Geometries, bonding nature, and electronic structures of (N<^>N)Ni(O-2) (N<^>N = beta-diketiminate), its cobalt(I) and copper(I) analogues, and (Ph3P)(2)Ni(O-2) were investigated by density functional theory (DFT) and multistate restricted active space multiconfigurational second-order perturbation (MS-RASPT2) methods. Only (N<^>N)Ni(O-2) takes a C-S symmetry structure, because of the pseudo-Jahn-Teller effect, while all other complexes take a C-2V structure. The symmetry lowering in (N<^>N)Ni(O-2) is induced by the presence of the singly occupied delta(dxy-pi x*) orbital. In all of these complexes, significant superoxo (O-2(-)) character is found from the occupation numbers of natural orbitals and the O-O pi* bond order, which is independent of the number of d electrons and the oxidation state of metal center. However, this is not a typical superoxo species, because the spin density is not found on the O-2 moiety, even in open-shell complexes, (N<^>N)Ni(O-2) and (N<^>N)Co(O-2). The M-O and O-O distances are considerably different from each other, despite the similar superoxo character. The M-O distance and the interaction energy between the metal and O-2 moieties are determined by the d energy of the metal moiety taking the valence state. The binding energy of the O-2 moiety is understood in terms of the d(yz) orbital energy in the valence state and the promotion energy of the metal moiety from the ground state to the valence state. Because of the participations of various charge transfer (CT) interactions between the metal and O-2 moieties, neither the d(yz) orbital energy nor the electron population of the O-2 moiety are clearly related to the O-O bond length. Here, the pi bond order of the O-2 moiety is proposed as a good measure for discussing the O-O bond length. Because the d electron configuration is different among these complexes, the CT interactions are different, leading to the differences in the pi bond order and, hence, the O-O distance among these complexes. The reactivity of dioxygen complex is discussed with the d(yz) orbital energy.