Bonding Analysis of N-Heterocyclic Carbene Tautomers and Phosphine Ligands in Transition-Metal Complexes: A Theoretical Study

DFT calculations at the BP86/TZ2P level were carried out to analyze quantitatively the metal-ligand bonding in transition-metal complexes that contain imidazole (IMID), imidazol-2-ylidene (nNHC), or imidazol-4-ylidene (aNHC). The calculated complexes are [Cl4TM(L)] (TM = Ti, Zr, Hf), [(CO)5TM(L)] (TM = Cr, Mo, W), [(CO)4TM(L)] (TM = Fe, Ru, Os), and [ClTM(L)] (TM = Cu, Ag, Au). The relative energies of the free ligands increase in the order IMID < nNHC < aNHC. The energy levels of the carbon sigma lone-pair orbitals suggest the trend aNHC > nNHC > IMID for the donor strength, which is in agreement with the progression of the metal-ligand bond-dissociation energy (BDE) for the three ligands for all metals of Groups 4, 6, 8, and 10. The electrostatic attraction can also be decisive in determining trends in ligand-metal bond strength. The comparison of the results of energy decomposition analysis for the Group 6 complexes [(CO)5TM(L)] (L = nNHC, aNHC, IMID) with phosphine complexes (L = PMe3 and PCl3) shows that the phosphine ligands are weaker sigma donors and better pi acceptors than the NHC tautomers nNHC, aNHC, and IMID.