Crabtree has made significant contributions to the field of carbene chemistry, particularly in the exploration of mesoionic carbenes (MICs), or so called "abnormal carbenes". These carbenes, offer advantages as ligand systems in organometallic complexes and catalytic applications. Unlike C2 coordinated imidazolylidenes, mesoionic carbenes possess only charge-separated electronic resonance structures, allowing for greater adaptability to metal centers within catalytic cycles. Crabtree has developed novel methods for generating and isolating abnormal carbenes, providing insights into their structures and stability under different conditions. Notably, he introduced the first example of an abnormal carbene complex incorporating an iridium complex with a C4 coordinated imidazolylidene, which found application in transfer hydrogenation catalysis.