Transition metal-dioxygen complexes, which have been studied intensively in relevance to the active species of oxygenation with dioxygen molecule, are classified according to the extent of electron transfer from the metal center(s) to the O-2 ligand. A new O-2-activation method via a higher valent bis(mu-oxo) dimetal species resulting from extensive electron transfer is proposed on the basis of the results of our bioinorganic studies on the dioxygen complexes supported by the hydrotris(3,5-diisopropylpyrazolyl)borate ligand (Tp(iPr)).