Dodecahydro-closo-dodecaborates of the heavy alkaline-earth metals from aqueous solution:: Ca(H2O)7[B12H12] • H2O, Sr(H2O)8[B12H12], and Ba(H2O)6[B12H12]

The crystalline hydrates of the heavy alkaline earth metal dodecahydro-closo-dodecaborates (M[B12H12]. nH(2)O, n = 6-8; M = Ca, Sr, Ba) are easily accessible by reaction of an aqueous (H3O)(2)[B12H12] solution with an alkaline earth metal carbonate (MCO3). By isothermic evaporation of the respective aqueous solution we obtained colourless single crystals which are characterized by X-ray diffraction at room temperature. The three compounds Ca(H2O)(7)[B12H12].H2O (orthorhombic, P2(1)2(1)2(1); a = 1161.19(7), b = 1229.63(8), c = 1232.24(8) pm; Z = 4), Sr(H2O)(8)[B12H12] (trigonal, R (3) over bar; a = 1012.71(6), c = 1462.94(9) pm.; Z = 3) and Ba(H2O)(6)[B12H12] (orthorhombic, Cmcm; a = 1189.26(7) pm, b = 919.23(5) pm, c = 1403.54(9) pm; Z = 4) are neither formula-equal nor isostructural. The structure of Sr(H2O)(8)[B12H12] is best described as a NaCl-type arrangement, Ba(H2O)(6)[B12H12] rather forms a layer-like and Ca(H2O)(7)[B12H12].H2O a channel-like structure. In first sphere the alkaline earth metal cations Ca2+ and Sr2+ are coordinated by just seven and eight oxygen atoms from the surrounding water molecules, respectively. A direct coordinative influence of the quasi-icosahedral [B12H12](2-) cluster an-ions becomes noticeable only for the Ba2+ cations (CN = 12) in Ba(H2O)(6)[B12H12]. The dehydratation of the alkaline earth metal dodecahydro-closo-dodecaborate hydrates has been shown to take place in several steps. Thermal treatment leads to the anhydrous compounds Ca[B12H12], Sr[B12H12] and Ba[B12H12] at 224, 164 and 116 degreesC, respectively.