Polyhalogen ions
Script error: No such module "Distinguish". 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.[1][2][3][4][5][6]
| Diatomic species | *Template:Chem2 |
| Triatomic species | Template:Chem2 |
| Tetraatomic species | Template:Chem2 |
| Pentaatomic species | Template:Chem2 |
| Heptaatomic species | †Template:Chem2 |
| Higher species | Template:Chem2 |
Page Template:Visible anchor/styles.css has no content.* Template:Chem2 can only exist as Template:Chem2 at low temperatures, a charge-transfer complex from Template:Chem2 to Template:Chem2.[2] Free Template:Chem2 is only known from its electronic band spectrum obtained in a low-pressure discharge tube.[3]
Page Template:Visible anchor/styles.css has no content.† The existence of Template:Chem2 is possible but still uncertain.[1]
| Triatomic species | Template:Chem2 |
| Pentaatomic species | Template:Chem2 |
| Heptaatomic species | Template:Chem2 |
| Triatomic species | Template:Chem2 |
| Tetraatomic species | Template:Chem2 |
| Pentaatomic species | Template:Chem2 |
| Heptaatomic species | Template:Chem2 |
| Octaatomic species | Template:Chem2 |
| Higher species | Template:Chem2 |
| Triatomic species | Template:Chem2 |
| Pentaatomic species | Template:Chem2 |
| Hexaatomic species | Template:Chem2 |
| Heptaatomic species | Template:Chem2 |
| Nonaatomic species | Template:Chem2 |
Structure
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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, Template:Chem2 has a structure of Template:Chem2 but not Template:Chem2.
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 Template:Chem2 and Template:Chem2, 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.[4] For example, in the solid state, Template:Chem2 is not regularly octahedral, as solid state structure of Template:Chem2 reveals loosely bound Template:Chem2 dimers. Significant cation-anion interactions were also found in Template:Chem2.[2]
| Linear (or almost linear) | Template:Chem2 |
| Bent | Template:Chem2 |
| Square planar | Template:Chem2 |
| Disphenoidal (or seesaw) | Template:Chem2 |
| Pentagonal planar | ‡Template:Chem2 |
| Octahedral | Template:Chem2, ¶Template:Chem2 |
| Square antiprismatic | Template:Chem2 |
Page Template:Visible anchor/styles.css has no content.‡ Template:Chem2 is one of the two Template:Chem2-type species known to have the rare pentagonal planar geometry, the other being Template:Chem2.
Page Template:Visible anchor/styles.css has no content.¶ Template:Chem2 is distorted octahedral as the stereochemical inert-pair effect is not significant in the chlorine atom.
The Template:Chem2 and Template:Chem2 ions have a trans-Z-type structure, analogous to that of Template:Chem2.
%2B_ion.png)
Higher polyiodides
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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 Template:Chem2, Template:Chem2 and Template:Chem2 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.[3]
As expected from the fact that an electron is removed from the antibonding orbital when Template:Chem2 is ionized to Template:Chem2, the bond order as well as the bond strength in Template:Chem2 gets higher, consequently the interatomic distances in the molecular ion is less than those in Template:Chem2.
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, Template:Chem2 can be viewed as the resonance hybrid of the following canonical forms:
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Evidence supporting this theory comes from the bond lengths (255 pm in Template:Chem2 and 232 pm in ICl(g)) and bond stretching wavenumbers (267 and 222 cm−1 for symmetric and asymmetric stretching in Template:Chem2 compared with 384 cm−1 in ICl), which suggests a bond order of about 0.5 for each I–Cl bonds in Template:Chem2, consistent with the interpretation using the resonance theory. Other triatomic species Template:Chem2 can be similarly interpreted.[2]
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.[7]
Synthesis
The formation of polyhalogen ions can be viewed as the self-dissociation of their parent interhalogens or halogens:
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.
- By an oxidative process, in which the halogen or interhalogen is reacted with an oxidizer and a Lewis acid to give the cation:
In some cases the Lewis acid (the fluoride acceptor) itself acts as an oxidant:
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 Template:Chem2, as their parent interhalogens, Template:Chem2 respectively, has never been isolated:
The preparation of some individual species are briefly summarized in the table below with equations:[1][2][3][4]
Page Template:Visible anchor/styles.css has no content.‡ In this reaction, the active oxidizing species is Template:Chem2, which is formed in situ in the Template:Chem2/Template:Chem2/HF system. It is an even more powerful oxidizing and fluorinating agent than [[platinum hexafluoride|Template:Chem2]].
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:
- By oxidation of simple halides:
The preparation of some individual species are briefly summarized in the table below with equations:[1][2][3][4]
The higher polyiodides were formed upon crystallization of solutions containing various concentrations of Template:Chem2 and Template:Chem2. For instance, the monohydrate of Template:Chem2 crystallizes when a saturated solution containing appropriate amounts of Template:Chem2 and KI is cooled.[8]Template:Rp
Properties
Stability
In general, a large counter cation or anion (such as Template:Chem2 and Template:Chem2) can help stabilize the polyhalogen ions formed in the solid state from lattice energy considerations, as the packing efficiency increases.
The polyhalogen cations are strong oxidizing agents, as indicated by the fact that they can only be prepared in oxidative liquids as a solvent, such as oleum. The most oxidizing and therefore most unstable ones are the species Template:Chem2 and Template:Chem2 (X = Cl, Br), followed by Template:Chem2 and Template:Chem2.
The stability of the Template:Chem2 salts (X = Br, I) are thermodynamically quite stable. However, their stability in solution depends on the superacid solvent. For example, Template:Chem2 is stable in fluoroantimonic acid (HF with 0.2 N Template:Chem2, H0 = −20.65), but disproportionates to Template:Chem2, Template:Chem2 and Template:Chem2 when weaker fluoride acceptors, like [[Niobium pentafluoride|Template:Chem2]], [[Tantalum pentafluoride|Template:Chem2]] or NaF, are added instead of Template:Chem2.[4]
For polyhalogen anions with the same number of atoms, the more stable ones are those with a heavier halogen at the center, symmetric ions are also more stable than asymmetric ones. therefore the stability of the anions decrease in the order:
Heteropolyhalogen ions with a coordination number larger than or equal to four can only exist with fluoride ligands.
Color
Most polyhalogen ions are intensely colored, with deepened color as the atomic weight of the constituent element increases. The well-known starch-iodine complex has a deep blue color due to the linear Template:Chem2 ions present in the amylose helix.[4] While the intensity of polyhalogen anion absorption bands in the UV–visible range are often influenced by the solvent dielectric constant, their location (determining the overall color) is relatively solvent-independent.[9] Some colors of common species are listed below:[3]
- fluorocations tend to be colorless or pale yellow, other heteropolyhalogen ions are orange, red or deep purple[4]
- compounds of Template:Chem2 are wine red to bright orange; while that of Template:Chem2 are dark brown to purplish black
- Template:Chem2 is yellow
- Template:Chem2 is blue[2]
- Template:Chem2 is cherry red
- Template:Chem2 is brown
- Template:Chem2 is dark brown
- Template:Chem2 is bright blue
- Template:Chem2 is dark brown to black
- Template:Chem2 is red to brown
- Template:Chem2 is green or black, the salt Template:Chem2 exists as greenish-black needles, but appears brown-red in thin sections
- Template:Chem2 is black, if its existence in the compound Template:Chem2 has been firmly established
- Template:Chem2 is black[5]
- Template:Chem2 is colorless[10]Template:Rp[11]
- Template:Chem2 is yellow[10]Template:Rp
- Template:Chem2 is red[10]Template:Rp
- Template:Chem2 is orange[10]Template:Rp to yellow[12]
- Template:Chem2 is golden-yellow[10]Template:Rp
- Template:Chem2 is red[10]Template:Rp
- polyiodides have very dark colors, either dark brown or dark blue
Chemical properties
The heteropolyhalogen cations are explosively reactive oxidants, and the cations often have higher reactivity than their parent interhalogens and decompose by reductive pathways. As expected from the highest oxidation state of +7 in Template:Chem2, Template:Chem2 and Template:Chem2, these species are extremely strong oxidizing agents, demonstrated by the reactions shown below:
Polyhalogen cations with lower oxidation states tend to disproportionate. For example, Template:Chem2 is unstable in solution and disproportionates completely in [[fluoroantimonic acid|HF/Template:Chem2]] mixture even at 197 K:
Template:Chem2 reversibly dimerizes at 193 K, and is observed as the blue color of paramagnetic Template:Chem2 dramatically shifts to the red-brown color of diamagnetic Template:Chem2, together with a drop in paramagnetic susceptibility and electrical conductivity when the solution is cooled to below 193 K:[2]
The dimerization can be attributed to the overlapping of the half-filled π* orbitals in two Template:Chem2.
Template:Chem2 in Template:Chem2 is structurally analogous to Template:Chem2, but decomposes at 195 K to give Template:Chem2, and salts of Template:Chem2 instead of Template:Chem2.[2]
Attempts to prepare Template:Chem2 and Template:Chem2 by fluorinating Template:Chem2 and Template:Chem2 using NOF have met with failure, because the following reactions occurred:[3]
The anions are less reactive compared to the cations, and are generally weaker oxidants than their parent interhalogens. They are less reactive towards organic compounds, and some salts are of quite high thermal stability. Salts containing polyhalogen anions of the type Template:Chem2, where m + n + p = {3, 5, 7, 9...}, tend to dissociate into simple monohalide salts between Template:Chem2 and the most electronegative halogen, so that the monohalide has the highest lattice energy. An interhalogen is usually formed as the other product. The salt Template:Chem2 decomposes at about 100 °C, and salts of Template:Chem2 are thermally unstable and can explode even at −31 °C.[4]
See also
References
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