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 sigma and pi-contributions in a series of complexes in which the formal d-electron count ranges from d(0) to d(10). Our results confirm the currently accepted idea that NHC are not pure a-donors. In the series of complexes examined here pi-contribution is 10% at least. Moreover, remarkable metal-to-ligand backdonation occurs also for d(0) complexes, and many systems present a substantial ligand-to-metal n donation. (c) 2006 Elsevier B.V. All rights reserved.