Chemistry:Tantalum pentafluoride

Tantalum(V) fluoride

Names
Other names tantalum pentafluoride
Identifiers
CAS Number	7783-71-3 Y
3D model (JSmol)	Interactive image
ChemSpider	74198
EC Number	232-022-3
PubChem CID	82218
RTECS number	WW5775000
UNII	QAG1W735WO Y
InChI Key: YRGLXIVYESZPLQ-UHFFFAOYSA-I InChI=1S/5FH.Ta/h5*1H;/q;;;;;+5/p-5
SMILES F[Ta](F)(F)(F)F
Properties
Chemical formula	TaF5
Molar mass	275.95 g/mol
Appearance	white powder
Density	4.74 g/cm3, solid
Melting point	96.8 °C (206.2 °F; 369.9 K)
Boiling point	229.5 °C (445.1 °F; 502.6 K)
Solubility in water	decomposes
Magnetic susceptibility (χ)	+795.0·10−6 cm3/mol
Structure
Dipole moment	0 D
Hazards
Main hazards	HF source
GHS pictograms
GHS Signal word	Danger
GHS hazard statements	H302, H314
GHS precautionary statements	P260, P261, P264, P270, P271, P280, P301+312, P301+330+331, P302+352, P303+361+353, P304+312, P304+340, P305+351+338, P310, P312, P321, P322, P330, P363, P405, P501
Flash point	Non-flammable
Related compounds
Related compounds	TaCl5 NbCl5 WF6
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Tantalum(V) fluoride is the inorganic compound with the formula TaF₅. It is one of the principal molecular compounds of tantalum. Characteristic of some other pentafluorides, the compound is volatile but exists as an oligomer in the solid state.

Preparation and structure

It is prepared by treating tantalum metal with fluorine gas. NbF₅ is prepared similarly.

Solid and molten TaF₅ is tetrameric, consisting of four TaF₆ centers linked via bridging fluoride centers. Gaseous TaF₅ adopts the trigonal pyramidal structure with D_3h symmetry.

Reactions and derivatives

The tendency of TaF₅ to form clusters in the solid state indicates the Lewis acidity of the monomer. Indeed, TaF₅ reacts with fluoride sources to give the ions [TaF₆]⁻, [TaF₇]²⁻, and [TaF₈]³⁻. With neutral Lewis bases, such as diethyl ether TaF₅ forms adducts.

TaF₅ is used in combination with HF as a catalyst for the alkylation of alkanes and alkenes and for the protonation of aromatic compounds. The TaF₅–HF system is stable in reducing environments, unlike SbF₅–HF. In the presence of fluoride, tantalum pentafluoride forms the anions [TaF₈]³⁻, [TaF₇]²⁻, or [TaF₆]⁻, depending on the nature of the counterion and the concentration of HF. High concentrations of HF favor the hexafluoride by virtue of the formation of HF−2:

[TaF₇]²⁻ + HF ⇌ [TaF₆]⁻ + HF−2

The salts M₃TaF₈ have been crystallized. For K⁺ = M⁺, the crystals consist of [TaF₇]²⁻ anions together with fluoride that does not coordinate to Ta(V). For M⁺ = M⁺, the crystals features [TaF₈]³⁻.

Section of the Na₃TaF₈ structure (Ta = turquoise, F = green).

Relevance to separation of Ta and Nb

In the Marignac process, Nb and Ta are separated by fractional crystallization of K₂TaF₇ from solutions of hydrofluoric acid. Under these conditions, niobium forms K₂NbOF₅, which is more soluble than K₂TaF₇. Reduction of K₂TaF₇ with sodium gives metallic Ta.