Density functional theory is used to probe the reaction Of O-2 with the unsaturated transition-metal fragments [M(bipy)(2)](2+), M = Cr, Fe. In both cases, calculations indicate that the O-2 molecule is initially trapped as an eta(2)-bound superoxide ion, where the unpaired electron in the out-of-plane pi* orbital Of O-2 is weakly coupled to those on the trivalent metal ion, In the chromium case, a cis-dioxo Cr-VI complex is found to be significantly more stable than the superoxo species. The two minima are, however, separated by a large barrier, along with a change in spin state, For the iron analogue, the relative energies of the two minima are reversed, the superoxo complex being the global minimum. The energetics of the O-2 activation processes are consistent with previously reported mass spectrometric experiments, where an adduct, [M(bipy)(2)(O-2)](2+), was detected only for chromium.