High spin tetracoordinate Co(II) complexes are interesting both from the biochemical point of view and due to possible nanotechnological application. Cobalt can replace zinc in some native metalloenzymes with partial or full retention of their activity giving thus the possibility to study the corresponding complexes by different experimental techniques. Some mononuclear tetrahedrally coordinated Co(II) complexes demonstrate a pronounced magnetic bistability that makes possible the potential application of these systems in quantum computing, spintronics and high-density storage devices. As a result, the investigation of magnetic anisotropy in these complexes and its dependence on the structural modifications is a subject of different studies.  In the present contribution a series of tetracoordinate Co(II) complexes is studied by a combination of magnetic circular dichroism (MCD) technique, magnetic measurements and theoretical calculations. The ground state of the four-coordinated Co(II) ion is orbitally non-degenerate and is characterized by the zero-field splitting (ZFS). The study was started from the magnetometric analysis. The fit of the magnetic susceptibility and magnetization data with the use of the spin Hamiltonian allows the extracting of the corresponding ZFS parameters. At the next stage of the study the MCD spectra of the investigated compounds were measured and analyzed. The positions of lines in these spectra were simulated in the framework of the exchange-charge model of the crystal field [1] and the values of the crystal field parameters were obtained. The ZFS tensors for all studied complexes were calculated (i) as the second-order spin-orbit coupling contribution [2] and (ii) as a contribution of the spin-orbit coupling to the effective Hamiltonian [3]. Finally, quantum chemical ab initio calculations were performed with the use of the ORCA package [4]. The complete active space self-consistent field (CASSCF) method improved with the second-order N-electron valence perturbation theory (NEVPT2) was used [5]. The CASSCF calculations were performed for 10 quartet (S=3/2) and 40 doublet (S=1/2) states. The values of the ZFS parameters obtained by different methods were compared.