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 Fe2+ atom is analyzed with respect to different spin states, and it is shown that the ion
size decreases with increasing spin state for the d6 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.