Interaction of tetracoordinated nickel(I) centers generated inside the channels of ZSM-5 zeolite with carbon monoxide ((CO)-C-12,13, p(CO) < 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 {Ni-I-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, nu(CO) = 2109 cm(-1), g and A(C-13) tensors (g(xx) = 2.018, g(yy) = 2.380, g(zz) = 2.436, A(xx) = +1.0 +/- 0.3 MHz, A(yy) = -3.6 +/- 0.9 MHz, A(zz) = -1.6 +/- 0.3 MHz) and Q(Al-27) parameters (e(2)Qq/h = -13 MHz and eta = 0.8) supported by quantum chemical modeling revealed that the Ni-CO bond results from the pi overlap between the low-laying pi(2p) CO states with the 3d(zz) and 3d(yz) orbitals, with a small sigma contribution due to the overlap of sigma(2p+2s) orbital and a protruding lobe of the in-plane 3d(xz) orbital. Two types of orbital channels (associated with the sigma and pi overlap) of the electron and spin density flows within the {Ni-I-CO} unit were identified. A bathochromic shift of the nu(CO) stretching vibration was accounted for by resolving quantitatively the separate contributions due to the sigma donation and pi back-donation, whereas the C-13 hyperfine coupling was rationalized by incongruent alpha and beta spin flows via the sigma and nu channels. As a result the very nature of the carbon-metal bond in the Ni-I-CO adduct and the molecular backbone of the corresponding spectroscopic parameters were revealed with unprecedented accuracy.