Antimony pentafluoride

Preparation

Antimony pentafluoride is prepared by the reaction of antimony pentachloride with anhydrous hydrogen fluoride:

It can also be prepared from antimony trifluoride and fluorine.

Structure and chemical reactions

In the gas phase, SbF₅ adopts a trigonal bipyramidal structure of D_3h point group symmetry (see picture). The material adopts a more complicated structure in the liquid and solid states. The liquid contains polymers wherein each Sb is octahedral, the structure being described with the formula [SbF₄(μ-F)₂]_n ((μ-F) denotes the fact that fluoride centres bridge two Sb centres). The crystalline material is a tetramer, meaning that it has the formula [SbF₄(μ-F)]₄. The Sb-F bonds are 2.02 Å within the eight-membered Sb₄F₄ ring; the remaining fluoride ligands radiating from the four Sb centers are shorter at 1.82 Å. The related species PF₅ and AsF₅ are monomeric in the solid and liquid states, probably due to the smaller sizes of the central atom, which limits their coordination number. BiF₅ is a polymer.

In the same way that SbF₅ enhances the Brønsted acidity of HF, it augments the oxidizing power of F₂. This effect is illustrated by the oxidation of oxygen:

Antimony pentafluoride has also been used in the first discovered chemical reaction that produces fluorine gas from fluoride compounds:

The driving force for this reaction is the high affinity of SbF₅ for F−
, which is the same property that recommends the use of SbF₅ to generate superacids.

Hexafluoroantimonate

SbF₅ is a strong Lewis acid, exceptionally so toward sources of F⁻ to give the very stable anion [SbF₆]⁻, called hexafluoroantimonate. [SbF₆]⁻ is a weakly coordinating anion akin to PF₆⁻. Although it is only weakly basic, [SbF₆]⁻ does react with additional SbF₅ to give a centrosymmetric adduct:

Safety

SbF₅ reacts violently with many compounds, often releasing dangerous hydrogen fluoride. It is corrosive to the skin and eyes.