Unlike simple NHCs, the canonical resonance structures of these carbenes are mesoionic: an MIC cannot be drawn without adding additional charges to some of the atoms.
[1][2][3] There are several mesoionic carbenes that cannot be generated as free compounds, but can be synthesized as a ligand in a transition metal complex.
They are stronger σ-donors than phosphines, as well as normal N-heterocyclic carbenes due to decreased heteroatom stabilization.
The strength of carbene ligands is attributed to the electropositive carbon center that forms strong bonds of a covalent nature with the metal.
[1][2] They have been shown to lower the frequency of CO stretching vibrations in metal complexes[4][5] and exhibit large trans effects.
The C2 carbenes are thermodynamically more stable than their C4 counterparts due to resonance and inductive carbon-nitrogen interactions.
[4][18][19] Mesoionic carbenes may be generated in situ with addition of a strong base to their salt forms.
The carbenes immediately form complexes with metals present in the reaction mixture through ligand exchange.
Activation of a C‒H bond leads to oxidative addition of the carbene ligand to the metal center.
Typically, direct metalation requires the blockage of sites that would lead to normal NHC complexes — phenyl and isopropyl groups are good blocking substituents, as discussed earlier.
Direct metalation by silver(I) with imidazolium salts can cause cleavage at the C2 position if methyl is used as the blocking group.
Interactions between the nitrogen substituents and the metal center prevent normal NHC complexes from forming.
If the carbene is part of a bidentate ligand with a forced geometry, the MIC complex may form preferentially as well.
NHC formation typically occurs by heterolytic bond cleavage, so small, coordinating anions favor this pathway.
MIC formation typically occurs by an oxidative addition pathway, so non-coordinating and apolar anions are preferred, such as BF4− or SbF6−.
This pathway results in the oxidative addition of the MIC carbene halide to a low valent metal center.
They are better able to provide electron density to promote hydrogen gas oxidative addition to the metal.
[31] Other MIC complexes have been used in hydroarylation, involving the addition of an electron-rich aryl group and a hydrogen across a multiple bond.