Antimony pentafluoride

Antimony pentafluoride is the chemical compound with the formula SbF5. This colourless, viscous liquid is a valuable Lewis acid and a component of the superacid fluoroantimonic acid, the strongest known acid. Some features that give the compound scientific interest are its Lewis acidity and that it also reacts with almost all known compounds.

Structure and chemical reactions
In the gas phase, SbF5 adopts a trigonal bipyramidal structure of D3h point group symmetry (see picture). The structure is more complex in the liquid and solid state. The liquid contains polymers wherein each Sb is octahedral, the structure being described with the formula [SbF4(&mu;-F)2]n. The crystalline material is tetrameric, i.e. it has the formula [SbF4(&mu;-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.

SbF5 is a strong Lewis acid, exceptionally so toward sources of F− to give the very stable anion [SbF6]−. The latter reacts with additional SbF5 to give [Sb2F11]−.

In the same way that SbF5 enhances the Brønsted acidity of HF, it enhances the oxidizing power of F2. This effect is illustrated by the oxidation of oxygen:
 * SbF5 + ½F2  +  O2  →  [O2]+[SbF6]-

Antimony pentafluoride has also been used in the first discovered chemical reaction that produces fluorine gas from fluoride compounds:
 * 2 SbF5 +  K2MnF6  →  2 KSbF6 + MnF3  +  0.5 F2

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

Safety
SbF5 reacts violently with many compounds, often releasing dangerous hydrogen fluoride.