CASPT2 Study of Inverse Sandwich-Type Dinuclear Cr(I) and Fe(I) Complexes of the Dinitrogen Molecule: Significant Differences in Spin Multiplicity and Coordination Structure between These Two Complexes

Inverse sandwich-type complexes (ISTCs), (μ-N2)[M(AIP)]2 (AIPH = (Z)-1-amino-3-imino-prop-1-ene; M = Cr and Fe), were investigated with the CASPT2 method. In the ISTC of Cr, the ground state takes a singlet spin multiplicity. However, the singlet to nonet spin states are close in energy to each other. The thermal average of effective magnetic moments (μeff) of these spin multiplicities is close to the experimental value. The η2-side-on coordination structure of N2 is calculated to be more stable than the η1-end-on coordination one. This is because the d-orbital of Cr forms a strong dπ–π* bonding interaction with the π* orbital of N2 in molecular plane. In the ISTC of Fe, on the other hand, the ground state takes a septet spin multiplicity, which agrees well with the experimentally reported μeff value. The η1-end-on structure of N2 is more stable than the η2-side-on structure. In the η1-end-on structure, two doubly occupied d-orbitals of Fe can form two dπ–π* bonding interactions. The negative spin density...