Many mesoionic carbenes may not be able to be isolated as a free carbene; however, these MICs can be generated as a ligand for transition metal complexes.  Numerous mesoionic carbene transition metal complexes are known with metals including Fe, Os, Rh, Ir, Ni, Pd, Pt, Cu, and Ag.  Metal complexes with Sm and Y are also known.  MIC complexes are formed by a variety of mechanisms.
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.
Direct metalation through C-H bond activation or C-H oxidative addition is one method often utilized.  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.  Smaller substituents may be cleaved.  Direct metalation by silver(I) with imidazolium salts can cause cleavage at the C2 position if methyl is used as the blocking group.  The result is formation of normal NHC carbenes.  n-alkyl and benzyl groups may undergo the same fate as the methyl group.  Steric bulk may also influence the formation of MIC complexes over NHC complexes.  For imidazolium salts, the C2 position may not need to be blocked if the nitrogen substituents (N1 or N3) are sterically-demanding.  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.  The counteranion of imidazolium salts participates in NHC vs. MIC formation.  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 BF or SbF.  Other techniques focus on the activation of the desired carbon rather than blocking undesired carbons.  A carbon may be activated by a halogen.  A C-X bond (X = halide) is more favorable for activation than a C-H bond.  This pathway results in the oxidative addition of the MIC carbene halide to a low valent metal center.
Transmetalation is another method commonly utilized.  Typically, a silver carbene complex is produced by direct metalation.  This silver complex is reacted via transmetalation with a salt of the desired metal.  The metal MIC complex is produced and silver salts generally precipitate.