text: In organometallic species, metal complexes with the formulae LMCRR' are often described as carbene complexes. Such species do not however react like free carbenes and are rarely generated from carbene precursors, except for the persistent carbenes.  The transition metal carbene complexes can be classified according to their reactivity, with the first two classes being the most clearly defined:
  *Fischer carbenes, in which the carbene is bonded to a metal that bears an electron-withdrawing group (usually a carbonyl).  In such cases the carbenoid carbon is mildly electrophilic.
  *Schrock carbenes, in which the carbene is bonded to a metal that bears an electron-donating group.  In such cases the carbenoid carbon is nucleophilic and resembles a Wittig reagent (which are not considered carbene derivatives).
  *Carbene radicals, in which the carbene is bonded to an open-shell metal with the carbene carbon possessing a radical character.  Carbene radicals have features of both Fischer and Schrock carbenes, but are typically long-lived reaction intermediates.
  *N-Heterocyclic (NHC), Arduengo or Wanzlick carbenes are C-deprotonated imidazolium or dihydroimidazolium salts.  They often are deployed as ancillary ligands in organometallic chemistry.  Such carbenes are usually very strong σ-donor spectator ligands, similar to phosphines.