Fluxional seven-coordinated fluorido- and oxofluoridotantalates.

Seven-coordinated (NH)TaF, RbTaF and RbTaOF were synthesized in single-crystal form and their structures were determined. A monocapped trigonal prism (CTP) or a pentagonal bipyramid (PB) of the TaF anion are stereochemically nonrigid and coexist in the first two compounds as a result of strong intraspheric dynamics. Upon cooling, tetragonal RbTaF undergoes a first-order phase transition at 145 K and the seven-coordinated polyhedron transforms into a regular CTP.