Metal-π Interactions Between M(CO)₃ and Halobenzene. Cooperative Effects on Halogen Bonding

The bonding between the metal atom of M(CO)₃ and the aromatic ring of benzene and halobenzene is examined by DFT calculations. Metals are drawn from the 3d and 4d periods of groups 6-12. The bonding mode is quite variable in strength, geometry, and fundamental nature: metals from the left side of the periodic table tend to form strong bonds, bordering on coordinate covalent, and can induce nonplanar distortions into the benzene ring, while the M atoms further to the right engage in weaker noncovalent bonds. Halogenation of the benzene ring causes only minor perturbations to this M··π bonding. The transfer of charge away from the ring to the metal species facilitates a deepening of the σ-hole on the halogen atom of the ring and a strengthening of its halogen bond to NH₃. This cooperativity is not large, amounting to less than 1.5 kcal/mol.