Kristallstrukturen und Phasenumwandlungen von Kalium‐ Tetrakis(trifluormethyl)borat, K[B(CF3)4]

Below room temperature K[B(CF3)4] was found to be trimorphic according to DSC measurements, single‐crystal X‐ray diffraction, and vibrational spectroscopy. In its modification below –65 °C K[B(CF3)4] crystallises in the orthorhombic space group P212121 (Nr. 19) with a = 9.2593(14), b = 9.7527(15), c = 10.2170(16) Å and four formula units in the unit cell. At this temperature the anion exhibit nearly T‐symmetry. Between –65 °C and –50 °C K[B(CF3)4] becomes trigonal in the space group P3221 (Nr. 154) / P3121 (Nr. 152) with a = 6.9064(7), c = 17.1509(18) Å, and three formula units per unit cell. In this temperature range the CF3 groups rotate hindered. At room temperature the salt transforms into a plastic phase in the cubic space group Fm\bar{3}m (Nr. 225) with a = 9.9145(28) Å, and four formula units per unit cell, and the anions rotate completely. The unusual properties of K[B(CF3)4] are in part due to the low coordinating ability of the spherical big anions. In comparison to other anions [B(CF3)4]– is very weakly coordinating as demonstrated by the high ν(N–H) vibrational frequency (3234 cm–1) of the ion pair (n‐C8H17)3N+H···[B(CF3)4]– in CCl4 solution.