Tantalum(V) fluoride

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 a tetramer in the solid state.

Preparation and structure TaF5 is prepared by treating tantalum metal with fluorine gas.

2 Ta + 5 F2 → 2 TaF5 NbF5 is prepared similarly. Solid and molten TaF5 is tetrameric, consisting of four octahedral TaF6 centers linked via bridging fluoride centers. Gaseous TaF5 is monomeric and adopts the trigonal bipyramidal structure with D3h 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]− (hexafluorotantalate(V)), [TaF7]2− (heptafluorotantalate(V)), and [TaF8]3− (octafluorotantalate(V)). 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 superacid 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 [HF2]−:

[TaF7]2− + HF ⇌ [TaF6]− + [HF2]− The salts M3TaF8 have been crystallized. For M = K, the crystals consist of [TaF7]2− anions together with fluoride that does not coordinate to Ta(V), and the salt is actually potassium fluoride heptafluorotantalate(V) (K+)3[TaF7]2−F−. For M = Na, the crystals features [TaF8]3−, and the salt is sodium octafluorotantalate(V) (Na+)3[TaF8]3−.

Relevance to separation of Ta and Nb In the Marignac process, Nb and Ta are separated by fractional crystallization of K2[TaF7] from solutions of hydrofluoric acid. Under these conditions, niobium forms K2[NbOF5], which is more soluble than K2[TaF7]. Reduction of K2[TaF7] with sodium gives metallic Ta.

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