Ligand properties of N-heterocyclic and Bertrand carbenes: A density functional study

In order to probe the ligand properties we have examined a series of Cr(CO)5L and Ni(CO)3L complexes using density functional theory (DFT). The ligands included in our study are N-heterocyclic carbenes (NHCs) and Bertrand-type carbenes. Our study shows that the carbene–metal bonds of imidazol-2-ylidenes (1), imidazolin-2-ylidenes (2), thiazo-2-ylidenes (3), and triazo-5-ylidenes (4) are significantly stronger than those of Bertrand-type carbenes (5–7). The force constants of C–O in complexes are related to the property of isolated carbenes such as proton affinity (PA), electronegativity (χ), and charge transfer (ΔN). NHCs and Bertrand-type carbenes are identified as nucleophilic, soft ligands. Carbene stabilization energy (CSE) computations indicate that carbenes 1 and 4 are the most stable species, while 2 and 3 are less stable. In contrast to NHCs, CSE of carbenes 5–7 are much smaller, and their relative stabilities are in the order (amino)(aryl) carbenes 7e–7g > (amino)(alkyl) carbenes 7a–7d > (phosphino)(aryl) 6d–6e, and (phosphino)(silyl) carbenes 5a–5c > (phosphino)(alkyl) carbenes 6a–6c.

Density functional theory (DFT) shows that the carbene stabilization energy (CSE) computed using the energy of reaction of N-heterocyclic carbene (NHC) are significantly larger than those of Bertrand-type carbenes. The carbene–metal bonds of imidazol-2-ylidenes, imidazolin-2-ylidenes, thiazo-2-ylidenes, and triazo-5-ylidenes are stronger than those of Bertrand-type carbenes. Bertrand-type carbenes are more nucleophilic, and are softer ligands compared with NHCs. The force constants of C–O in Cr(CO)5L and Ni(CO)3L are related to the property of isolated carbenes such as PA, electronegativity (χ), and charge transfer (ΔN).Figure optionsDownload as PowerPoint slide