Carbene is simply written as CH2, no need to put negative charge. It's a neutral compound.
carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R-(C:)-R' or R=C:.
The term "carbene" may also refer to the specific compound H2C:, also called methylene.carbene only has 6 valence electrons and is neutral,
Carbanion has 8 valrnce electron and is charged.
A carboanion is an organic molecular species with a negative electrical charge located on a carbon atom. In other words, it is an anion in which a carbon atom possess an unshared pair of electrons with three substituents.