Sterically induced differences in N-heterocyclic carbene transition metal complexes

The steric implications of the wingtip groups and tether length in bidentate N-heterocyclic carbene ligands on the structural features of NHC complexes are still not fully understood. Steric crowding causes pitch and yaw angles of unsymmetrically coordinated NHC ligands, equatorial or axial coordination of the carbene in trigonal bipyramidal complexes and the close approach of halogen atoms to the C2 atom in some complexes, although electronic factors have been implicated in all instances.