N-Heterocyclic Carbenes:  Synthesis, Structures, and Electronic Ligand Properties^†,‡

N-Heterocyclic carbene (NHC) rhodium(I) complexes of general formula Rh(COD)X(NHC) are synthesized and structurally characterized (spectroscopy, X-ray diffraction). The relative σ-donor/π-acceptor quality of various NHC ligands was examined and classified by means of IR spectroscopy at the corresponding Rh(CO)₂I(NHC) complexes. The first single-crystal X-ray diffraction studies of rhodium pyrazolin- and tetrazolinylidene complexes are reported. A wide variety of different azolium salts was applied to obtain rhodium and iridium complexes with two and four carbene ligands.