In coordination chemistry, a transition metal NHC complex is a metal complex containing one or more N-heterocyclic carbene ligands.  Such compounds are the subject of much research, in part because of prospective applications in homogeneous catalysis.  One such success is the second generation Grubbs catalyst.
Historically, N-heterocyclic carbenes were thought to mimic properties of tertiary phosphines.  Many steric and electronic differences exist between the two ligands. Compared to phosphine ligands, NHC ligands' cone angle is more complex. The imidazole ring of the NHC ligand is angled away from the metal center, yet the substituents at the 1,3 positions of the imidazole ring are angled towards it. The presence of the ligand inside of the metal coordination sphere affects the metal reactivity. In terms of electronic effects, NHC are often stronger sigma donation.