Activation of dioxygen molecules on transition metal centers, especially dinuclear metal centers was systematically investigated using hydrotris(3,5-dialkylpyrazolyl)borate ligands from both organometallic and bioinorganic viewpoints. Physicochemical data and X-ray crystallography of synthesized dicopper-O2 complexes revealed aμ-η2:η2 coordination mode which was proposed as the most probable model ofoxyhemocyanin. Dioxygen species bound to dinuclear cobalt and nickel centers are quite active and react rapidly with C-H groups in ligands giving oxygenated products even at room temperature. The stability of (μ-η2:η2-O2)M2 complexes depends on the electron-donating ability of the central metal atoms and the difference arises mainly from the extent of back donation to the antibonding O-O orbital.
