Compounds in which an anionic site Y⁻ (originally on carbon, but now including other atoms) is attached directly to a heteroatom X⁺ (usually nitrogen, phosphorus or sulfur) carrying a formal positive charge. They are thus 1,2-dipolar species of the type R_mX⁺–Y⁻R_n. If X is a saturated atom of an element from the first row of the periodic system, the ylide is commonly represented by a charge-separated form; if X is a second, third, etc. row element uncharged canonical forms are available R_mX=YR_n. If X is an unsaturated atom, doubly bonded to another first row element Z, the negative charge on Y may be stabilized by π-conjugation, Z=X⁺–Y⁻R_n↔︎ Z⁻–X⁺=YR_n. Such ylides belong to the class 1,3 dipolar compounds. However, 1,3-dipolar compounds with only sextet-containing canonical forms (e.g. vinylcarbenes) are not ylides. E.g. Ph₃P⁺–C⁻H₂↔︎ Ph₃P⁺=CH₂ (often called a Wittig reagent), (CH₃)₃N⁺–C⁻H₂, RC≡N⁺N⁻–R, (CH₃)₂S=CHPh↔︎ (CH₃)₂S⁺–C⁻HPh. Note that ylide is a complete word, not to be confused with the suffix -ylide, used for some radical anions. Subclasses of ylides: Ylides R_mX⁺–C⁻R₂ having the negative charge on carbon are classified by citing the name of the element X before the word ylide. E.g. nitrogen ylide, phosphorus ylide, oxygen ylide, sulfur ylide. A further specification may be achieved by citing the class name of R_mX before the word ylide. Thus nitrogen ylides include amine ylides, R₃N⁺–C⁻R₂, azomethine ylides R₂C=N⁺R–C⁻R₂, nitrile ylides, RC≡N⁺–C⁻R₂. Some authors, who wish to express the positive charge on X, prefer e.g. ammonium ylides over amine ylides; such usage varies according to the heteroatom X and to national custom. The ylides R_mX⁺–Y⁻↔︎ R_mX=Y (Y = O, S, Se, Te, NR) are usually named by citing the name of R_mX followed by the additive nomenclature term for Y (oxide, sulfide, selenide, telluride, imide, respectively). E.g. amine imides; use of the less systematic synonyms amine imines and aminimines is discouraged. Some classes of ylides are known by trivial names e.g. nitrones, Wittig reagents (synonymous with phosphonium ylides).