The synthesis and characterization of a series of cobalt (II) complexes which contain coordinated linear pentadentate thioiminato ligands will be completed. The reactivity of these new complexes and the related ketoiminato species toward molecular oxygen will be evaluated using visible spectral and EPR techniques. Thermodynamic parameters for the oxygen adducts will be determined. Differences in Co-O2 bond stabilities will be correlated with different electronic and structural features of the ligands. In particular, the relationship between oxygen adduct stability and chelate ring substituents will be investigated. The trends observed for different substituents will be used to estimate the relative importance of pi bonding (resonance) effects vs. sigma bonding (inductive) effects in the Schiff base complexes. Further attempts will be made to clarify the role of sulfur in these systems.