Tantalum(V) fluoride is the inorganic compound with the formula TaF₅. It is one of the principal molecular compounds of

tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that ...

. 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 , , and . With neutral Lewis bases, such as

diethyl ether Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable li ...

TaF₅ forms

adduct An adduct (from the Latin ''adductus'', "drawn toward" alternatively, a contraction of "addition product") is a product of a direct addition of two or more distinct molecules, resulting in a single reaction product containing all atoms of all co ...

s. is used in combination with HF as a catalyst for the

alkylation Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecti ...

of alkanes and

alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

s and for the

protonation In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), (H+) to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brønsted–Lowry acid ...

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

compounds. The system is stable in reducing environments, unlike . In the presence of fluoride, tantalum pentafluoride forms the anions , , or , 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 + The salts M₃TaF₈ have been crystallized. For K⁺ = M⁺, the crystals consist of anions together with fluoride that does not coordinate to Ta(V). For M⁺ = M⁺, the crystals features . : 260875-ICSD

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.Klaus Andersson, Karlheinz Reichert, Rüdiger Wolf "Tantalum and Tantalum Compounds" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH. Weinheim.

References

{{fluorine compounds Fluorides Tantalum compounds Metal halides