The majority of chemical and phisical properties of any compound are determined by the structure of the highest valence electronic levels. Electronic structure of nickel(II) acetylacetonate and its amino- and tio-substituted analogues was studied using UV spectroscopy and quantum chemical calculations. The spectra were obtained in our previous work [1]. Chelate complexes of d8 metals have high density of states, that makes hard to assign spectra bands definitely. Quantum chemical calculations were performed by Firefly 8.0.1 program using density functional theory approximation with B3LYP functional and def2-tzvpp basis set. Chosing of the calculation model is described in our work [2], in which band assignment problem was succesfully solved. O→S and O→N substitution provides destabilizing effect on electron levels (the largest for π2 and π3). The results of calculation have shown that metal-ligand bonding implements by mixing of nickel d-orbitals and ligand orbitals. Based on calculated energies, molecular orbital structure and features of electron density localization, the nature of the bands of gas-phase photoelectron spectra of Ni(acac)2, of Ni(acim)2, NiEcim, Ni(Sacac)2 and Ni(S2acac)2 was established, and interpretation of these spectra was introduced. The experimental values of vertical ionization energies were correlated to the calculated energies of the Kohn–Sham orbitals in the approximation of the extended Koopmans theorem.