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.[2] 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 D3hsymmetry.[3]
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 [TaF 6]− , [TaF 7]2− , and [TaF 8]3− . With neutral Lewis bases, such as diethyl ether TaF5 forms adducts.
TaF 5 is used in combination with HF as a catalyst for the alkylation of alkanes and alkenes and for the protonation of aromatic compounds. The TaF 5–HF system is stable in reducing environments, unlike SbF 5–HF.[4] In the presence of fluoride, tantalum pentafluoride forms the anions [TaF 8]3− , [TaF 7]2− , or [TaF 6]− , 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:[5]
[TaF 7]2− + HF ⇌ [TaF 6]− + HF− 2
The salts M3TaF8 have been crystallized. For K+ = M+, the crystals consist of [TaF 7]2− anions together with fluoride that does not coordinate to Ta(V).[6] For M+ = M+, the crystals features [TaF 8]3− .[7]
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.[8]
^Klaus Andersson, Karlheinz Reichert, Rüdiger Wolf "Tantalum and Tantalum Compounds" in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH. Weinheim. doi:10.1002/14356007.a26_071
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