Polyhalogen ions

Polyhalogen ions are a group of polyatomic cations and anions containing halogens only. The ions can be classified into two classes, isopolyhalogen ions which contain one type of halogen only, and heteropolyhalogen ions with more than one type of halogen.

Numerous polyhalogen ions have been found, with their salts isolated in the solid state and structurally characterized. The following tables summarize the species found so far.

Most of the structures of the ions have been determined by IR spectroscopy, Raman spectroscopy and X-ray crystallography. The polyhalogen ions always have the heaviest and least electronegative halogen present in the ion as the central atom, making the ion asymmetric in some cases. For example, [Cl₂F]⁺ has a structure of [Cl–Cl–F]⁻ but not [Cl–F–Cl]⁻.

In general, the structures of most heteropolyhalogen ions and lower isopolyhalogen ions were in agreement with the VSEPR model. However, there were exceptional cases. For example, when the central atom is heavy and has seven lone pairs, such as [BrF₆]⁻ and [IF₆]⁻, they have a regular octahedral arrangement of fluoride ligands instead of a distorted one due to the presence of a stereochemically inert lone pair. More deviations from the ideal VSEPR model were found in the solid state structures due to strong cation-anion interactions, which also complicates interpretation of vibrational spectroscopic data. In all known structures of the polyhalogen anion salts, the anions make very close contact, via halogen bridges, with the counter-cations. For example, in the solid state, [IF₆]⁻ is not regularly octahedral, as solid state structure of [Me₄N]⁺[IF₆]⁻ reveals loosely bound [I₂F₁₁]²⁻ dimers. Significant cation-anion interactions were also found in [BrF₂]⁺[SbF₆]⁻, [ClF₂]⁺[SbF₆]⁻, [BrF₄]⁺[Sb₆F₁₁]⁻.

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