Potassium heptafluoroniobate

Potassium heptafluoroniobate is the inorganic compound with the formula K2NbF7. A white water-soluble salt, it is one of the more important fluorides of niobium. It is often invoked as an intermediate in the separation of niobium from tantalum. These two elements are generally found together in ores (e.g. columbite) and their separation presents a challenge since they form very similar compounds. The basis of their separation hinges on the stability and solubility of the fluorides and oxyfluorides. Reduction of this salt gives niobium metal.

Synthesis and structure
The salt is prepared by dissolving niobium pentoxide in a solution of potassium bifluoride in 40% hydrofluoric acid:
 * Nb2O5 + 4 KHF2  + 6 HF  ->  2K2NbF7  +  5 H2O

Potassium heptafluoroniobate and potassium heptafluorotantalate are isostructural. The anion has approximate C2v symmetry, being a face-capped trigonal prism. The Nb-F distances fall in the range 1.940--1.978 Å.

Spectroscopic measurements indicate that the heptafluoride converts to the hexafluoride in solution.