Carbene
In organic chemistry, a 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: where the R represents substituents or hydrogen atoms.
The term "carbene" may also refer to the specific compound :CH2, also called methylene, the parent hydride from which all other carbene compounds are formally derived.
There are two types of carbenes: singlets or triplets, depending upon their electronic structure. The different classes undergo different reactions.
Most carbenes are extremely reactive and short-lived. A small number (the dihalocarbenes, carbon monoxide, and carbon monosulfide) can be isolated, and can stabilize as metal ligands, but otherwise cannot be stored in bulk. A rare exception are the persistent carbenes, which have extensive application in modern organometallic chemistry.