Dioxygen complexes are coordination compounds that contain O as a ligand. The study of these compounds is inspired by oxygen-carrying proteins such as myoglobin, hemoglobin, hemerythrin, and hemocyanin.  Several transition metals form complexes with O, and many of these complexes form reversibly.  The binding of O is the first step in many important phenomena, such as cellular respiration, corrosion, and industrial chemistry.  The first synthetic oxygen complex was demonstrated in 1938 with cobalt(II) complex reversibly bound O.