A variety of organic compounds which contain two adjacent atoms bearing formal positive and negative charges, and in which both atoms have full octets of electrons. Heteroatoms most commonly utilized as the positive atom are phosphorus, nitrogen, sulfur, selenium, and oxygen, and the negative atom usually involves carbon, nitrogen, oxygen, or sulfur. The ylide may be in an alicyclic or cyclic environment, and in the latter the ylide function may be endocyclic or exocyclic. Ylides most useful in organic synthesis are those containing phosphorus or sulfur and an adjacent carbanion.