π-Acidity and π-basicity of N-heterocyclic carbene ligands. A computational assessment

In this contribution we report on a systematic analysis of the bond between transition metals and N-heterocyclic carbene (NHC) ligands. We compare the σ and π-contributions in a series of complexes in which the formal d-electron count ranges from d0 to d10. Our results confirm the currently accepted idea that NHC are not pure σ-donors. In the series of complexes examined here π-contribution is 10% at least. Moreover, remarkable metal-to-ligand backdonation occurs also for d0 complexes, and many systems present a substantial ligand-to-metal π donation.