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This article reports on the activation of dioxygen on nickel(I) dispersed inside the nanopores of the ZSM-5 zeolite, which can be regarded as a heterogeneous mimetic system (zeozyme) for Ni-bearing enzymes. The side-on η²-coordination of the resulting nickel-bound superoxo adduct was ascertained by detailed analysis of the EPR spectra of both 16O2 and 17O2 species supported by computer simulations of the spectra and relativistic DFT calculations of the EPR signatures. Molecular analysis of the g and A(17O) tensors (gxx = 2.0635, gyy = 2.0884, gzz = 2.1675; ∣Axx∣ ≈ 1.0 mT, ∣Ayy∣ = 5.67 mT, ∣Azz∣ ≈ 1.3 mT) and quantum chemical modeling revealed an unusual electronic and magnetic structure of the observed adduct (with gzz(gmax) > gyy(gmid) > gxx(gmin) and the largest O-17 hyperfine splitting along the gmid direction) in comparison to the known homogeneous and enzymatic nickel-superoxo systems. It is best described as a mixed metalloradical with two supporting oxygen donor ligands and even triangular spin-density redistribution within the η2-{NiO2}11 magnetophore. The semioccupied molecular orbital (SOMO) is constituted by highly covalent δ overlap between the out-of-plane 2p(πg*) MO of dioxygen and the 3dx²-y² MO of nickel. By means of the extended transition state-natural orbitals for the chemical valence approach (ETS-NOCV), three distinct orbital channels (associated with σ,π, and δ overlap) of congruent and incongruent charge and spin density flows within the η²-{NiO2}11 unit, contributing jointly to activation of the attached dioxygen, were identified. Their individual energetic relevance was quantified, which allowed for explaining the oxygen binding mechanism with unprecedented accuracy. The nature and structure sensitivity of the g tensor was rationalized in terms of the contributions due to the magnetic field-induced couplings of the relevant molecular orbitals that control the g-tensor anisotropy. The calculated O-17 hyperfine coupling constants correspond well with the experimental parameters, supporting assignment of the adduct. To the best of our knowledge, the η²-{NiO2}11 superoxo adducts have not been observed yet for digonal mononuclear nickel(I) centers supported by oxygen donor ligands. [ABSTRACT FROM AUTHOR]