Density functional study of N-heterocyclic and diamino carbene complexes:: Comparison with phosphines

In this study we have examined 27 Cr(CO)(5)L complexes, ligands including N-heterocyclic carbenes (NHCs), acyclic diaminocarbenes, Fischer- and Schrock-type carbenes, and phosphines. We found that NHC-metal bonds are significantly stronger than those of phosphines. Imidazol-2-ylidenes (1a-g) and their C-C saturated imidazolin-2-ylidene counterparts (2a-g) demonstrate similar ligand-metal binding and gas-phase proton affinity (PA). CO exchange from Cr(CO)(6) by NHCs and carbenes is energetically favorable, while CO exchange for phosphines is unfavorable. NHC ligands facilitate carbonyl dissociation from the complexes to a larger extent than phosphines. It was found that the geometrical parameters and force constants of trans C-O in Cr(CO)(5)L complexes are closely related to ligand properties such as PA, electronegativity (chi), and charge transfer (DeltaN). In general, NHCs and phosphines are nucleophilic and soft ligands, while the nucleophilicity and softness of NHCs are slightly larger. Among them, acyclic diaminocarbenes, imidazolin-2-ylidenes with large substituent groups on nitrogens, and PCy3 and PPh3 are ligands that are the softest and most nucleophilic. The nucleophilicity of ligands decrease in the order: :C(N(i-Pr)(2))(2) > :C(NMe2)(2) > PCy3, :C(Me)(NMe2) > PPh3, 2c-g > 1c-g, :C(Me)(OMe), P(alkyl)(3) > PH3, :C(OH)(2) > :CH2 > PF3 > :CF2.