The popularization of NHC ligands can be traced to Arduengo, who reported the deprotonation of dimesitylimidazolium cation to give IMes. IMes is a free NHC that can be used as a ligand.  Other NHCs have been isolated as the free ligands. Aside from IMes, another important NHC ligand is IPr, which features diisopropylphenyl groups in place of the mesityl groups.  NHCs with saturated backbones include SIMes and SIPr.