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Binding oxygen: A four-coordinate chromium(II) complex binds O2 to yield the first structurally characterized chromium(III) superoxo complex. The superoxide ligand is coordinated in a “side-on” fashion (see structure; BARF=tetrakis(3,5-bis(trifluoromethyl)phenyl)borate). This bonding mode seems to be more widespread than commonly appreciated.
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Qin et al. (2002) studied this question.
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