Theoretical study of linear and bent CrCO, NiCO, and CuCO

I performed density functional calculations on the electronic ground state of the monocarbonyls of chromium, nickel, and copper. CrCO and CuCO have bent equilibrium geometries with angles equal to 139.9° and 143.9° respectively, while NiCO is linear. The linear structures of CrCO and CuCO are less stable by 3 kcal/mol and are not minima, but have two imaginary frequencies. An analysis of the bonding suggests that the bent structures of CrCO and CuCO are more stable because the repulsion between the 5σ orbital of CO (essentially a lone pair on the carbon atom) and the half-filled orbital on the metal is smaller in that geometry. The calculated Ni–CO bond energy 53 kcal/mol is 13 kcal/mol larger than the most recent experimental determination [L. S. Sunderlin et al., J. Am. Chem. Soc. 114, 2788 (1992)]. The calculated bond energies of CrCO and CuCO are 14 and 20 kcal/mol, respectively.