Dioxygen is produced in tonnage quantities by the distillation of air at cryogenic temperatures. In recent years, alternative technologies have emerged that employ coordination complexes to offer technical and economic potential for air separation which yet may prove competitive with cryogenic separation. New transition metal complexes that can bind dioxygen reversibly and with high , specificity may provide the basis for even better processes for dioxygen recovery. In this paper, we review the structure and property of synthetic dioxygen complexes containing the elements iron, cobalt, and manganese. We describe the application of these complexes in air separation.