Four pentacoordinate complexes, 1–4, of the type [Co(L1)X₂] and [Co(L2)X₂] (where L1 = 2,6-bis(1-octyl-1H-benzimidazol-2-yl)pyridine for 1 and 2, L2 = 2,6-bis(1-dodecyl-1H-benzimidazol-2-yl)-pyridine for 3 and 4; X = Cl⁻ for 1 and 3, X = Br⁻ for 2 and 4) have been synthesized, and their structures have been determined by X-ray analysis. The DC magnetic investigation confirmed the high-spin and anisotropic behavior of the metal centers of the reported compounds, and magnetic data were analyzed with respect to the spin Hamiltonian and the Griffith–Figgis Hamiltonian. CASSCF-NEVPT2 analysis enabled us to identify triaxial magnetic anisotropy for 1 and 2 and axial anisotropy for 3 and 4, and we discuss thoroughly the relationship between geometry and orbital ordering. The g-tensor components of the ground Kramers doublet extracted from EPR spectroscopy are compatible with this prediction of magnetic anisotropy. Simulated FIRMS experimental data of 2, 3 and 4 show very good agreement with theoretical calculations and provide precise values of zero-field splitting. The AC susceptibility measurements confirmed that the reported complexes were field-induced single-ion magnets. The slow relaxation of magnetization in 1 and 2 is mediated through two relaxation channels that are unusually close to each other. On the other hand, complexes 3 and 4 show the single-channel relaxation of magnetization, and their isostructural characteristic allowed us to study the relaxation changes caused solely by the replacement of terminal halido ligands. Finally, a wet lithographic technique has been used to evaluate both the processability of the complexes in solution and the preparation of microstructured films.

A series of spin crossover iron(III) complexes with the general composition [Fe(4OH-L6)]X (H₂-4OH-L6 = 1,8-bis(4-hydroxysalicylaldiminato)-3,6-diazaoctane; X = Cl, 1a; Br, 1b; I, 1c) was prepared. A combination of the results following the single crystal X-ray analysis, infrared and EPR spectroscopy, and temperature dependent magnetic experiments revealed that the Fe(III) atoms occur in the low-spin state below room temperature and the crystal structures of the complexes involve rich networks of non-covalent intermolecular contacts resulting in two-dimensional supramolecular structures. Alterations in the halide anions influence the strength of the non-covalent contacts and affect the magnetic properties of the studied complexes. The antiferromagnetic exchange interaction between the non-covalently bound cations is the most obvious in the case of 1a and it weakens with the growing anionic volume of X. The 1D and 2D spin Hamiltonian models were applied to quantitatively extract the information about the intermolecular magnetic exchange (fit on 1D infinite chain gives J(1a) = −2.86 cm⁻¹, J(1b) = −2.02 cm⁻¹, J(1c) = −1.16 cm⁻¹). Furthermore, gradual spin crossover behaviour for all of the compounds of the series was observed above room temperature in the solid state. Spin crossover accompanied by thermochromism was also demonstrated by EPR experiments in solution.

A tridentate ligand L (2,6-bis(1-(3,5-di-tert-butylbenzyl)-1H-benzimidazol-2-yl)pyridine) was synthesized and used for the preparation of three pentacoordinated Co(II) complexes of formula [Co(L)X₂] (where X = NCS⁻ for 1, X = Cl⁻ for 2 and X = Br⁻ for 3) and one ionic compound 4 ([Co(L)₂]Br₂·2CH₃OH·H₂O) containing a hexacoordinated Co(II) centre. Static magnetic data were analysed with respect to the spin (1–3) or the Griffith–Figgis (4) Hamiltonian. Ab initio calculations enable us to identify the positive axial magnetic anisotropy parameter D accompanied by a significant degree of rhombicity in the reported complexes. Also, magneto-structural correlation was outlined for this class of compounds. Moreover, all four compounds exhibit slow relaxation of magnetisation at an applied static magnetic field with either both low- and high-frequency relaxation channels (3) or a single high-frequency relaxation process (1, 2 and 4). The interplay between the stereochemistry of coordination polyhedra, magnetic anisotropy and the relaxation processes was investigated and discussed in detail.