Potassium heptafluorotantalate

Potassium heptafluorotantalate
IUPAC name Dipotassium heptafluorotantalum(2-)
Other names Dipotassium heptafluorotantale
Identifiers
CAS number	16924-00-8 Y
Properties
Molecular formula	K2TaF7
Molar mass	392.13 g/mol
Appearance	white solid
Density	4.56 g/mL at 25°C
Melting point	630-820°C
Solubility in water	0.5 g/100 mL (15 °C)[1]
Y heptafluorotantalate (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Potassium heptafluorotantalate is an inorganic compound with the formula K₂TaF₇. This white, water-soluble solid is an intermediate in the purification of tantalum from its ores and is the precursor to the metal.^[2]

Structure

The structure of solid potassium heptafluorotantalate consists of potassium ions and the complex anion TaF₇^2-. The seven-coordinate coordination sphere for Ta(V), established by X-ray crystallography, can be viewed as a trigonal prism defined by six fluoride ions with the seventh fluoride ion positioned above the center of the one the three rectangular faces. The anion has C_2v symmetry. The salt undergoes a series of phase transitions at lower temperatures. The Ta-F distances vary in the range of 1.976-1.919 Å. Two types of fluoride polyhedra surround the two potassium cations. The first polyhedron has K-F distance range of 2.905-2.646 Å, and the second one has 2.956-2.651 Å.^[3]

Preparation

Potassium heptafulorotantalate can be produced by both anhydrous and wet methods. The anhydrous method involves the reaction of tantalum oxide with potassium bifluoride or ammonium bifluoride according to the following equation:^[1][4]

Ta₂O₅ + 4 KHF₂ 6 HF → 2 K₄TaF₇ + 5 H₂O

The basic method was reported by Berzelius.^[5]

K₂TaF₇ can also be precipitated from solutions in hydrofluoric acid provided that the concentration of HF is below about 42%. Solutions having higher concentrations of HF yield potassium hexafluorotantalate (KTaF₆). The K-salt can be also precipitated from a solution in hydrofluoric acid of tantalum pentachloride:

5 HF + 2 KF + TaCl₅ → K₂TaF₇ + 5 HCl

K₂TaF₇ is susceptible to hydrolysis. For example, a boiling aqueous solution of K₂TaF₇ yields potassium oxyfluorotantalate (K₂Ta₂O₃F₆), known as “Marignac’s salt”. In order to prevent hydrolysis and co-precipitation of potassium oxyfluorotantalate, a small excess of HF is added to the solution.

Reference

1. ^ ^{a b} Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 256.
2. ^ Agulyanski, A., The Chemistry of Tantalum and Niobium Fluoride Compounds, Elsevier: 2004, p 316-317, ISBN - 0444516042
3. ^ Langer, V. Smrčok, L. Boča, M. "Dipotassium heptafluorotantalate(V), β-K₂TaF₇, at 509K" Acta Crystallographica Section E 2006, E62, i91-i93. doi:10.1107/S1600536806009147
4. ^ Agulyansky, A. "Potassium fluorotantalate in solid, dissolved and molten conditions" J. Fluorine Chemistry 2003, 155-161. doi:10.1016/S0022-1139(03)00190-8
5. ^ J. J. Berzelius Pogg. Ann. 4, 6 (1825).