Fluxional seven-coordinated fluorido- and oxofluoridotantalates

Seven-coordinated (NH₄)₂TaF₇, Rb₂TaF₇ and Rb₃TaOF₆ were synthesized in single-crystal form and their structures were determined. A monocapped trigonal prism (CTP) or a pentagonal bipyramid (PB) of the TaF₇^2- anion are stereochemically nonrigid and coexist in the first two compounds as a result of strong intraspheric dynamics. Upon cooling, tetragonal Rb₂TaF₇ undergoes a first-order phase transition at 145 K and the seven-coordinated polyhedron transforms into a regular CTP. The seven-coordinated polyhedron in (NH₄)₂TaF₇ approaches the PB configuration as the temperature decreases. Cubic elpasolite-like Rb₃TaOF₆ is characterized by the simultaneous two-state coexistence of TaOF₆^3- of the PB shape as rigidly reoriented and as fluxional. In the former case, the central atom is disordered over the octahedron in the unit cell, allowing the determination of the short Ta-O distance, whereas in the latter case, tantalum remains in the polyhedron center, resulting in synchronous Ta-O and Ta-F stretching vibrations appearing as the infrared band at 723 cm^-1.