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| - Symmetry and Bonding in Metalloporphyrins. A ModernImplementation for the Bonding Analyses of Five- andSix-Coordinate High-Spin Iron(III)−Porphyrin Complexesthrough Density Functional Calculation and NMRSpectroscopy
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| - Bonding interactions between the iron and the porphyrin macrocycle of five- and six-coordinatehigh-spin iron(III)−porphyrin complexes are analyzed within the framework of approximate density functionaltheory with the use of the quantitative energy decomposition scheme in combination with removal of thevacant π* orbitals of the porphyrin from the valence space. Although the relative extent of the iron−porphyrininteractions can be evaluated qualitatively through the spin population and orbital contribution analyses,the bond strengths corresponding to different symmetry representations can be only approximatedquantitatively by the orbital interaction energies. In contrast to previous suggestions, there are only limitedFe → P π* back-bonding interactions in high-spin iron(III)−porphyrin complexes. It is the symmetry-allowedbonding interaction between dz2 and a2u orbitals that is responsible for the positive π spin densities at themeso-carbons of five-coordinate iron(III)−porphyrin complexes. Both five- and six-coordinate complexesshow significant P → Fe π donation, which is further enhanced by the movement of the metal toward thein-plane position for six-coordinate complexes. These bonding characteristics correlate very well with theNMR data reported experimentally. The extraordinary bonding interaction between dz2 and a2u orbitals infive-coordinate iron(III)−porphyrin complexes offers a novel symmetry-controlled mechanism for spin transferbetween the axial ligand σ system and the porphyrin π system and may be critical to the electron transferpathways mediated by hemoproteins.
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