The most studied mesoionic carbenes are based on imidazole and are referred to as imidazolin-4-ylidenes. These complexes were first reported by Crabtree in 2001. The formation of imidazolin-4-ylidenes (MIC) instead of imidazolin-2-ylidenes (NHC) is typically a matter of blocking the C2 position.  Most imidazolin-4-ylidenes are trisubstituted in the N1, C2, and N3 positions or tetrasubstituted.  Electron-withdrawing groups in the N3 and C5 positions stabilize the carbenes more than electron-donating groups.  Free carbenes as well as numerous transition metal complexes have been synthesized.