Carbene Ligands in Organometallic Chemistry
In organometallic species, metal complexes with the formulae LnMCRR' 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 groups. In such cases the carbenoid carbon is nucleophilic and resembles Wittig reagent (which are not considered carbene derivatives).
- Persistent carbenes, also known as Arduengo or Wanzlick carbenes. These include the class of N-heterocyclic carbenes (NHCs) and are often are used as ancillary ligands in organometallic chemistry. Such carbenes are spectator ligands of low reactivity.
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