Carbanions

Chemical species bearing a negative charge on carbon and possessing eight electrons in its valence shell are called carbonions. These are produced by heterolylic cleavage of covalent bonds in which the shared pair of electrons remain with the carbon atom.

(a) Structure:

A carbanion posses an unshared pair of electron and thus represents a base. The best likely description is that the central carbon atom is sp3 hybridized with the unshared pair occupying one apex of the tetrahedron. Carbonions would thus have pyramidal structures similar to those of amines. It is believed that carbanions undergo a rapid interconversion between two pyramidal forms.

There is evidence for the sp3 nature of the central carbon and for its tetrahedral structure.
At bridgeheads carbon does not undergo reaction in which it is converted to a carbocation. However, the reactions which involve carbanions at such centre take place with ease, and stable bridgehead carbanion are known. In case this structure is correct and if all three R groups on a carbanion are different, the carbanion should give retention of configuration. However, this never happens and has been explained due to an umbrella effect as in amines. Thus the unshared pair and the central carbon rapidly oscillate from one side of the plane to the other.

(b) Stability and Generation:

The Grignard regent is the best known member of a broad class of substances, called organometallic compounds where carbon is bonded to a metal lithium, potassium sodium, zinc, mercury, lead, thallium almost any  metal known. Whatever the metal it is less electronegative than carbon and the carbon metal bond like the one in the Grignard reagent is highly polar. Although the organic group is not a full fledged carbanion an anion in which carbon carries negative charge, it however, has carbanion character organometallic compounds can serve as a source form which carbon is readily transferred with its electrons. On treatment with a metal, in RX the direction of the original dipole moment is reversed (reverse polarization)

  Also acetylide ion

(c) Properties:

carbanions are nucleophilic and basic and in this behaviour these are similar to amines, since the carbanion has a negative charge on its carbon, to make it a powerful base and a stronger nucleophile than an amine. Consequently is enough basic to remove a proton from ammonia.