Structure of the Wittig reagent

One of the simplest ylides is methylenetriphenylphosphorane (Ph₃P=CH₂).  The Wittig reagent may be written in the phosphorane form or the ylide form:

The ylide form is a significant contributor, and the carbon is nucleophilic. It has been noted that dipolar phosphorus compounds are stabilized by p-d bonding. This bonding stabilization extends to carbanions adjacent to phosphonium centers.  An ylide is defined as a compound with opposite charges on adjacent atoms, both of which have complete octets.

Reactivity

Simple phosphoranes (Wittig reagents) are reactive towards air and water, so they are usually handled under nitrogen.  These will react easily with most aldehydes and ketones.

However, phosphoranes some are stabilized by electron-withdrawing groups, as in Ph₃P=CHCO₂R and Ph₃P=CHPh. These ylides are sufficiently stable to be sold commercially

From the phosphonium salts, these reagents are formed more readily, requiring only a moderate base such as NaOH, and they are usually more air-stable. Stabilized Wittig reagents are less reactive than simple ylides, and so they usually fail to react with ketones, and they usually give rise to an E-alkene product when they react, rather than the more usual Z-alkene.