Carbodicarbenes or Bent Allenes

The pursuit of unusual bonding environments of carbon species with non-octet electronic configurations has been quite active in fundamental organic and theoretical chemistry for many decades. In this respect, a distinct carbone species has been successfully isolated recently and defined as "carbodicarbene" or "bent allene." This neutral two-coordinated central carbon atom possesses two electron lone pairs, while its octet deficiency is overcome by sigma donation of N-heterocyclic carbenes (NHCs) bound to the central carbon. The present review is focused on the rich chemistry of carbodicarbenes in the aspects of synthesis, characterization, and ligand science of metal complex as well as their intrinsic reactivities.