The Nature of the Metal-Carbene Bond in Normal and Abnormal Pyridylidene, Quinolylidene and Isoquinolylidene Complexes

Quantum chemical calculations using DFT at the BP86/TZ2P level of theory are reported for the complexes (PH3)(2)ClM-L where L is an N-heterocyclic ligand and M a group-10 metal Ni, Pd and Pt. The ligands comprise pyridyl groups or carbenes derived from the pyridine, quinolidine or isoquinolidine systems wherein the nitrogen atom is either adjacent to the carbene carbon atom or it is in a remote (meta or para, or in the adjacent ring) position. Comparative calculations include the isomeric ligands of the well-known five-membered N-heterocyclic carbene. The nature of the metal-ligand interactions are investigated by energy decomposition analysis (EDA). The EDA results suggest that the nature of the metal-carbene bonds in the complexes shows little variation when the position of the nitrogen atom in pyridylidenes is adjacent (ortho) or remote (meta or para). It changes even very little when the nitrogen atom is in an adjacent ring to the cyclic carbene moiety. The most significant differences between the bond strengths come from the energy level of the sigma-HOMO of the carbene ligand which depends largely on the position of the nitrogen atom. An energetically higher-lying sigma lone-pair orbital of the carbene ligand yields stronger orbital interactions but also stronger electrostatic attraction because of better overlap with the metal nucleus. This holds also for the isomers of the five-membered N-heterocyclic carbenes. An excellent correlation is established between the epsilon(HOMO) values of the ligands and the metal-ligand interaction energies, Delta E-int. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)