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.

Department of Inorganic Chemistry Faculty of Chemical and Food Technology Slovak University of Technology in Bratislava Bratislava SK 81237 Slovakia

Department of Inorganic Chemistry Faculty of Chemical and Food Technology Slovak University of Technology in Bratislava Bratislava SK 81237 Slovakia and Department of Inorganic Chemistry Faculty of Science Palacký University 17 listopadu 12 771 46 Olomouc Czech Republic and Central European Institute of Technology Brno University of Technology Purkyňova 123 61200 Brno Czech Republic

Department of Inorganic Chemistry Faculty of Science Palacký University 17 listopadu 12 771 46 Olomouc Czech Republic

Institut für Nanotechnologie Karlsruher Institut für Technologie Postfach 3640 Karlsruhe 76021 Germany

Institut für Nanotechnologie Karlsruher Institut für Technologie Postfach 3640 Karlsruhe 76021 Germany and Institute de Physique et Chimie de Matériaux de Strasbourg CNRS Université de Strasbourg 23 rue du Loess BP 43 67034 Strasbourg cedex 2 France

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Co(II) single-ion magnets: synthesis, structure, and magnetic properties