The application of Hartree–Fock and configuration interaction calculations to transition metal nitrosyls

This is a study of the Hartree–Fock single configuration wave function as a reasonable description of the electronic structure of molecules or molecular fragments containing transition metals. For the linear species MnNO and CoNO, intentional reduction of the electronic symmetry from C∞v to C2v resulted in an asymmetric charge distribution which was physically meaningless but lower in calculated total energy than the proper Hartree–Fock result. Configuration interaction was required to restore the cylindrically symmetric charge density. An analysis of the origin of this observed saddle point in the Hartree–Fock energy surface suggests such situations will tend to arise when the lower virtual orbitals are in energetic proximity to the higher occupied orbitals and when the virtual orbital or orbitals delocalize the electron density within the species. The implications of the effects observed in the triatomic species are examined with respect to the known molecules Mn(CO)4(NO) and Co(CO)3(NO). In these latter systems, the orbital degeneracies are removed by the geometries of the species and not by a calculational artifact. The effects on the Hartree–Fock results, however, are quite analogous as clearly evidenced by the configuration interaction results.