Structural and Spectroscopic Evidence for a Side-on Fe(III)–Superoxo Complex Featuring Discrete O–O Bond Distances

[Image: see text] The O–O bond length is often used as a structural indicator to determine the valence states of bound O(2) ligands in biological metal–dioxygen intermediates and related biomimetic complexes. Here, we report very distinct O–O bond lengths found for three crystallographic forms (1.229(4), 1.330(4), 1.387(2) Å at 100 K) of a side-on iron–dioxygen species. Despite their different O–O bond distances, all forms possess the same electronic structure of Fe(III)–O(2)(•–), as evidenced by their indistinguishable spectroscopic features. Density functional theory and ab initio calculations, which successfully reproduce spectroscopic parameters, predict a flat potential energy surface of an η(2)-O(2) motif binding to the iron center regarding the O–O distance. Therefore, the discrete O–O bond lengths observed likely arise from differential intermolecular interactions in the second coordination sphere. The work suggests that the O–O distance is not a reliable benchmark to unequivocally identify the valence state of O(2) ligands for metal–dioxygen species in O(2)-utilizing metalloproteins and synthetic complexes.