Molecular oxygen's ready ability to form stable complexes with transition metals is crucial for cellular respiration, profitable for the industrial use of many catalysts, and interesting for understanding the fundamental forces at play in metal-ligand chemical bonding. Voluminous studies report the reactivity of heme 5 and Vaska's complex with small molecules, particularly O 2 , attesting to the high level of biochemical and organometallic interest in dioxygen catalysis. The breadth of these metal-O 2 species is notable. Various elements, oxidation states, and d-occupancies can be observed in the transition metal centers and diverse reactivity, binding modes, and a distribution in the extent of O -O bond activation (typically 1.2 to 1.5 Å) in the O 2 ligands. Nonetheless, on the basis of spectroscopic, structural, and magnetic measurements, metal-O 2 compounds can generally be dichotomized according to the electronic character of the dioxygen: superoxo (O 2 -) or peroxo (O 2 2 -). Peroxo-metal adducts constitute the majority of this family and tend to feature elongated, side-on bound O -O moieties, 1.4 to 1.5 Å. Superoxo-metal species, on the other hand, often have shorter, end-on O -O units, 1.2 to 1.3 Å. And yet despite the versatility of dioxygen as a ligand, the maximum known ratio of peroxide ligands to metal center is four in the homoleptic tetraperoxo metal complexes of the form M(η 2 -O 2 ) 4 .