Electronic structure of trigonal-planar transition-metal - Imido complexes: Spin-state energetics, spin-density profiles, and the remarkable performance of the OLYP functional
Journal of Chemical Theory and Computation
Center for Theoretical and Computational Chemistry, Department of Chemistry, University of Tromsø, N-9037 Tromsø, Norway; Department of Chemistry, University of the Free State, 9300 Bloemfontein, South Africa
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 (d x 2-z 2) ≤ a2 (d xy) ≤ a1 (dy2) < b2 (d yz) < 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 d z 2-z 2 orbital, one of whose lobes points directly at the imido nitrogen, is considerably lower in energy than the dπ orbitale. We have determined that the remarkable stability of the dσ-type orbitale 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. © 2007 American Chemical Society.