Crystal structures and properties of divalent transition metal decahydro-closo-decaborate hydrates [M(H2O)6][B10H10] • 2H2O (M = Mn, Fe, Co, Ni, Zn)

Transition metal(II) decahydro-closo-decaborate hydrates [M(H2O)(6)][B10H10] center dot 2H(2)O (M = Fe, Co, Ni) were synthesized by reacting an aqueous solution of (H3O)(2)[B10H10] with the corresponding metal powders. The manganese(II) compound [Mn(H2O)(6)][B10H10] center dot 2 H2O could be obtained by the reaction of an aqueous solution of Ba[B10H10] with Mn(SO4) center dot H2O after the precipitation of Ba(SO4). For the zinc salt [Zn(H2O)(6)][B10H10] center dot 2 H2O an aqueous solution of (H3O)(2)[B10H10] was reacted with the basic zinc(II) carbonate [Zn(CO3)](2) center dot [Zn(OH)(2)](3). All five compounds crystallize isotypically in the monoclinic space group C2/c with four formula units per unit cell. Their crystal structure contains crystallographically unique transition metal(II) cations, which are octahedrally coordinated by oxygen atoms of six water molecules. Another six water molecules are bound to each hexaaqua-transition-metal(II) cation through hydrogen bonds, again erecting a distorted octahedron. The second octahedra are linked to each others via common edges to build up a layer containing [M(H2O)(6)](2+) cations and second-sphere crystal water molecules. Between these layers the bicapped square-antiprismatically shaped decahydro-closo-decaborate anions [B10H10](2-) are situated and held there through non-classical hydrogen bonds between the positively polarized hydrogen atoms of water molecules and the negatively polarized hydrogen atoms of boron-cluster anions. The closo-decaborate clusters themselves show only slight distortions away from their ideal D-4d symmetry as vibrational spectroscopy studies prove. Stepwise thermal decomposition takes place not only under water removement, but finally also under hydrogen evolution.