Cobalt(II) complexes containing various types of Schiff base ligands are being evaluated as synthetic oxygen carriers. Ligands under study include the tetradentate ketoimines p-XbenacenH2 and m-XbenacenH2, and thioimines, p-XbensacenH2, and m-XbensacenH2, where X2 equal C1, Br, CH3 and CH3 alpha. Results of thermodynamic, EPR and electrochemical studies are being compared and correlated. Relationships between oxygen adduct stability and ring substituent effects and oxidation potentials are being investigated in order to clarify the role of coordinated sulfur in these systems. Cobalt plus (II) complexes with linear pentadentate ketoimine ligands with N3O2 donor sets and with macrocyclic Schiff base ligands with N4 and N2P2 donor sets are also being synthesized, characterized by structure techniques, and evaluated for oxygen reactivity.