The Spin Dependence of the Spatial Size of Fe(II) and of the Structure of Fe(II)-Porphyrins

The question of why the iron displacement out of the porphyrin plane is enhanced in quintet states of singly ligated iron-porphyrin complexes compared to lower spin states and unligated iron-porphyrin is addressed. The spatial size of the Fe 2+ atom is analyzed with respect to different spin states, and it is shown that the ion size decreases with increasing spin state for the d 6 electronic configuration. This contradicts the common belief that the iron out-of-plane location in the quintet state of ligated Fe(II)-porphyrins is due to an increased required space of the iron within the porphyrin ring. Therefore, the singlet, triplet, and quintet ground states of imidazole-ligated iron-porphyrin have been calculated employing density functional theory, and the relevant molecular orbitals have been analyzed. Additional comparison with the unligated iron-porphyrin molecules reveals that the enhanced doming in the quintet state is the result of a combination of the weakening of the iron-ring nitrogen bonds by occupying antibonding orbitals and the repulsion between the imidazole ligand and the porphyrin ring.