Ylides

A neutral dipolar molecule called a ylide or ylids has two formally positively charged heteroatoms (often nitrogen, phosphorus, or sulphur) directly connected, and both atoms have complete octets of electrons. The resulting structure may be thought of as two neighbouring atoms joined by both an ionic and a covalent connection. Ylides are a category of zwitterions and 1,2-dipolar molecules. They show up as reagents or reactive intermediates in organic chemistry. The Wittig reaction, which transforms ketones and in particular aldehydes to alkenes, uses phosphonium ylides. In these Wittig reagents, a phosphorus atom with three phenyl substituents and a link to a carbanion carries the positive charge.