Orientational disorder in crystals of (NH4)3MoO3F3 and (NH4)3WO3F3

Triammonium trioxotrifluoromolybdate (NH4)3MoO3F3 (I) and trioxotrifluorotungstate (NH4)3WO3F3 (II) were synthesized in a single-crystal form and their structures were refined by X-ray diffraction. These two isostructural compounds belong to the elpasolite-type structure (cubic system, space group Fm3m, Z = 4). O and F atoms are randomly distributed in two independent positions [24(e) and 96(j)] of the cubic unit cell, and the central atoms and the ammonium cation containing N2 are shifted from the symmetry centers into the 32(f) position. As a consequence, O and F atoms in the equilibrium structure were identified on a local scale by the metal–ligand distance and hydrogen atomic coordinates of the disordered ammonium cation N2H4 were determined. The slightly compressed MX6 (M = Mo, W; X = O, F) octahedron has a fac configuration with the central atom shifted toward the face occupied by three O atoms. The true geometry of this polyhedron permits us to explain the observed vibrational spectra of the compounds examined and to eliminate the contradiction in interpretation of the vibrational spectra of [MO3F3]n− species reported in the literature. Both complexes reveal a dynamic disordering: the [MX6]3− anions are disordered on 48 equivalent orientations and the N2H4 groups have eight orientations.