title: Electronic structure and ligand vibrations in FeNO, CoNO, and FeOO porphyrin adducts.
abstract: The gaseous ligands, CO, NO, and O2 interact with the Fe ion in heme proteins largely via backbonding of Fe electrons to the π* orbitals of the XO (X = C, N, O) ligands. In these FeXO adducts, the Fe-X stretching frequency varies inversely with the X-O stretching frequency, since increased backbonding strengthens the Fe-X bond while weakening the X-O bond. Inverse frequency correlations have been observed for all three ligands, despite differing electronic and geometric structures, and despite variable composition of the "FeX" vibrational mode, in which Fe-X stretching and Fe-X-O coordinates are mixed for bent FeXO adducts. We report experimental data for 5-coordinate Co(II)(NO) porphyrin adducts (isoelectronic with Fe(II)(OO) adducts), and the results of density functional theory (DFT) modeling for 5-coordinate Fe(II)(NO), Co(II)(NO), and Fe(II)(OO) adducts. Inverse ν(MX)/ν(XO) correlations are obtained computationally, using model porphyrins with graded electron-donating and -withdrawing substituents to modulate the backbonding. Computed slopes agree satisfactorily with experiment, provided nonhybrid functionals are used, which avoid overemphasizing high-spin states. The BP86 functional gives correct ground states, a closed-shell singlet for Co(II)(NO) and an open-shell singlet for the isoelectronic Fe(II)(OO), as corroborated by structural data for Co(II)(NO), and the ν(MX)/ν(XO) slope agreement with experiment for both adducts. However, for Fe(II)(OO) adducts, the computed inverse ν(MX)/ν(XO) correlation applies only to porphyrins with electron-donating and withdrawing substituents of moderate strength. For substituents more donating than -CH3, a direct correlation is obtained, the Fe-O and O-O bonds weakening in concert. This effect is ascribed to the dominance of σ bonding via the in-plane dxz(+dz(2))-π* orbital, when electron-donating substituents raise the d orbital energies sufficiently to render backbonding (dyz-π*) unimportant.
authors.affiliation: Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195, USA.
authors.forename: Alexandra V
authors.initials: AV
authors.lastname: Soldatova
authors.forename: Mohammed
authors.initials: M
authors.lastname: Ibrahim
authors.forename: Thomas G
authors.initials: TG
authors.lastname: Spiro
mesh_terms: D003035:Cobalt / Q000737:chemistry*; D004583:Electrons; D006418:Heme / Q000737:chemistry; D007501:Iron / Q000737:chemistry*; D008958:Models, Molecular; D009569:Nitric Oxide / Q000737:chemistry*; D011166:Porphyrins / Q000737:chemistry*
chemical_list: D011166:Porphyrins; D009569:Nitric Oxide; D003035:Cobalt; D006418:Heme; D007501:Iron