Crystal structures of trisammonium bisperoxotetrafluoroniobate(V) and analogous tantalate(V)

Abstract

(NH 4) 3[Nb(O 2) 2F 4] (I) and (NH 4) 3[Ta(O 2) 2F 4] (II) are isostructural, and belong to the cubic Fm3m space group with four molecules in the unit cell. The unit cell parameters are a = 9.4442(4) (I) and a = 9.4512(4) Å (II). The structures were solved by the Patterson method and were refined by the least-squares method to the conventional R factors of 0.036 for 86 reflections (I) and 0.043 for 103 reflections (II) (in both structures having I ⩾ 2σ(I)). The disordered distributions of fluorine and peroxo oxygens with partially occupied sites are observed. The disordered NH 4 + tetrahedra appear in the structures. The metal atoms exhibit an octahedral coordination with two corners of a polyhedron at the centre of the peroxo bonds. Inter-atomic distances are NbF, 1.95(2), NbO, 1.94, TaF, 1.91(4) and TaO, 2.07 Å. The structures (I, II) are composed of [M υ(O 2) 2-F 4] 3− octahedra and two symmetrically-independent ammonium cations connected by NH⋯O and NH⋯F hydrogen bonds. These two structures are compared with the structure of (NH 4) 3Ti(O 2)F 5].