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.

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

Structure

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 [(CH3)4N]+[IF6]− reveals loosely bound [I2F11]2− dimers. Significant cation-anion interactions were also found in [BrF2]+[SbF6]−, [ClF2]+[SbF6]−, [BrF4]+[Sb6F11]−.

The [I3Cl2]+ and [I3Br2]+ ions have a trans-Z-type structure, analogous to that of [I5]+.

Higher polyiodides

The polyiodide ions have much more complicated structures. Discrete polyiodides usually have a linear sequence of iodine atoms and iodide ions, and are described in terms of association between I2, I− and [I3]− units, which reflects the origin of the polyiodide. In the solid states, the polyiodides can interact with each other to form chains, rings, or even complicated two-dimensional and three-dimensional networks.

Bonding The bonding in polyhalogen ions mostly invoke the predominant use of p-orbitals. Significant d-orbital participation in the bonding is improbable as much promotional energy will be required, while scant s-orbital participation is expected in iodine-containing species due to the inert-pair effect, suggested by data from Mössbauer spectroscopy. However, no bonding model has been capable of reproducing such wide range of bond lengths and angles observed so far. As expected from the fact that an electron is removed from the antibonding orbital when X2 is ionized to [X2]+, the bond order as well as the bond strength in [X2]+ gets higher, consequently the interatomic distances in the molecular ion is less than those in X2. Linear or nearly-linear triatomic polyhalides have weaker and longer bonds compared with that in the corresponding diatomic interhalogen or halogen, consistent with the additional repulsion between atoms as the halide ion is added to the neutral molecule. Another model involving the use of resonance theory exists, for example, [ICl2]− can be viewed as the resonance hybrid of the following canonical forms:

Evidence supporting this theory comes from the bond lengths (255 pm in [ICl2]− and 232 pm in ICl(g)) and bond stretching wavenumbers (267 and 222 cm−1 for symmetric and asymmetric stretching in [ICl2]− compared with 384 cm−1 in ICl), which suggests a bond order of about 0.5 for each I–Cl bonds in [ICl2]−, consistent with the interpretation using the resonance theory. Other triatomic species [XY2]− can be similarly interpreted. Even though they have a reduced bond order, all three halogen atoms are tightly bound. The fluorine–fluorine bond of trifluoride, with bond order 0.5, has a bond-strength is 30 kcal/mol, only 8 kcal/mol less than the fluorine–fluorine bond in difluorine whose bond order is 1.

Synthesis The formation of polyhalogen ions can be viewed as the self-dissociation of their parent interhalogens or halogens:

2 XYn ⇌ [XYn−1]+ + [XYn+1]− 3 X2 ⇌ [X3]+ + [X3]− 4 X2 ⇌ [X5]+ + [X3]− 5 X2 ⇌ 2 [X2]+ + 2 [X3]−

Polyhalogen cations There are two general strategies for preparing polyhalogen cations:

By reacting the appropriate interhalogen with a Lewis acid (such as the halides of B, Al, P, As, Sb) either in an inert or oxidizing solvent (such as anhydrous HF) or without one, to give a heteropolyhalogen cation. XYn + MYm → [XYn−1]+ + [MYm+1]− By an oxidative process, in which the halogen or interhalogen is reacted with an oxidizer and a Lewis acid to give the cation: Cl2 + ClF + AsF5 → [Cl3]+[AsF6]− In some cases the Lewis acid (the fluoride acceptor) itself acts as an oxidant:

3 I2 + 3 SbF5 → 2 [I3]+[SbF6]− + SbF3 Usually the first method is employed for preparing heteropolyhalogen cations, and the second one is applicable to both. The oxidative process is useful in the preparation of the cations [IBr2]+, [ClF6]+, [BrF6]+, as their parent interhalogens, IBr3, ClF7, BrF7 respectively, has never been isolated:

Br2 + IOSO2F → [IBr2]+[SO3F]− 2 ClF5 + 2 PtF6 → [ClF6]+[PtF6]− + [ClF4]+[PtF6]− BrF5 + [KrF]+[AsF6]− → [BrF6]+[AsF6]− + Kr The preparation of some individual species are briefly summarized in the table below with equations:

Polyhalogen anions For polyhalogen anions, there are two general preparation strategies as well:

By reacting an interhalogen or halogen with a Lewis base, most likely a fluoride: [(CH3CH2)4N]+Y− + XYn → [(CH3CH2)4N]+[XYn+1]− X2 + X− → [X3]− By oxidation of simple halides: KI + Cl2 → K+[ICl2]− The preparation of some individual species are briefly summarized in the table below with equations:

The higher polyiodides were formed upon crystallization of solutions containing various concentrations of I− and I2. For instance, the monohydrate of K+[I3]− crystallizes when a saturated solution containing appropriate amounts of I2 and KI is cooled.

Properties

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