Tetraazaporphyrin is used as a primitive fragment to examine theoretically the electron donor-acceptor properties of a variety of highly conjugated planar ligands of the porphyrin and phthalocyanine type. Calculations employing the atom superposition electron delocalization MO theory indicate that for iron macrocycles the energy gap between the occupied dxz, dyz orbitals and the empty eg(π*) ring orbitals plays a key role in controlling the extent of metal to ring back-bonding. This provides a framework for a rational search of structural modifications of chelating ligands that may afford an optimized activation of axially coordinated π electron acceptor molecules such as dioxygen, nitric oxide, and carbon monoxide. The higher reactivity of iron(II) porphyrins toward such π acceptor species, compared to iron(II) phthalocyanine, is attributed to the longer Me-N bond length in the porphyrins, which leads to less metal to ligand back-bonding and thus to more electron charge at the metal center.
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