O adducts derived from cobalt(II) and iron(II) complexes of porphyrin (and related anionic macrocyclic ligands) exhibit this bonding mode. Myoglobin and hemoglobin are famous examples, and many synthetic analogues have been described that behave similarly.  Binding of O is usually described as proceeding by electron transfer from the metal(II) center to give superoxide () complexes of metal(III) centers. As shown by the mechanisms of cytochrome P450 and alpha-ketoglutarate-dependent hydroxylase, Fe-η-O bonding is conducive to formation of Fe(IV) oxo centers. O can bind to one metal of a bimetallic unit via the same modes discussed above for mononuclear complexes. A well-known example is the active site of the protein hemerythrin, which features a diiron carboxylate that binds O at one Fe center. Dinuclear complexes can also cooperate in the binding, although the initial attack of O probably occurs at a single metal.