Two hexacoordinate Co(II) complexes [Co(hfac)(etpy)] (1) and [Co(hfac)(bzpyCl)] (2) were synthesized and spectrally and structurally characterized. The {CoON} chromophore adopts a geometry of the elongated tetragonal bipyramid with a small -rhombic component. This less common arrangement causes the magnetic data to need be analysed using the Griffith-Figgis model, instead of the commonly used spin-Hamiltonian with zero-field splitting parameters and . In the case of the elongated bipyramid for d complexes, the source of the magnetic anisotropy of an easy-axis type is the axial crystal field splitting . The CASSCF calculations followed by the NEVPT2 module confirm that the ground electronic term is quasi-degenerate owing to the splitting of the E () mother term. The lowest spin-orbit multiplets appear as four Kramers doublets belonging to the Γ irreducible representation of the double point group '. They exhibit a serious mixing of the |±1/2〉 and |±3/2〉 spins which reflects a sizable effect of the spin-orbit coupling. Both complexes exhibit field-supported slow magnetic relaxation governed by the Raman process.
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