Complex compounds of 3d transition metals with β-diketonate ligands are widely used for obtaining metal and metaloxide coatings, as analytical reagents for d-element detecting and as reagents in a nuclear magnetic resonance spectroscopy. High biological activity of a series of β-diketonates allow them to be applied as antiviral, antiallergenic, antineoplastic therapeutics, tranquilizers, insecticides and herbicides, to be used in polymerization and polycondensation reactions, and for metal purification. The majority of these properties are determined by the structure of the highest valence electronic levels. Commonly, electronic structure is studied by ultra-violet photoelectron spectroscopy. In our work spectrum of Ni(acac)2 was measured with electron spectrometer ES-3201 [1], spectra of Co(acac)2 and Mn(acac)2 were obtained from the literature [2]. Quantum chemical calculations were performed by Firefly 7.1.G program. In our calculations we choose density functional theory method with hybrid exchange-correlation functional B3LYP5 and Ahlrichs’s basis set def2-TZVPP. Choice of the basis set including polarization functions for using with β-diketonate complexes of 3d-metals was justified by our previous work with Zn(acac)2 [3]. Correlation diagram illustrating the interaction between acetylacetonate ligands and metal 3d electronic levels was composed for Ni(acac)2 complex. Based on calculated energies, molecular orbital structure and features of electron density localization, the nature of the bands of gas-phase photoelectron spectra of Mn(acac)2, Co(acac)2 and Ni(acac)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, which makes it possible to obtain good correspondence of the calculated energies to the experimental ionization energies.