Enhanced π-Back-Donation as a Way to Higher Coordination Numbers in d¹⁰ [M(NHC)_n ] Complexes: A DFT Study

We aim to understand the electronic factors determining the stability and coordination number of d¹⁰ transition-metal complexes bearing N-heterocyclic carbene (NHC) ligands, with a particular emphasis on higher coordinated species. In this DFT study on the formation and bonding of Group 9-12 d¹⁰ [M(NHC)_n ] (n=1-4) complexes, we found that all metals form very stable [M(NHC)₂ ] complexes, but further coordination depends on the specific interplay of 1) the interaction energy (ΔE_int ) between the [M(NHC)_n-1 ] (n=2-4) fragment and the incoming NHC ligand, and 2) the strain energy (ΔE_strain ) associated with bending of the linear NHC-M-NHC arrangement. The key observation is that ΔE_strain , which is an antagonist for higher coordination numbers, can significantly be lowered by M→NHC π*-back-donation. This leads to favorable thermodynamics for n=3-4 for highly electrophilic metals in our study, and thus presents a general design motif to achieve coordination numbers beyond two. The scope of our findings extends beyond the NHC model systems and has wider implications for the synthesis of d¹⁰ [ML_n ] complexes and their catalytic activity.