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, transient intermediates whose intermediacy is inferred through mechanistic experiments. A small number (e.g., dihalocarbenes, carbon monosulfide) can be isolated (sometimes only at low temperatures), and can stabilize as metal ligands, but otherwise cannot be stored in bulk. A handful of persistent carbenes (including carbon monoxide, isocyanides, and some N-heterocyclic carbenes) are indefinitely stable and have extensive application in modern organometallic chemistry.