Tantalum(V) fluoride

Tantalum(V) fluoride
Other names

tantalum pentafluoride
Identifiers
CAS number 7783-71-3
RTECS number WW5775000
Properties
Molecular formula TaF5
Molar mass 275.95 g/mol
Appearance white powder
Density 4.74 g/cm3, solid
Melting point

96.8 °C
Boiling point

229.5 °C
Solubility in water decomposes
Structure
Dipole moment 0 D
Hazards
EU classification not listed
R-phrases 34
S-phrases 26-27-28-36/37/39-45
Main hazards HF source
Flash point nonflammable
Related compounds
Related compounds TaCl5
NbCl5
WF6
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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.[1] 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.[2]

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.[3] In the presence of fluoride, tantalum pentafluoride forms the anions [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:[4]

[TaF7]2− + HF [TaF6]− + 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.[5]

References

  1. ^ Priest, H. F. “Anhydrous Metal Fluorides” Inorganic Syntheses McGraw-Hill: New York, 1950; Vol. 3, pages 171-183.doi:10.1002/9780470132340.ch47
  2. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  3. ^ Arpad Molnar; G. K. Surya Prakash; Jean Sommer (2009). Superacid Chemistry (2nd ed.). Wiley-Interscience. p. 60. ISBN 047159668X. 
  4. ^ Anatoly Agulyanski (2004). The chemistry of tantalum and niobium fluoride compounds. Amsterdam: Elsevier. p. 134. ISBN 0444516042. 
  5. ^ 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