Compounds in which an anionic site $\ce{Y^{−}}$ (originally on carbon, but now including other atoms) is attached directly to a heteroatom $\ce{X^{+}}$ (usually nitrogen, phosphorus or sulfur) carrying a formal positive charge. They are thus 1,2-dipolar species of the type $\ce{R_{m}X^{+}–Y^{−}R^{n}}$. If $\ce{X}$ 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 $\ce{X}$ is a second, third, etc. row element uncharged canonical forms are available $\ce{R_{m}X=YR_{n}}$. If $\ce{X}$ is an unsaturated atom, doubly bonded to another first row element $\ce{Z}$, the negative charge on $\ce{Y}$ may be stabilized by $\ce{\uppi\!\mbox{-}}$conjugation, $\ce{Z=X^{+}–Y^{−}Rn <-> Z^{−}–X^{+}=YR_{n}}$. 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. $\ce{Ph3P^{+}–C^{−}H2 <-> Ph3P^{+}=CH2}$ (often called a Wittig reagent), $\ce{(CH3)3N^{+}–C^{−}H2}$, $\ce{RC#N^{+}N^{−}–R}$, $\ce{(CH3)2S=CHPh <-> (CH3)2S^{+}–C^{−}HPh}$. Note that ylide is a complete word, not to be confused with the suffix -ylide, used for some radical anions. Subclasses of ylides: Ylides $\ce{R_{m}X^{+}–C^{−}R2}$ having the negative charge on carbon are classified by citing the name of the element $\ce{X}$ 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 $\ce{R_{m}X}$ before the word ylide. Thus nitrogen ylides include amine ylides, $\ce{R3N^{+}–C^{−}R2}$, azomethine ylides $\ce{R2C=N^{+}R–C^{−}R2}$, nitrile ylides, $\ce{RC#N^{+}–C^{−}R2}$. Some authors, who wish to express the positive charge on $\ce{X}$, prefer e.g. ammonium ylides over amine ylides; such usage varies according to the heteroatom $\ce{X}$ and to national custom. The ylides $\ce{R_{m}X^{+}–Y^{−} <-> R_{m}X=Y}$ ($\ce{Y} = \ce{O}, \ce{S}, \ce{Se}, \ce{Te}, \ce{NR}$) are usually named by citing the name of $\ce{R_{m}X}$ followed by the additive nomenclature term for $\ce{Y}$ (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).