Overlapping of empty p- orbital(carbocation) or singly occupied p orbital (free radical) with adjacent sigma bond is called hypercongugation. ... But carbanion contains filled p-orbital so overlapping does not take place. That's why carbanions do not show hypercongugation.

Why there is no hyperconjugation in carbanion?

The carbon holding negative charge in carbanion comprises one sp3 hybrid orbital that contains a pair of electrons and thus, due to repulsion between the electrons, there is no risk that it will undergo bonding contact with the adjacent C-H bond.

Does carbanion have resonance?

Carbocations, radicals, and carbanions can be stabilized by resonance. For example, if a carbon atom with a π bond is bonded to the trivalent carbon atom of a reaction intermediate, the empty orbital of that carbon atom can interact with the 2p orbitals of the π bond. The result is a resonance-stabilized intermediate.