The reaction of 3,6-diaza-2S,7S-di(2-pyridyl)- octane, S,S-peaen, with Co(II) and O2 in aqueous solution yields a mixture, from which may be isolated three chelate diastereomers after the addition of HCl and HClO4. These are δ-α-[Co(S,S-peaen)Cl2]- ClO4 and its Λ-α and Δ-β analogues. Previous workers had reported that a second β-diastereoisomer could be obtained but it has been shown that this is in fact an isomeric mixture of both Δ-α- and Λ-α- [Co(S,S-peaen)Cl2]ClO4. All three isomers react with oxalate anions to form an apparently unique product Λ-α-[Co(S,S-peaen)ox]+ in aqueous solution and which can be crystallized as its perchlorate salt. Two of the reactions reported represent unusual examples of octahedral inversions.