In chemistry, mesoionic carbenes (MICs) are a type of reactive intermediate that are related to N-heterocyclic carbenes (NHCs); thus, MICs are also referred to as abnormal N-heterocyclic carbenes (aNHCs) or remote N-heterocyclic carbenes (rNHCs). 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. 
A variety of free carbenes can be isolated and are stable at room temperature.  Other free carbenes are not stable and are susceptible to intermolecular decomposition pathways.  MICs do not dimerize according to Wanzlick equilibrium as do normal NHCs. This results in relaxed steric requirements for mesoionic carbenes as compared to NHCs. 
There are several mesoionic carbenes that cannot be generated as free compounds, but can be synthesized as a ligand in a transition metal complex. Most MIC transition metal complexes are less sensitive to air and moisture than phosphine or normal NHC complexes.  They are also resistant to oxidation. The robust nature of MIC complexes is due to the ligand’s strong σ-donating ability.  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.  They have been shown to lower the frequency of CO stretching vibrations in metal complexes and exhibit large trans effects.