Transition metal carbene complex
A transition metal carbene complex in organometallic chemistry is a compound bearing a formal carbon-metal bond. The ligands coordinated to the metal center are carbenes. All transition metals are able to form these types of complexes. Many methods for synthesizing them and reactions utilizing them have been reported.
When the development of transition metal carbene complexes started in the mid-1970s, two different patterns of reactivity were discovered. At that time, these compounds were divided into two types. One of these types is called a Fischer Carbene  named after Ernst Otto Fischer with strong -acceptors at the metal. This complex is electrophilic at the carbene carbon atom which as considered in a singlet state. Following Arduengo's isolation of a stable free carbene in 1991, a new field of transition metal complexes with N-heterocyclic carbenes (NHCs) has been opened.

# Fischer carbenes
Fischer carbenes are found with :
- low oxidation state metals
- middle and late transition metals Fe(0), Mo(0), Cr(0)
- pi electron acceptor metal ligands
- pi-donor substituents on methylene group such as alkoxy and alkylated amino groups
The chemical bonding (scheme 1) is based on electron -type donation of the filled metal d-orbital to the empty p-orbital of the methylene group and pi electron back bonding of the filled methylene lone pair orbital to an empty metal d-orbital. An example is the complex (CO)5Cr=C(NR2)Ph.
Fischer carbenes can be likened to ketones, with the carbene carbon being nucleophilic, much like the carbonyl carbon of a ketone. Like ketones, Fischer carbene species can undergo Aldol-like reactions. The hydrogen atoms attached to the carbon  to the carbene carbon are acidic, and can be deprotonated by a base such as n-butyl lithium, to give a nucleophile which can undergo further reaction.
This type of carbenes was discovered by E. O. Fischer and together for other achievements in organometalic chemistry he was awarded the Noble prize.

# Schrock carbenes
Schrock carbene, named after Richard R. Schrock do not have -accepting ligands. This complex is nucleophilic at the carbene carbon atom in an unpaired triplet state.
Schrock carbenes are found with:
- high oxidation states
- early transition metals Ti(IV), Ta(V)
- non pi-acceptor ligands
- non pi-donor substituents
Bonding in this complex takes place when two methylene p-orbitals each containing a radical form two covalent bonds. These bonds are polarized towards carbon and therefore the methylene group is a nucleophile. An example of a Schrock carbene is the compound Cp2(Me)Ta=CH2 with a tantalum atom coordinated to a methylene group, a methyl group and two cyclopentadienyl groups. Another example is Tebbe's reagent.
Today, many carbene complexes bearing a broad range of different reactivities have been prepared. Often it is no longer possible to predict whether a carbene complex will behave as an electrophile or as a nucleophile. Thus, a reactivity-based nomenclature would be difficult to apply consistently.

# N-heterocyclic carbenes
Typically, N-heterocyclic carbenes are good -donors but poor -acceptors. Due to the poor -backbonding in NHC-metal complexes, the bond between the carbon and the metal center is usually formally represented by a single bond compared with Fischer and Schrock carbenes.
In catalytic studies, NHCs are often compared with well-established phosphine-based complexes. The advantages of NHCs over phosphines include the lack of toxicity compared with phosphines and the ease of handling.