Editing Interhalogen

{{Short description|Molecule containing only halogen elements of two or more kinds}}

In [[chemistry]], an '''interhalogen compound''' is a [[molecule]] which contains two or more different [[halogen]] atoms ([[fluorine]], [[chlorine]], [[bromine]], [[iodine]], or [[astatine]]) and no atoms of elements from any other group. 

Most interhalogen compounds known are binary (composed of only two distinct elements). Their formulae are generally {{chem2|XY_{''n''} }}, where ''n'' = 1, 3, 5 or 7, and X is the less [[electronegativity|electronegative]] of the two halogens. The value of ''n'' in interhalogens is always odd, because of the odd valence of halogens. They are all prone to [[hydrolysis]], and ionize to give rise to polyhalogen ions. Those formed with astatine have a very short half-life due to astatine being intensely radioactive.

No interhalogen compounds containing three or more different halogens are definitely known,<ref name = "Chemistry of Interhalogens"/> although a few books claim that {{chem2|IFCl2}} and {{chem2|IF2Cl}} have been obtained,<ref>{{Greenwood&Earnshaw2nd|page=824}}</ref><ref name=Meyers/><ref name=Murthy/><ref name=Sahoo/> and theoretical studies seem to indicate that some compounds in the series {{chem|BrClF|''n''}} are barely stable.<ref name=Ignatiev/>

Some interhalogens, such as {{chem2|BrF3}}, {{chem2|IF5}}, and {{chem2|ICl}}, are good [[halogenation|halogenating]] agents. {{chem2|BrF5}} is too reactive to generate fluorine. Beyond that, [[iodine monochloride]] has several applications, including helping to measure the saturation of fats and oils, and as a [[catalyst]] for some [[chemical reaction|reactions]]. A number of interhalogens, including [[Iodine heptafluoride|{{chem2|IF7}}]], are used to form [[polyhalide]]s.<ref name = "Chemistry of Interhalogens"/>

Similar compounds exist with various [[pseudohalogen]]s, such as the halogen [[azide]]s ({{chem2|[[fluorine azide|FN3]]}}, {{chem2|[[chlorine azide|ClN3]]}}, {{chem2|[[bromine azide|BrN3]]}}, and {{chem2|[[iodine azide|IN3]]}}) and [[cyanogen]] halides ({{chem2|[[cyanogen fluoride|FCN]]}}, {{chem2|[[cyanogen chloride|ClCN]]}}, {{chem2|[[cyanogen bromide|BrCN]]}}, and {{chem2|[[cyanogen iodide|ICN]]}}).

==Types of interhalogens==
{{Expand section|with=This table needs more explanation and context.|date=October 2024}}
{| class="wikitable" style="margin: 1em auto 1em auto"
|-
! F 
| {{center|[[fluorine|F<sub>2</sub>]]}}
|-
! Cl 
| {{center|[[chlorine monofluoride|ClF]], [[chlorine trifluoride|ClF<sub>3</sub>]], [[chlorine pentafluoride|ClF<sub>5</sub>]]}} || {{center|[[chlorine|Cl<sub>2</sub>]]}}
|-
! Br 
| {{center|[[bromine monofluoride|BrF]], [[bromine trifluoride|BrF<sub>3</sub>]], [[bromine pentafluoride|BrF<sub>5</sub>]]}} || {{center|[[bromine monochloride|BrCl]]}} 
|[[Bromine|Br<sub>2</sub>]]
|-
! I 
| {{center|[[iodine monofluoride|IF]], [[iodine trifluoride|IF<sub>3</sub>]], [[iodine pentafluoride|IF<sub>5</sub>]], [[iodine heptafluoride|IF<sub>7</sub>]]}} || {{center|[[iodine monochloride|ICl]], [[iodine trichloride|(ICl<sub>3</sub>)<sub>2</sub>]]}} || [[Iodine monobromide|IBr]], [[Iodine tribromide|IBr<sub>3</sub>]]
|{{center|[[iodine|I<sub>2</sub>]]}}
|-
! At 
| {{center|''unknown''}} || {{center|[[astatine monochloride|AtCl]]}} || [[Astatine bromide|AtBr]]|| {{center|[[astatine monoiodide|AtI]]}}
|{{center|[[astatine|At<sub>2</sub>]](?)}}
|-
!style="border: none; background: none;"| !! F !! Cl !! Br !! I
!At
|}
===Diatomic interhalogens===
[[File:Iodine-monochloride-3D-vdW.png|thumb|176x176px|[[Iodine monochloride]]]]
The interhalogens of form XY have physical properties intermediate between those of the two parent halogens. The [[covalent]] bond between the two atoms has some [[Ionic bond|ionic]] character, the less [[electronegative]] halogen, X, being oxidised and having a partial positive charge. All combinations of fluorine, chlorine, bromine, and iodine that have the above-mentioned general formula are known, but not all are stable. Some combinations of astatine with other halogens are not even known, and those that are known are highly unstable.

*'''[[Chlorine monofluoride]]''' (ClF) is the lightest interhalogen compound. ClF is a colorless gas with a [[normal boiling point]] of −100&nbsp;°C.
*'''[[Bromine monofluoride]]''' (BrF) has not been obtained as a pure compound — it dissociates into the trifluoride and free [[bromine]]. It is created according to the following equation:
::Br<sub>2</sub>(l) + F<sub>2</sub>(g) → 2&nbsp;BrF(g)
:Bromine monofluoride dissociates like this:
::3&nbsp;BrF → Br<sub>2</sub> + BrF<sub>3</sub>

[[File:Astatine-chloride-3D-vdW.png|thumb|177x177px|Astatine chloride]][[File:Astatine-bromide-3D-vdW.png|thumb|[[Astatine bromide]]|179x179px]][[File:Astatine-iodide-3D-vdW.svg|thumb|[[Astatine iodide]]|177x177px]]

*'''[[Iodine monofluoride]]''' (IF) is unstable and decomposes at 0&nbsp;°C, [[Disproportionation|disproportionating]] into elemental iodine and [[iodine pentafluoride]].
*'''[[Bromine monochloride]]''' (BrCl) is a yellow-brown gas with a boiling point of 5&nbsp;°C. 
*'''[[Iodine monochloride]]''' (ICl) exists as red transparent crystals that melt at 27.2&nbsp;°C to form a choking brownish liquid (similar in appearance and weight to [[bromine]]). It reacts with HCl to form the strong acid HICl<sub>2</sub>. The [[crystal structure]] of iodine monochloride consists of puckered zig-zag chains, with strong interactions between the chains. 
*'''Astatine monochloride'''<ref><sup>211</sup>At+Cl<sub>2</sub> at room temperature: {{cite book |url= https://inis.iaea.org/collection/NCLCollectionStore/_Public/12/609/12609573.pdf |page= 32 |title= Organic Chemistry of Astatine |first1= Klara |last1= Berei |first2= L. |last2= Vasáros |isbn= 9633717876 |year= 1981 |publisher= Hungarian Academy of Sciences, Central Research Institute for Physics }}</ref> (AtCl) is made either by the direct combination of gas-phase [[astatine]] with chlorine or by the sequential addition of astatine and dichromate ion to an acidic chloride solution.
*'''[[Iodine monobromide]]''' (IBr) is made by the direct combination of the elements to form a dark red crystalline solid. It melts at 42&nbsp;°C and boils at 116&nbsp;°C to form a partially dissociated vapour. 
*'''[[Astatine bromide|Astatine monobromide]]''' (AtBr) is made by the direct combination of astatine with either bromine vapour or an aqueous solution of iodine monobromide.
*'''[[Astatine iodide|Astatine monoiodide]]''' (AtI) is made by direct combination of astatine and iodine.

No astatine fluorides<!--AtF, AtF5, or whatever else, "no" means "no" (not AtF, not AtF5, not At45F387, or whatever)--> have been discovered yet. Their absence has been speculatively attributed to the extreme reactivity of such compounds, including the reaction of an initially formed fluoride with the walls of the glass container to form a non-volatile product.{{efn|An initial attempt to fluoridate astatine using [[chlorine trifluoride]] resulted in formation of a product which became stuck to the glass. Chlorine monofluoride, chlorine, and [[tetrafluorosilane]] were formed. The authors called the effect "puzzling", admitting they had expected formation of a volatile fluoride.<ref>{{cite journal |first1=E. H.|last1=Appelman|first2=E. N.|last2=Sloth|first3=M. H. |last3=Studier |year=1966 |title=Observation of astatine compounds by time-of-flight mass spectrometry |journal=Inorganic Chemistry |volume=5 |issue = 5|pages=766–769 |doi=10.1021/ic50039a016}}</ref> Ten years later, the compound was predicted to be non-volatile, out of line with the other halogens but similar to [[radon difluoride]];<ref>{{cite journal |first1=K. S.|last1=Pitzer|year=1975 |title=Fluorides of radon and element 118 |journal=Journal of the Chemical Society, Chemical Communications |volume=5 |issue = 18|pages=760b–761 |doi=10.1039/C3975000760B|url=https://escholarship.org/uc/item/8xz4g1ff}}</ref> by this time, the latter had been shown to be ionic.<ref>{{cite encyclopedia | last1 = Bartlett | first1 = N. | last2 = Sladky | first2 = F. O. | editor1-first = J. C. | editor1-last = Bailar | editor2-first= H. J. | editor2-last = Emeléus | editor3-first = R. | editor3-last = Nyholm | editor4-first = A. F. | editor4-last = Trotman-Dickenson | display-editors=3| encyclopedia =Comprehensive Inorganic Chemistry | title = The chemistry of krypton, xenon and radon | year = 1973 | publisher = Pergamon | volume = 1 | isbn = 0-08-017275-X | pages =213–330}}</ref>}} Thus, although the synthesis of an astatine fluoride is thought to be possible, it may require a liquid halogen fluoride solvent, as has already been used for the characterization of radon fluorides.{{sfn|Zuckerman|Hagen|1989|p=31}}{{sfn|Kugler|Keller|1985|pp=112, 192–193}}

In addition, there exist analogous molecules involving [[pseudohalogen]]s, such as the [[cyanogen halide]]s.

===Tetratomic interhalogens===
[[File:Chlorine-trifluoride-3D-vdW.png|thumb|175x175px|[[Chlorine trifluoride]]]]
*'''[[Chlorine trifluoride]]''' (ClF<sub>3</sub>) is a colourless gas that condenses to a green liquid, and freezes to a white solid. It is made by reacting chlorine with an excess of fluorine at 250&nbsp;°C in a [[nickel]] tube. It reacts more violently than fluorine, often explosively. The molecule is planar and [[T-shaped molecular geometry|T-shaped]]. It is used in the manufacture of [[uranium hexafluoride]].
*'''[[Bromine trifluoride]]''' (BrF<sub>3</sub>) is a yellow-green liquid that conducts electricity — it self-ionises to form [BrF<sub>2</sub>]<sup>+</sup> and [BrF<sub>4</sub>]<sup>−</sup>. It reacts with many metals and metal oxides to form similar ionised entities; with other metals, it forms the metal fluoride plus free bromine and [[oxygen]]; and with water, it forms hydrofluoric acid and hydrobromic acid. It is used in [[organic chemistry]] as a fluorinating agent. It has the same molecular shape as chlorine trifluoride.
*'''[[Iodine trifluoride]]''' (IF<sub>3</sub>) is a yellow solid that decomposes above −28&nbsp;°C. It can be synthesised from the elements, but care must be taken to avoid the formation of [[iodine pentafluoride|IF<sub>5</sub>]]. [[fluorine|F<sub>2</sub>]] attacks [[iodine|I<sub>2</sub>]] to yield IF<sub>3</sub> at −45&nbsp;°C in [[trichlorofluoromethane|CCl<sub>3</sub>F]]. Alternatively, at low temperatures, the fluorination reaction
::I<sub>2</sub> + 3&nbsp;[[xenon difluoride|XeF<sub>2</sub>]] → 2&nbsp;IF<sub>3</sub> + 3&nbsp;[[xenon|Xe]]
:can be used. Not much is known about iodine trifluoride as it is so unstable.
*'''[[Iodine trichloride]]''' (ICl<sub>3</sub>) forms lemon yellow crystals that melt under pressure to a brown liquid. It can be made from the elements at low temperature, or from iodine pentoxide and hydrogen chloride. It reacts with many metal chlorides to form tetrachloroiodides ({{chem|ICl|4|-}}), and hydrolyses in water. The molecule is a planar [[dimer (chemistry)|dimer]] (ICl<sub>3</sub>)<sub>2</sub>, with each iodine atom surrounded by four chlorine atoms.
*'''[[Iodine tribromide]]''' (IBr<sub>3</sub>) is a dark brown liquid.

===Hexatomic interhalogens===
[[File:Bromine-pentafluoride-3D-vdW.png|thumb|179x179px|[[Bromine pentafluoride]]]]
All stable hexatomic and octatomic interhalogens involve a heavier halogen combined with five or seven fluorine atoms. Unlike the other halogens, fluorine atoms have high electronegativity and small size which is able to stabilize them.
*'''[[Chlorine pentafluoride]]''' (ClF<sub>5</sub>) is a colourless gas, made by reacting chlorine trifluoride with fluorine at high temperatures and high pressures. It reacts violently with water and most [[metals]] and [[Nonmetal (chemistry)|nonmetal]]s.
*'''[[Bromine pentafluoride]]''' (BrF<sub>5</sub>) is a colourless fuming liquid, made by reacting bromine trifluoride with fluorine at 200&nbsp;°C. It is physically stable, but decomposes violently on contact with water, organic substances, and most metals and [[Nonmetal (chemistry)|nonmetal]]s.
*'''[[Iodine pentafluoride]]''' (IF<sub>5</sub>) is a colourless liquid, made by reacting iodine pentoxide with fluorine, or iodine with [[silver(II) fluoride]]. It is highly reactive, even slowly with glass. It reacts with water to form [[hydrofluoric acid]] and with fluorine gas to form [[iodine heptafluoride]]. The molecule has the form of a [[Square pyramidal molecular geometry|tetragonal pyramid]].

===Octatomic interhalogens===
[[File:Iodine-heptafluoride-3D-vdW.png|thumb|180x180px|[[Iodine heptafluoride]]]]
*'''[[Iodine heptafluoride]]''' (IF<sub>7</sub>) is a colourless gas and a strong fluorinating agent. It is made by reacting iodine pentafluoride with fluorine gas. The molecule is a [[Pentagonal bipyramidal molecular geometry|pentagonal bipyramid]]. This compound is the only known interhalogen compound where the larger atom is carrying seven of the smaller atoms.
*All attempts to [[chemical synthesis|synthesize]] bromine or chlorine heptafluoride have met with failure; instead, [[bromine pentafluoride]] or [[chlorine pentafluoride]] is produced, along with fluorine gas.

==Properties==
Typically, interhalogen bonds are more reactive than diatomic halogen bonds, because interhalogen bonds are weaker than diatomic halogen bonds, except for F<sub>2</sub>. If interhalogens are exposed to water, they convert to [[halide]] and [[oxyhalide]] ions. With BrF<sub>5</sub>, this reaction can be [[explosion|explosive]]. If interhalogens are exposed to [[silicon dioxide]], or [[metal]] oxides, then silicon or metal respectively bond with one of the types of halogen, leaving free diatomic halogens and diatomic oxygen. Most interhalogens are halogen fluorides, and all but three (IBr, AtBr, and AtI) of the remainder are halogen chlorides. Chlorine and bromine can each bond to five fluorine atoms, and iodine can bond to seven. AX and AX<sub>3</sub> interhalogens can form between two halogens whose [[electronegativity|electronegativities]] are relatively close to one another. When interhalogens are exposed to metals, they react to form metal halides of the constituent halogens. The oxidation power of an interhalogen increases with the number of halogens attached to the central atom of the interhalogen, as well as with the decreasing size of the central atom of the compound. Interhalogens containing fluorine are more likely to be [[volatility (chemistry)|volatile]] than interhalogens containing heavier halogens.<ref name = "Chemistry of Interhalogens"/>

Interhalogens with one or three halogens bonded to a central atom are formed by two elements whose electronegativities are not far apart. Interhalogens with five or seven halogens bonded to a central atom are formed by two elements whose sizes are very different. The number of smaller halogens that can bond to a large central halogen is guided by the ratio of the [[atomic radius]] of the larger halogen over the atomic radius of the smaller halogen. A number of interhalogens, such as IF<sub>7</sub>, react with all metals except for those in the [[platinum group]]. IF<sub>7</sub>, unlike interhalogens in the XY<sub>5</sub> series, does not react with the fluorides of the [[alkali metals]].<ref name = "Chemistry of Interhalogens"/>

ClF<sub>3</sub> is the most reactive of the XY<sub>3</sub> interhalogens. ICl<sub>3</sub> is the least reactive. BrF<sub>3</sub> has the highest thermal stability of the interhalogens with four atoms. ICl<sub>3</sub> has the lowest. Chlorine trifluoride has a boiling point of −12&nbsp;°C. Bromine trifluoride has a boiling point of 127&nbsp;°C and is a liquid at [[room temperature]]. Iodine trichloride melts at 101&nbsp;°C.<ref name = "Chemistry of Interhalogens"/>

Most interhalogens are [[covalent]] gases. Some interhalogens, especially those containing bromine, are [[liquid]]s, and most iodine-containing interhalogens are solids. Most of the interhalogens composed of lighter halogens are fairly colorless, but the interhalogens containing heavier halogens are deeper in color due to their higher [[molecular weight]]. In this respect, the interhalogens are similar to the halogens. The greater the difference between the electronegativities of the two halogens in an interhalogen, the higher the boiling point of the interhalogen. All interhalogens are [[diamagnetic]]. The [[bond length]] of interhalogens in the XY series increases with the size of the constituent halogens. For instance, ClF has a bond length of 1.628&nbsp;[[angstrom|Å]], and IBr has a bond length of 2.47&nbsp;Å.<ref name = "Chemistry of Interhalogens"/>

==Production==
It is possible to produce larger interhalogens, such as ClF<sub>3</sub>, by exposing smaller interhalogens, such as ClF, to pure diatomic halogens, such as F<sub>2</sub>. This method of production is especially useful for generating halogen [[fluoride]]s. At temperatures of 250 to 300&nbsp;°C, this type of production method can also convert larger interhalogens into smaller ones. It is also possible to produce interhalogens by combining two pure halogens at various conditions. This method can generate any interhalogen save for IF<sub>7</sub>.<ref name = "Chemistry of Interhalogens">{{cite book|first = P. B. |last=Saxena|url = https://books.google.com/books?id=nvatWdX1ZWcC|title = Chemistry Of Interhalogen Compounds|year = 2007|publisher=Discovery Publishing House |access-date = February 27, 2013|isbn=978-81-8356-243-0}}</ref>

Smaller interhalogens, such as ClF, can form by direct reaction with pure halogens. For instance, F<sub>2</sub> reacts with Cl<sub>2</sub> at 250&nbsp;°C to form two molecules of ClF. Br<sub>2</sub> reacts with diatomic fluorine in the same way, but at 60&nbsp;°C. I<sub>2</sub> reacts with diatomic fluorine at only 35&nbsp;°C. ClF and BrF can both be produced by the reaction of a larger interhalogen, such as ClF<sub>3</sub> or BrF<sub>3</sub> and a diatomic molecule of the element lower in the [[periodic table]]. Among the hexatomic interhalogens, IF<sub>5</sub> has a higher [[boiling point]] (97&nbsp;°C) than BrF<sub>5</sub> (40.5&nbsp;°C), although both compounds are liquids at [[room temperature]]. The interhalogen IF<sub>7</sub> can be formed by reacting [[palladium iodide]] with fluorine.<ref name = "Chemistry of Interhalogens"/>

==See also==
* [[Interchalcogen]]
* [[Hydrogen halide]]

==Notes==
{{reflist|group=lower-alpha}}

==References==
{{reflist|refs=

<ref name=Murthy>
{{cite book|first=C. Parameshwara |last=Murthy |date=2008 |url=https://books.google.com/books?id=0xl17YU8WzQC&pg=PA217 |title=University Chemistry |volume=1 |page=675 |publisher=New Age International |isbn=978-81-224-0742-6 |quote=The only two interhalogen compounds are {{chem|IFCl|2}} and {{chem|IF|2|Cl}} [no source given].}}
</ref>

<ref name=Sahoo>
{{cite book|first1=Balaram |last1=Sahoo |first2=Nimai Charan |last2=Nayak |first3=Asutosh |last3=Samantaray |first4=Prafulla Kumar |last4=Pujapanda |date=2012 |url=https://books.google.com/books?id=QBkLQzaJXc4C&pg=PA534 |title=Inorganic Chemistry |publisher=PHI Learning |isbn=978-81-203-4308-5 |quote=Only a few ternary interhalogen compounds such as {{chem|IFCl|2}} and {{chem|IF|2|Cl}} have been prepared [no source given].}}
</ref>

<ref name=Meyers>
{{cite book|editor-first=Robert A. |editor-last=Meyers |date=2001 |url=http://www.sciencedirect.com/science/referenceworks/9780122274107 |title=Encyclopedia of Physical Science and Technology: Inorganic Chemistry |edition=3rd |publisher=Academic Press |isbn=978-0-12-227410-7 |quote=A few ternary compounds, such as {{chem|IFCl|2}} and {{chem|IF|2|Cl}}, are also known [no source given].}}
</ref>

<ref name=Ignatiev>{{cite journal | last1 = Ignatyev | first1 = Igor S. | last2 = Schaefer | first2 = Henry F. III | year = 1999 | title = Bromine Halides: The Neutral Molecules {{chem|BrClF|''n''}} (''n'' = 1–5) and Their Anions — Structures, Energetics, and Electron Affinities | journal = Journal of the American Chemical Society | volume = 121 | issue = 29| pages = 6904–6910 | doi = 10.1021/ja990144h }}</ref>
}}

==Bibliography==
* {{Greenwood&Earnshaw}}
* {{cite book | series = Gmelin Handbook of Inorganic and Organometallic Chemistry | title = 'At, Astatine', system no. 8a | edition=8th | year = 1985 | publisher = Springer-Verlag | isbn = 3-540-93516-9 | last1 = Kugler | first1 = H. K. | last2 = Keller | first2 = C. | volume = 8 }}
* {{cite book | title = Inorganic Reactions and Methods, the Formation of Bonds to Halogens | first1 = J. J. | last1 = Zuckerman | first2 = A. P. | last2 = Hagen | year = 1989 | publisher = [[John Wiley & Sons]] | isbn = 978-0-471-18656-4 }}

==External links==
{{commons category|Interhalogen compounds}}
<!-- Added by Ryan Jones. Think we could do with some more description of how the reaction occurs, uses etc. -->
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[[Category:Interhalogen compounds| ]]