Metal carbene and carbyene complexes

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Carbene:-
Synopsis:-Definition,different type of carbene,structure,reactivity and synthesis.

Definition:-• A carbene is a divalent carbon species linked to two adjacent groups by a covalent bond • It possess two non-bonding electrons and six valence electrons • If the non-bonding electrons have anti-parallel spins then singlet carbene • If the non-bonding electrons have parallel spins in different orbitals then triplet carbene • Generally carbenes are expected to be triplet carbenes (Hund's rule) but substituents can change this and in organic chemistry we normally use singlet carbenes • They are electron deficient like carbocations • But they possess a non-bonding pair like carbanion hence can be represented as shown above • The nature of substituents R have profound effects on the electronics of the carbenes and their reactions .


Different type of carbene:-
1.Fischer carbenes
Fischer carbenes are found with: 
- low oxidation state metal center
-middle and late transition metals Fe(0), Mo(0), Cr(0) 
-π-acceptor metal ligands 
-π-donor substituents on the carbene atom such as alkoxy and alkylated amino groups.

2.Schrock carbenes
Schrock carbenes do not have π-accepting ligands. These complexes are nucleophilic at the carbene carbon atom. Schrock carbenes are typically found with: 
 -high oxidation state metal center 
-early transition metals Ti(IV), Ta(V) 
-π-donor ligands 
-hydrogen and alkyl substituents on carbenoid carbon.
3.N-Heterocyclic carbenes
 -IMes is a common NHC ligand. 
N-Heterocyclic carbenes (NHCs) are particularly common carbene ligands. They are popular because they are more readily prepared than Schrock and Fischer carbenes. In fact many NHCs are isolated as the free ligand, since they are persistent carbenes.

4.Carbene radicals
Carbene radicals are long-lived reaction intermediates found with
 -low oxidation state metal center with singly occupied dz2 orbital 
-middle and late transition metal, e.g. Co(II) 
-σ-donor and π-acceptor ligand 
-π-acceptor substituents on the ligand such as carbonyl or sulfonyl groups. The chemical bond -present in carbene radicals is described as aspects of both Fischer and Schrock carbenes.

Carbenoids • A slightly confusing class of compounds • Includes intermediates that exhibit reactions similar to carbenes without necessarily having any structures defined previously • For the purposes of this course we will limit ourselves to the following: • Decomposition of diazo-compounds in the presence of Rh, Cu, Pd 

Structure:-The LnM=CR2 type Fischer carbene complexes comprise of two dative covalent interactions that include (i) a LnM←CR2 type ligand to metal σ−donation and (ii) a LnM→CR2 type metal to ligand π−back donation. The Fischer type carbene complexes are usually formed with metal centers at a low oxidation state. These are also commonly observed for the more electron rich late−transition metals that participate in the LnM→CR2 type metal to ligand π−back donation. Another characteristic of the Fischer type carbene complex is the presence of the heteroatom substituents like R = OMe or NMe2 on the carbene CR2 moiety which makes the carbene carbon significantly cationic (δ+) to facilitate the LnM→CR2 type metal to ligand π−back donation. 
Similarly, the LnM=CR2 type Schrock carbene complexes comprise of two covalent interactions that involve one electron donation towards the σ−bond from each of the metal LnM and the carbene CR2 fragments. Schrock carbene complexes are thus formed with the metal centers having high oxidation state and are usually observed for electron deficient early−transition metals 

Reactivity and Synthesis:Fischer Carbene Complexes • Emphasize that this is a simplified view as we are interested in their use in organic synthesis 


Prepration:-
• The most common means to synthesise Fischer carbene complexes is from metal carbonyl compounds (OC)5Cr CO + R Li (OC)5Cr R O (OC)5Cr R

Uses in synthesis:-






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