphorus pentafluoride.[2]
Phosphoranes of the type R3P=CR2 are more common and more important. These compounds feature a tetrahedral phosphorus center including a phosphorus carbon double bond or ylide. These compounds are used as reagents in the Wittig reaction, for instance methylenetriphenylphosphorane or Ph3P=CH2.