Dodecahydro-closo-dodecaborate halides of the heavy alkali metals with the formula M3X[B12H12] (M = K-Cs, NH4; X = Cl and Br)

The solvent-free dodecahydro-closo-dodecaborate chlorides M3Cl[B12H12] (M+ = Rb+, Cs+) and bromides M3Br[B12H12] (M = K+ - Cs+, NH4+) of the heavy alkali metals are easily accessible by recrystallization of the corresponding M-2[B12H12] salts from aqueous solutions of the respective alkali-metal chlorides (MCI) or bromides (MBr). After precipitation colourless, polyhedral-shaped single crystals were obtained and characterized by X-ray diffraction at room temperature. The compounds are all isostructural and crystallize in the trigonal space group R3m with Z = 3 (Rb3Cl[B12H12]: a = 1009.73(7), c = 1139.14(9) pm; Cs3Cl[B12H12]: a = 1038.02(7), c = 1179.59(9) pm; K3Br[B12H12]: a = 1002.34(7), c = 1117.68(9) pm; (NH4)(3)Br[B12H12]: a = 1015.61(7), c = 1138.70(9) pm; Rb3Br[B12H12]: a = 1016.89(7), c = 1141.82(9) pm; Cs3Br[B12H12]: a = 1045.53(7), c = 1185.47(9) pm). Their structures are best described as a trigonally distorted variant of an anti-perowskite arrangement, the anti-LaAlO3-type structure. Together, the quasi-icosahedral [B12H12](2-) cluster anions and the M+ cations build up a cubic close-packed host structure where the halide anions (X-) occupy all those octahedral interstices which are exclusively formed by the cations. The thermal decomposition of the salts was investigated by thermal analysis with DTA/TG methods in the temperature range between 30 and 1200 degrees C. The solid salts were also characterized using IR and Raman spectroscopy. The observed splitting of the B-H and B-B absorption bands clearly indicates a loss of symmetry of the quasi-icosahedral dianionic [B12H12](2-) clusters.