Crystal structure of bis(triethylammonium)closo-decahydrodecaborate, [(C2H5)3NH]2[B10H10]

The crystal structure of bis(triethylammonium)closo-decahydrodecaborate [bis(triethylammonium)decaboranate(10)], [(C2H5)(3)NH](2)[B10H10], was determined and refined (space group Pmmn, no. 59, n = 989.7, b = 1333.7, c = 903.7 pm). The compound is a versatile starting material for many substances containing the [B10H10](2-) entity and its derivatives. The closo-[B10H10](2-) cluster is a bicapped square antiprism which is only slightly distorted. Its deviation from D-4d symmetry is smaller than that of the B-10 cages in every other compound containing this entity that have been structurally characterised. The presence of additional (N)H---B-3 interactions in form of multiple-centre bonds between the cations and the anions, which were postulated earlier and which should influence the cage symmetry, could not be confirmed. At 55 degrees C, the transition into a high temperature phase was investigated by X-ray powder diffraction. The high temperature phase crystallises in the tetragonal crystal system (a = 946.9, c = 1351.0 pm).