The solvent free dodecahydro-closo-dodecaborates M-2[B12H12] of the heavy alkali metals (M = K - Cs, NH4) and their formal iodide adducts M3I[B12H12] (drop MI . M-2[B12H12]) are easily accessible by reaction of an aqueous (H3O)(2)[B12H12] solution either with the respective alkali metal hydroxide (MOH) and ammonia (NH3) or with the desired alkali metal iodide (MI). After precipitation and recrystallization from aqueous solution, colourless single crystals are obtained and characterized by X-ray diffraction at room temperature. The isostructural compounds M-2[B12H12] crystallize in the cubic space group Fm (3) over bar (K: a = 1062.90(8) pm; Rb: a = 1086.74(8) pm; NH4: a = 1087.81(9) pm; Cs: a = 1128.12(7) pm; Z = 4). The crystal structures are best described as an anti-CaF2-type arrangement with the M+ cations in all tetrahedral interstices of the cubic closest-packed host lattice of the quasi-icosahedral [B12H12](2-)-cluster dianions. The mixed alkali metal dodecahydro-closo-dodecaborate iodides M3I[B12H12] (drop MI . M-2[B12H12]) are all isostructural and crystallize in the trigonal space group R (3) over barm (K: a = 1023.96(7), c = 1127.83(9) pm; NH4: a = 1036.43(7), c = 1144.38(9) pm; Rb: a = 1040.43(7), c = 1155.48(9) pm; Cs: a = 1067.80(7), c = 1197.20(9) pm; Z = 3). Their structures are best described as a trigonal distorted variation of an anti-perowskite arrangement, the anti-LaAlO3-type structure. The quasi-icosahedral [B12H12](2-) anions and the M+ cations build a cubic closest-packed host lattice together, where the I- anions occupy all the octahedral interstices, which are only formed by the M+ cations. The thermal decomposition of the heavy alkali metal dodecahydro-closo-dodecaborates M-2[B12H12] and their anion mixed salts M3I[B12H12] was investigated thermoanalytically with DTA/TG methods within a temperature range of 30 and 1200 degreesC.
