Bonding capabilities of imidazol-2-ylidene ligands in group-10 transition-metal chemistry

In this contribution, we give a brief overview of studies on the bonding between transition metals (TM) and N-heterocyclic carbenes (NHC) and report on a systematic bond analysis of the bonding of 1,3-diorganyl-imidazol-2-ylidenes (R2Im) in a series of nickel, palladium and platinum complexes D2h- and D2d-M(H2Im)2 to exemplify the dependence of the TM-NHC bonding on the group-10 transition metal. Furthermore complexes with seemingly different complex fragment group electronegativities, i.e., [Ni(R2Im)3], [Ni(R2Im)2], [Ni(R2Im)(CO)], [Ni(R2Im)(CO)2], and [Ni(R2Im)(CO)3] have been analyzed, a series that provides theoretical evidence that the bonding of 1,3-diorganyl-imidazol-2-ylidene ligands to metal-complex fragments strongly depends on the nature of the ligand environment. Our results confirm the currently accepted idea that NHCs are not pure σ-donors. In the series of complexes examined here π-contribution is at least 10% and up to 40%, depending on the transition metal complex fragment bonded to the carbene. The dependence of the bonding mechanism on the R substituent in R2Im has also been investigated.