K2B12F12: A rare A2X structure for an ionic compound at ambient conditions

The anhydrous salt K2B12F12 crystallized from aqueous solution and its structure was determined by single crystal X-ray diffraction. The Ni2In-type structure it exhibits is rare for an A2X ionic compound at 25 °C and 1 atm., consisting of an expanded hexagonal close-packed array of B12F122− centroids (cent⋯cent distances: 7.204–8.236 Å) with half of the K+ ions filling all of the Oh holes and half of the K+ ions filling all of the D3h trigonal holes in the close-packed layers that are midway between two “empty” Td holes. The structure is also unusual in that the bond-valence sum for the K+ ions in Oh holes is less than or equal to 0.73 (the bond-valence sum for the other type of K+ ion is 1.16). A variation of the Ni2In structure is exhibited by the previously published monohydrate Cs2(H2O)B12F12, for which an improved structure is also reported here. For K2B12F12: monoclinic, C2/c, a = 8.2072(8), b = 14.2818(7), c = 11.3441(9) Å, β = 92.832(5)°, Z = 4, T = 120(2) K. For Cs2(H2O)B12F12: orthorhombic, P212121, a = 9.7475(4), b = 10.2579(4), c = 15.0549(5) Å, Z = 4, T = 110(1) K.