Syntheses, Crystal Structures, and Properties of the Isotypic Pair [Cr(H2O)6]2[B12H12]3•15H2O and [In(H2O)6]2[B12H12]3•15H2O

Single crystals of [Cr(H2O)(6)](2)[B12H12](3)center dot 15H(2)O and [In(H2O)(6)](2)[B12H12](3)center dot 15H(2)O were obtained by reactions of aqueous solutions of the acid (H3O)(2)[B12H12] with chromium(III) hydroxide and indium metal shot, respectively. The title compounds crystallize isotypically in the trigonal system with space group R (3) over barc (a = 1157.62(3), c = 6730.48(9) pm for the chromium, a = 1171.71(3), c = 6740.04(9) pm for the indium compound, Z = 6). The arrangement of the quasi-icosahedral [B12H12](2-) dianions can be considered as stacking of two times nine layers with the sequence ...ABCCABBCA... and the metal trications arrange in a cubic closest packed ...abc... stacking sequence. The metal trications are octahedrally coordinated by six water molecules of hydration, while another fifteen H2O molecules fill up the structures as zeolitic crystal water or second-sphere hydrating species. Between these free and the metal-bonded water molecules, bridging hydrogen bonds are found. Furthermore, there is also evidence of hydrogen bonding between the anionic [B12H12](2-) clusters and the free zeolitic water molecules according to B-H delta-center dot center dot center dot delta+H-O interactions. Vibrational spectroscopy studies prove the presence of these hydrogen bonds and also show slight distortions of the dodecahydro-closo-dodecaborate anions from their ideal icosahedral symmetry (I-h). Thermal decomposition studies for the example of [Cr(H2O)(6)](2)[B12H12](3)center dot 15H(2)O gave no hints for just a simple multistepwise dehydration process.