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 (R 2 Im) in a series of nickel, palladium and platinum complexes D 2h - and D 2d -M(H 2 Im) 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(R 2 Im) 3 ], [Ni(R 2 Im) 2 ], [Ni(R 2 Im)(CO)], [Ni(R 2 Im)(CO) 2 ], and [Ni(R 2 Im)(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 R 2 Im has also been investigated.