Crystal Structures and Phase‐transitions of Potassium Tetrakis(trifluoromethyl)borate, K[B(CF 3 ) 4 ]

Below room temperature K[B(CF 3 ) 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(CF 3 ) 4 ] crystallises in the orthorhombic space group P 2 1 2 1 2 1 (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(CF 3 ) 4 ] becomes trigonal in the space group P 3 2 21 (Nr. 154) / P 3 1 21 (Nr. 152) with a = 6.9064(7), c = 17.1509(18) Å, and three formula units per unit cell. In this temperature range the CF 3 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(CF 3 ) 4 ] are in part due to the low coordinating ability of the spherical big anions. In comparison to other anions [B(CF 3 ) 4 ] – is very weakly coordinating as demonstrated by the high ν(N–H) vibrational frequency (3234 cm –1 ) of the ion pair (n‐C 8 H 17 ) 3 N + H···[B(CF 3 ) 4 ] – in CCl 4 solution.