AbstractBinuclear cobalt chelates with O2 as bridging ligand have been prepared, using ethylenediamine, triethylenetetramine, tetraethylenepentamine and tris‐(2‐aminoethyl)‐amine as chelating agents and SCN− and ClO4− as anions. They form well characterized crystals which give off molecular oxygen spontaneously in acid solution and, except in the case of the cobalt tetra‐ethylenepentamine chelate, reveal the general composition [(Z)Co · μ(O2, OH) · Co(Z)]X3, where Z represents the polydentate complexing agents and X the anion. In the presence of ammonia different complexes are produced, in which NH3 occupies one coordination site of each cobalt, replacing the hydroxo bridge.