The first mesoionic carbenes based on pyrazole have been reported by Huynh in 2007.  These carbenes are referred to as pyrazolin-3(or 4)-ylidenes.  Pyrazolin-4-ylidenes are often tetrasubstituted with alkyl or aryl groups; however, the C3 and C5 positions could be substituted with nitrogen- or oxygen-based groups.  The electronic properties of the groups in the C3 and C5 positions affect the overall electron properties of the ligand and influence catalytic activity.  Free carbene have been produced as well as transition metal complexes.