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

Abstract

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 (Δ N). 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.