Basic properties Carbene’ complexes

Heterocyclic imidazolylidene carbenes proved to be very interesting ligands of transition metal complexes. Their electronic properties have very often been compared to those of basic phosphines. They are indeed good a-donors but weak n-acceptors. A number of carbene complexes have been involved in catalytic reactions such as metathesis (Ru) or C-C coupling (Pd, Ni). They can be generated easily by deprotonation of an imidazoUum salt in the presence of an organic base. 

The reaction of carbenes with X2 (X = F, Cl, Br, or I) can result in three potential complexes depending on the electronic properties of the carbene ligand and the halogen used. Initially, the carbene lone pair interacts with the o orbital of the X-X bond to form a charge transfer complex (C, Scheme 5.34). This charge transfer complex is typically in equilibrium with a cationic halo-substituted complex with a halide counterion D. If the halide counterion is strongly Lewis basic, or if cation D is sufficiently Lewis acidic,... 

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