The significance of π interactions in group 11 complexes with N-heterocyclic carbenes

The nature of the chemical bond in mixed carbene-halogen complexes (1)TMX (X = F-I) and bis(carbene) complexes (1)₂TM⁺ of group 11 metals (TM = Cu, Ag, Au) with imidazol1-ylidene (1) as ligand has been investigated at the BP86 level of theory using an energy decomposition analysis (EDA). The metal-carbene bonds are mainly held together by classical electrostatic attraction, which contributes >65% of the binding interactions. The metal-carbene bonds are very strong. In the bis(carbene) complexes, the N-heterocyclic carbene ligand 1 is bonded even more strongly than in the mixed carbene-halogen complexes. In the bis(carbene) complexes, orbital interactions are slightly more important than in the mixed carbene-halogen complexes but the covalent contribution is always <35% of the total attractive interaction. The orbital interaction part of the bonding has only ∼20% π back-bonding. The calculated data are not very different from previous EDA results for the Fischer carbene complex (CO)₅W-C(OH)₂. The EDA results suggest that R₂C←ML_n π back-donation in complexes with N-heterocyclic carbenes is not substantially smaller than in classical Fischer carbene complexes bearing two π donor groups R.