Related compounds Molar mass 275.95 g/mol Density 4.74 g/cm³ Appearance white powder | Formula TaF5 Melting point 96.8 °C Boiling point 229.5 °C | |
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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.
Contents
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 TaF4 centers linked together via bridging fluoride centers. Gaseous TaF5 adopts the trigonal pyramidal 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 [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. In the presence of fluoride, tantalum pentafluoride forms the anions [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:
7]2−
+ HF ⇌ [TaF
6]−
+ HF−
2
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.