Cobalt(II) complexes with linear potentially pentadentate Schiff base ligands have been prepared and their reaction with dioxygen studied. The ligands were synthesized from salicylaldehyde and diamino compounds containing a central donor atom. Various derivatives were produced by using a substituted salicylaldehyde or by placing different groups on the compounds containing a central nitrogen donor. Those ligands containing weak central donors produce both four and five-coordinate complexes. The coordination number is determined by the electron density on the metal as well as the basicity of the central donor. The five-coordinate complexes assume a distorted trigonal bipyramidal structure, but the phosphorus containing complexes are believed to involve a different ligand arrangement from that of the other complexes. Most of the cobalt(II) complexes are stable in the solid state, but all react with oxygen in solution to some extent. A monomeric superoxo adduct was isolated with one of the complexes containing an unsubstituted central nitrogen donor. Other cobalt(II) complexes of this type react with oxygen to form dimeric peroxo adducts. An unusual dimeric dioxygen complex was obtained with a compound in which the central nitrogen donor is substituted with a methyl group. This adduct is believed to involve both superoxo and unreacted species. Comparison of the structure of precursors to that of a dimeric peroxo adduct suggests a different mode of bonding than that which occurs in dioxygen complexes with other types of ligands. The significance of this is explored in terms of the prevention of dimerization. / Ph. D.