Mononuclear metal−dioxygen species (M−O 2 ) are key intermediates in a variety of the oxidative transformation processes mediated by heme and nonheme metalloenzymes. Understanding their O 2 -activation mechanism at an orbital level is still of scientific significance. In the present work, we spectroscopically analyzed the biomimetic Ni II −superoxo δ-bond: the origin of its abnormal electrophilic reactivity. We prepared the biomimetic Ni II −superoxo species through the room temperature reaction of the Ni I site with O 2 in MFI zeolite. Under vacuum condition, this isolated species acts like a metal oxide molecule in the gas phase, and it gives the vibronically resolved spectrum that is generally seen in the vapor phase. This type of spectroscopic probe has never been observed for past M−O 2 complexes. Through a combination of an isotopic experiment and quantum chemical calculations, we successfully assigned the observed vibrational fine structure as the vibronic progression in a stretching O−O vibrational structure associated with the excitation from doubly occupied δ orbital to singly occupied δ* orbital (SOMO). This spectroscopic probe provides information on how the δ channel interaction contributes to the activation of O−O bond in an O 2 molecule. The recondite vibronic progression feature was well reproduced by DFT cluster calculation assuming the square planar Ni II −superoxo site, by which we successfully obtained the spectroscopically calibrated DFT cluster model that well describes Ni II −superoxo δ-bond. This model revealed that Ni II −superoxo δ-bond is ionic rather than covalent. The high ionicity of the δ bond results in the high oxygen character in the important frontier molecular orbital (FMO) for the electrophilic oxidative reaction, i.e., the unoccupied β-spin O 2 -π* orbital. This is why that Ni ion stabilizes superoxo ligand having abnormal electrophilic reactivity. The findings in the present study provide general description of the Ni−superoxo δbond and maybe can help us to uncover the structure−reactivity relationships of the metalloprotein.