Persistent carbene

A persistent carbene (also known as stable carbene) is a class of organic molecules admitting a formal resonance structure with incomplete octet on a carbon atom (a carbene), but without the tremendous instability typically associated with such moieties. Modern theoretical analysis suggests that the ground state electronic structure in such compounds is not in fact a carbene, but instead an ylide stabilized by aromatic resonance or steric shielding; excitation to a carbene structure then accounts for the carbene-like dimerization that some persistent carbenes undergo over the course of days.

The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC) (sometimes called Arduengo carbenes), in which the formal carbene is flanked by nitrogen atoms. Persistent carbenes in general, and Arduengo carbenes in particular, are popular ligands in organometallic chemistry.