Potassium heptafluorotantalate

Potassium heptafluorotantalate
Names

IUPAC name Dipotassium heptafluorotantalate
Systematic IUPAC name Dipotassium heptafluorotantalum(2-)
Other names Potassium heptafluorotantalate(V) Potassium fluorotantalate
Identifiers
CAS Number	16924-00-8^Y
3D model (JSmol)	Interactive image
ChemSpider	28541740^N
ECHA InfoCard	100.037.245
EC Number	240-986-1
PubChem CID	28146
CompTox Dashboard (EPA)	DTXSID00884943
InChI InChI=1S/7FH.2K.Ta/h71H;;;/q;;;;;;;2+1;+5/p-7
SMILES F[Ta-2](F)(F)(F)(F)(F)F.[K+].[K+]
Properties
Chemical formula	K₂[TaF₇]
Molar mass	392.13 g/mol
Appearance	white solid
Density	4.56 g/mL at 25 °C
Melting point	630 to 820 °C (1,166 to 1,508 °F; 903 to 1,093 K)
Solubility in water	0.5 g/100 mL (15 °C)^[1]
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H301, H315, H319, H331, H335
Precautionary statements	P261, P264, P270, P271, P280, P301+P310, P302+P352, P304+P340, P305+P351+P338, P311, P312, P321, P330, P332+P313, P337+P313, P362, P403+P233, P405, P501
Lethal dose or concentration (LD, LC):
LD₅₀ (median dose)	110 mg/kg (Oral: rat)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is ^YN ?) Infobox references

Chemical compound

Potassium heptafluorotantalate is an inorganic compound with the formula K₂[TaF₇]. It is the potassium salt of the heptafluorotantalate anion [TaF₇]²⁻. This white, water-soluble solid is an intermediate in the purification of tantalum from its ores and is the precursor to the metal.^[2]

Preparation

Industrial

Potassium heptafluorotantalate is an intermediate in the industrial production of metallic tantalum. Its production involves leaching tantalum ores, such as columbite and tantalite, with hydrofluoric acid and sulfuric acid to produce the water-soluble hydrogen heptafluorotantalate.^[2]

Ta₂O₅ + 14 HF → 2 H₂[TaF₇] + 5 H₂O

This solution is subjected to a number of liquid-liquid extraction steps to remove metallic impurities (most importantly niobium) before being treated with potassium fluoride to produce K₂[TaF₇]

H₂[TaF₇] + 2 KF → K₂[TaF₇] + 2 HF

Lab-scale

Hydrofluoric acid is both corrosive and toxic, making it unappealing to work with; as such a number of alternative processes have been developed for small-scale syntheses. Potassium heptafluorotantalate 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]^[3]

Ta₂O₅ + 4 KHF₂ + 6 HF → 2 K₂[TaF₇] + 5 H₂O

The method was originally reported by Berzelius.^[4]

K₂[TaF₇] 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 → K₂[TaF₇] + 5 HCl

Structure

Potassium heptafluorotantalate exists in at least two polymorphs. α-K₂[TaF₇] is the most common form and crystallises in the monoclinic P2₁/c space group.^[5] The structure is composed of [TaF₇]²⁻ units interconnected by potassium ions. [TaF₇]²⁻ polyhedra may be described as monocapped trigonal prisms with the capping atom located on one of the rectangular faces. Potassium atoms are 9-coordinated and may be viewed as distorted monocapped square prisms.

At temperatures above 230 °C this converts to β-K₂[TaF₇], which is orthorhombic (space group: Pnma). This structure also consists of potassium ions and the complex anion [TaF₇]²⁻. The structure of the 7-coordinate [TaF₇]²⁻ units is essentially unchanged. However the potassium atoms now exist in 2 environments where they coordinate to either 11 or 8 fluorine atoms.^[6]^[7]

Reactions

K₂[TaF₇] is primarily used to produce metallic tantalum by reduction with sodium. This takes place at approximately 800 °C in molten salt and proceeds via a number of potential pathways.^[8]

K₂[TaF₇] + 5 Na → Ta + 5 NaF + 2 KF

K₂[TaF₇] is susceptible to hydrolysis. For example, a boiling aqueous solution of K₂[TaF₇] 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.

References

^ ^a ^b Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 256.
^ ^a ^b Anthony Agulyanski (2004). "Fluorine chemistry in the processing of tantalum and niobium". In Anatoly Agulyanski (ed.). Chemistry of Tantalum and Niobium Fluoride Compounds (1st ed.). Burlington: Elsevier. ISBN 9780080529028.
^ Agulyansky, A. "Potassium fluorotantalate in solid, dissolved and molten conditions" J. Fluorine Chemistry 2003, 155-161. doi:10.1016/S0022-1139(03)00190-8
^ J. J. Berzelius Pogg. Ann. 4, 6 (1825#.
^ Torardi, C.C.; Brixner, L.H.; Blasse, G. (1987). "Structure and luminescence of K₂TaF₇ and K₂NbF₇". Journal of Solid State Chemistry. 67 (1): 21–25. doi:10.1016/0022-4596#87)90333-1.
^ 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
^ Smrčok, Ľubomír; Brunelli, Michela; Boča, Miroslav; Kucharík, Marian (8 April 2008). "Structure of K₂TaF₇ at 993 K: the combined use of synchrotron powder data and solid-state DFT calculations". Journal of Applied Crystallography. 41 (3): 634–636. doi:10.1107/S0021889808005876.
^ Okabe, Toru H.; Sadoway, Donald R. (1998). "Metallothermic reduction as an electronically mediated reaction". Journal of Materials Research. 13 (12): 3372–3377. Bibcode:1998JMatR..13.3372O. doi:10.1557/JMR.1998.0459.