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Cobalt(II) complexes of poly(ethyleneimine)s 1 in aqueous solution are able to form with oxygen a μ-peroxo adduct as is evidenced from stoichiometry and spectral properties. The oxygen adduct of Co/1 is characterized by its higher rate constant and activation entropy of the oxygen dissociation step, in comparison with those of the low-molecular-weight analogue Co/tetraethylenepentamine (2). This is related with the smaller affinity of Co/1 to oxygen and the reversibility in oxygen-binding. The dissociation of the oxygen molecule coordinated to and bridged with Co/1 is facilitated by a distortion of its μ-peroxo structure due to electrostatic repulsion of polycationic 1.