A key step in dioxygen evolution during photosynthesis is the oxidative generation of the O-O bond from water by a manganese cluster consisting of M(2)(mu-O)(2) units (where M is manganese). The reverse reaction, reductive cleavage of the dioxygen O-O bond, is performed at a variety of dicopper and di-iron active sites in enzymes that catalyze important organic oxidations. Both processes can be envisioned to involve the interconversion of dimetal-dioxygen adducts, M(2)(O-2), and isomers having M(2)(mu-O)(2) cores. The viability of this notion has been demonstrated by the identification of an equilibrium between synthetic complexes having [Cu-2(mu-eta(2):eta(2)-O-2)](2+) and [Cu-2(mu-O)(2)](2+) cores through kinetic, spectroscopic, and crystallographic studies.