Editing Carbanion (section)

== Geometry ==
Absent [[Conjugated system|π delocalization]], the negative charge of a carbanion is localized in an [[Orbital hybridisation#spx hybridisation|sp<sup>''x''</sup>]] hybridized orbital on carbon as a [[lone pair]]. As a consequence, ''localized'' alkyl, alkenyl/aryl, and alkynyl carbanions assume trigonal pyramidal, bent, and linear geometries, respectively. By [[Bent's rule]], placement of the carbanionic lone pair electrons in an orbital with significant s character is favorable, accounting for the pyramidalized and bent geometries of alkyl and alkenyl carbanions, respectively. [[VSEPR theory|Valence shell electron pair repulsion (VSEPR) theory]] makes similar predictions. This contrasts with carbocations, which have a preference for unoccupied nonbonding orbitals of pure atomic p character, leading to planar and linear geometries, respectively, for alkyl and alkenyl carbocations.[[File:Carbanion Structural Formulae V.1.svg|thumb|250x250px|An alkyl carbanion is trigonal pyramidal.]]
[[File:Ez-isomerismofvinylanions.png|thumb|Vinyl anions are bent. 1,2-Disubstituted vinyl anions have ''E'' and ''Z'' isomers that undergo inversion through a linear transition state.|300x300px]]
However, ''delocalized'' carbanions may deviate from these geometries. Instead of residing in a hybrid orbital, the carbanionic lone pair may instead occupy a p orbital (or an orbital of high p character). A p orbital has a more suitable shape and orientation to overlap with the neighboring π system, resulting in more effective charge delocalization. As a consequence, alkyl carbanions with neighboring conjugating groups (e.g., allylic anions, enolates, nitronates, etc.) are generally planar rather than pyramidized. Likewise, delocalized alkenyl carbanions sometimes favor a linear instead of bent geometry. More often, a bent geometry is still preferred for substituted alkenyl anions, though the linear geometry is only ''slightly'' less stable, resulting in facile equilibration between the (''E'') and (''Z'') isomers of the (bent) anion through a linear [[transition state]].<ref>{{Cite journal|last1=Caramella|first1=Pierluigi|last2=Houk|first2=K. N.|date=1981-01-01|title=The influence of electron-withdrawing substituents on the geometries and barriers to inversion of vinyl anions|url=https://dx.doi.org/10.1016/0040-4039%2881%2980005-6|journal=Tetrahedron Letters|language=en|volume=22|issue=9|pages=819–822|doi=10.1016/0040-4039(81)80005-6|issn=0040-4039|url-access=subscription}}</ref> For instance, calculations indicate that the parent vinyl anion or ethylenide, {{chem2|H2C\dCH−}}, has an inversion barrier of {{cvt|27|kcal/mol|kJ/mol}}, while allenyl anion or allenide, {{chem2|H2C\dC\dCH− ↔ H2C−\sC\tCH}}), whose negative charge is stabilized by delocalization, has an inversion barrier of only {{cvt|4|kcal/mol|kJ/mol}}, reflecting stabilization of the linear transition state by better π delocalization.<ref>{{Cite book|last=Alabugin|first=Igor V.|url=http://doi.wiley.com/10.1002/9781118906378|title=Stereoelectronic Effects: A Bridge Between Structure and Reactivity|date=2016-09-19|publisher=John Wiley & Sons, Ltd|isbn=978-1-118-90637-8|location=Chichester, UK|language=en|doi=10.1002/9781118906378}}</ref>