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Interaction of tetracoordinated nickel(I) centers generated inside the channels of ZSM-5 zeolite with carbon monoxide (12,13CO, pCO < 1 Torr) led to the formation of T-shaped, top-on monocarbonyl adducts with a unique trigonal nickel core, supported by two oxygen donor ligands. The mechanism of the formation of the {NiI−CO}ZSM-5 species was accounted for by a quantitative molecular orbital correlation diagram of CO ligation. Detailed electronic and magnetic structure of this adduct was obtained from comprehensive DFT calculations, validated by quantitative reproduction of its continuous wave electron paramagnetic resonance (CW-EPR), hyperfine sublevel correlation (HYSCORE), and IR fingerprints, using relativistic Pauli and ZORA-SOMF/B3LYP methods. Molecular analysis of the stretching frequency, νCO = 2109 cm−1, g and A(13C) tensors (gxx = 2.018, gyy = 2.380, gzz = 2.436, Axx = +1.0 ± 0.3 MHz, Ayy = −3.6 ± 0.9 MHz, Azz = −1.6 ± 0.3 MHz) and Q(27Al) parameters (e2Qq/h = −13 MHz and η = 0.8) supported by quantum chemical modeling revealed that the Ni−CO bond results from the π overlap between the low-laying π(2p) CO states with the 3dxz and 3dyz orbitals, with a small σ contribution due to the overlap of σ(2p+2s) orbital and a protruding lobe of the in-plane 3dxz orbital. Two types of orbital channels (associated with the σ and π overlap) of the electron and spin density flows within the {NiI−CO} unit were identified. A bathochromic shift of the νCO stretching vibration was accounted for by resolving quantitatively the separate contributions due to the σ donation and π back-donation, whereas the 13C hyperfine coupling was rationalized by incongruent α and β spin flows via the σ and π channels. As a result the very nature of the carbon−metal bond in the NiI−CO adduct and the molecular backbone of the corresponding spectroscopic parameters were revealed with unprecedented accuracy. [ABSTRACT FROM AUTHOR]