Phosphorus-containing groups carbanion stabilization

Stabilization by Sulfur or phosphorus. Attachment to the carbanionic carbon of a sulfur or phosphorus atom causes an increase in carbanion stability, though the reasons for this are in dispute. One theory is that there is overlap of the unshared pair with an empty d orbital" (pn-dn bonding, see p. 45). For example, a carbanion containing the SO2R group would be written... 

Another useful reagent in nucleophilic additions contains a carbanion that is stabilized by an adjacent, positively charged phosphorus group. Such a species is called a phosphorus ylide, and its attack on aldehydes and ketones is called the Wittig reaction. The Wittig reaction is a powerful method for the selective synthesis of alkenes from aldehydes and ketones. 

---