These binding modes include μ-η,η-, μ-η,η-, and μ-η,η-.  Depending on the degree of electron-transfer from the dimetal unit, these O ligands can again be described as peroxo or superoxo. Hemocyanin is an O-carrier that utilizes a bridging O2 binding motif. It features a pair of copper centers.
Salcomine, the cobalt(II) complex of salen ligand is the first synthetic O carrier. Solvated derivatives of the solid complex bind 0.5 equivalent of O:
:2 Co(salen) + O → [Co(salen)]O
Reversible electron transfer reactions are observed in some dinuclear O complexes.