Die unerwartete Kristallstruktur des Cäsium-Dodekahydro-Monocarba-closo-Dodekaborats Cs[CB11H12]

Abstract

Abstract The cesium dodecahydro-monocarba-closo-dodecaborate Cs[CB11H12] crystallizes with an unexpected trigonal crystal structure having the lattice parameters a = 2094.73(3) and c = 1324.56(2) pm (c/a = 0.632) for Z = 18. The non-centrosymmetric space group R3 allows an ordering of the unsymmetric [CB11H12]– anions in a way that the least electronegative vertices of the pseudo-icosahedral cages avoid close proximity to the Cs+ cations. Hence there are channels at [0 0 z], [1/3 2/3 z] and [2/3 1/3 z], into which the C–H bonds of the [CB11H12]– units are pointing. There are two crystallographically independent Cs+ cations and [CB11H12]– anions present with unsurprising interatomic distances (d(C–B) = 166–181 pm, d(B–B) = 170–183 pm, d(B–H) = d(C–H) ≈ 110 pm) for the latter. Both Cs+ cations have contact to 18 hydrogen atoms (d(Cs–H) = 296–427 pm) stemming from six unevenly face-grafting [CB11H12]– anions, where only B–H bonds are involved. This fact is nicely reflected by IR and Raman spectroscopy. According to a 6/6 motif of the ions with highly distorted mutual octahedral coordination spheres of their centres of gravity, the crystal structure of Cs[CB11H12] follows roughly a rock salt-like arrangement. This becomes even more evident, when order-disorder transitions starting at T = 60 °C lead to more highly symmetrical structures with orientationally disordered [CB11H12]– anions.