Quantifying the Donor-Acceptor Properties of Phosphine and N-Heterocyclic Carbene Ligands in Grubbs' Catalysts Using a Modified EDA Procedure Based on Orbital Deletion

The sigma-donation and pi-acceptor properties of phosphine and N-heterocyclic carbene (NHC) ligands in first- and the second-generation Grubbs' catalysts have been theoretically evaluated by means of a modified version of the energy decomposition analysis (EDA). According to the calculations, the NHC ligand is significantly more tightly bonded than the phosphine ligand as a consequence of the larger energetic contributions of the sigma-donation, pi-back-donation, and synergic effects. Despite its larger sigma-donor strength, the NHC ligand is a slightly poorer charge sigma-donor and better pi-acceptor than phophine, explaining the recently observed overall poorer charge donation of the NHC ligand.