Electronic Structure of Trigonal-Planar Transition-Metal-Imido Complexes:  Spin-State Energetics, Spin-Density...

We have carried out a detailed multifunctional density functional theory study of first-row transition-metal (Cr to Cu) β-diketiminato ("nacnac") imido and oxo complexes. All the complexes studied exhibit essentially the same d-orbital energy ordering, which is a1 (dx(2)-z(2)) ≤ a2 (dxy) ≤ a1 (dy2) < b2 (dyz) < b1 (dxz), where the metal-imido vector is identified with the z axis and metal-N3 plane is identified with the xz plane. A curious aspect of this orbital ordering is that the metal dx(2)-z(2) orbital, one of whose lobes points directly at the imido nitrogen, is considerably lower in energy than the dπ orbitals. We have determined that the remarkable stability of the dσ-type orbitals owes largely to the way these orbitals hybridize or "shape-shift" as a result of the absence of ligands trans or equatorial with respect to the imido (or oxo) group. Of the many functionals examined, OLYP and OPBE, based on the Handy-Cohen OPTX exchange functional, appear to provide the best overall description of the spin-state energetics of the various complexes. In particular, these two functionals predict an S = (3)/2 ground state for Fe(III) nacnac imido complexes and an S = 0 ground state for Co(III) nacnac imido complexes, as observed experimentally. In contrast, classic pure functionals such as PW91 predict S = (1)/2 ground states or at best equienergetic S = (1)/2 and S = (3)/2 states for the Fe(III) imido complexes, while hybrid functionals such as B3LYP and O3LYP predict S = 1 or 2 ground states for the Co(III) nacnac imido complexes.