Magnesium and 3d transition metals closo-dodecaborates were prepared by mechanosynthesis (ball milling) of the mixtures Na2B12H12 + MCl2 (M=Ti, Cr, Mn, Fe, Co, Cu, Zn, Mg) and CuCl, followed by hydration and drying under dynamic vacuum. The crystal structures of hydrated and anhydrous closo-dodecaborates have been characterized by temperature dependent synchrotron radiation X-ray powder diffraction, ab initio calculations, thermal analysis and infrared spectroscopy. Three different complexes containing water were found: a homoleptic octahedral complex in M(H2O)6B12H12 crystallizing in three different deformation variants of the complex centred closo-dodecaborate cube, a heteroleptic octahedral complex in M(H2O)4B12H12 containing four water molecules and two hydrogens from a closo-dodecaborate centring also a deformed closo-dodecaborate cube, and a heteroleptic octahedral complex in Fe(H2O)2B12H12 containing two water molecules where four hydrogen atoms are bridging two closo-dodecaborates. Anhydrous cobalt(II) and copper(I) closo-dodecaborates were obtained by drying the hydrated sample under dynamic vacuum. Both compounds crystallize with bcc packing of closo-dodecaborates where the metal is in octahedral coordination by hydrogen H6 for cobalt, and a tetrahedral coordination H4 is found for copper, bridging two closo-dodecaborates in an infinite chain. The orbital overlap between transition metal and closo-anion leads to directional covalent bonding. While CoB12H12 has trigonal symmetry (R3) allowing for an ordered metal position, Cu2B12H12 follows the cubic symmetry of bcc packing, which leads to a disordered metal position. The structure of NiB12H12, originally published as monoclinic (I2), is revised based on the trigonal structural model of CoB12H12.