A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons. The carbanion exists in a trigonal planar geometry. Formally a carbocation is the conjugate base of a carbon acid
R3C-H + B → R3C- + H-B
where B stands for the base.
A Carbanion is a nucleophile. The stability and reativity of a carbanion is determined by the inductive effect .
A carbanion is a reactive intermediate and is encountered in organic chemistry for instance in a elimination reaction and in organometallic chemistry in for instance a Grignard reaction or in alkyl lithium chemistry.
Stable carbanions do however exist. In 1984 Olmstead presented the lithium crown ether salt of the diphenylmethyl carbanion from diphenylmethane, butyl lithium and 12-crown-4 at low temperatures [1].
references
[1] The isolation and x-ray structures of lithium crown ether salts of the free phenyl carbanions [CHPh2]- and [CPh3]- Marilyn M. Olmstead, Philip P. Power; J. Am. Chem. Soc.; 1985; 107(7); 2174-2175.