| Abstract
| - The question of why the iron displacement out of the porphyrin plane is enhanced in quintet states of singlyligated iron-porphyrin complexes compared to lower spin states and unligated iron-porphyrin is addressed.The spatial size of the Fe2+ atom is analyzed with respect to different spin states, and it is shown that the ionsize decreases with increasing spin state for the d6 electronic configuration. This contradicts the commonbelief that the iron out-of-plane location in the quintet state of ligated Fe(II)-porphyrins is due to an increasedrequired space of the iron within the porphyrin ring. Therefore, the singlet, triplet, and quintet ground statesof imidazole-ligated iron-porphyrin have been calculated employing density functional theory, and the relevantmolecular orbitals have been analyzed. Additional comparison with the unligated iron-porphyrin moleculesreveals that the enhanced doming in the quintet state is the result of a combination of the weakening of theiron-ring nitrogen bonds by occupying antibonding orbitals and the repulsion between the imidazole ligandand the porphyrin ring.
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