| Summary: | Magnesium and 3<i>d</i> transition metals <i>closo</i>-borates were prepared by mechanosynthesis (ball milling) of the mixtures Na<sub>2</sub>B<sub>12</sub>H<sub>12</sub> + MCl<sub>2</sub> (M = Mn, Fe, Co, Ni, Mg), followed by addition of ethanol or methanol and drying under dynamic vacuum. The dead mass of NaCl is partly removed by filtration. The crystal structures of solvent-coordinated and solvent-free <i>closo</i>-borates have been characterized by temperature dependent synchrotron radiation X-ray powder diffraction, ab initio calculations, thermal analysis and infrared spectroscopy. Various solvated complexes containing six, four, three, two or one solvent molecules were obtained by successive removal of the solvent until in most case the solvent-free metal <i>closo</i>-borates were obtained with the exception of Mg whose hypothetical crystal structure, however, could have its prototype in MnB<sub>12</sub>H<sub>12</sub>. The 3<i>d</i> transition metal <i>closo</i>-borates were studied in the view of their potential use as Na- or Li-ion battery electrodes in combination with Na or Li <i>closo-</i>borate solid electrolytes. The metal oxidation state (II) obtained in compounds presented here does not allow such application.
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