Triiodide

{{Short description|Ion}} {{Chembox |ImageFileL1 = Triiodide.svg | ImageClassL1 = skin-invert-image |IUPACName=Triiodide anion |ImageFileL1_Ref = {{chemboximage|correct|??}} |ImageSizeL1 = 121 |ImageNameL1 = Skeletal formula of triiodide |ImageFileR1 = Triiodide-anion-3D-vdW.png | ImageClassR1 = bg-transparent |ImageFileR1_Ref = {{chemboximage|correct|??}} |ImageSizeR1 = 121 |ImageNameR1 = Spacefill model of triiodide |Section1={{Chembox Identifiers |CASNo_Ref = {{cascite|correct|CAS}} |CASNo = 14900-04-0 |UNII_Ref = {{fdacite|correct|FDA}} |UNII = DA1N05631Q |PubChem = 105054 |ChemSpiderID = 94786 |ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} |ChEMBL = 1233501 |ChEMBL_Ref = {{ebicite|correct|EBI}} |SMILES = I[I-]I |StdInChI = 1S/I3/c1-3-2/q-1 |StdInChI_Ref = {{stdinchicite|correct|chemspider}} |StdInChIKey = WRTMQOHKMFDUKX-UHFFFAOYSA-N |StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} }} |Section2={{Chembox Properties |Formula = {{Chem|I|3|-}} |I=3 }} }}

In chemistry, '''triiodide''' usually refers to the triiodide ion, {{chem|I|3|−}}. This anion, one of the polyhalogen ions, is composed of three iodine atoms. It is formed by combining aqueous solutions of iodide salts and iodine. Some salts of the anion have been isolated, including thallium(I) triiodide (Tl⁺[I₃]⁻) and ammonium triiodide ([NH₄]⁺[I₃]⁻). Triiodide is observed to be a red colour in solution.<ref>{{cite web|url=http://www.chemguide.co.uk/inorganic/group7/halogensasoas.html |title= Halogens as oxidising agents - Chemguide}}</ref>

==Nomenclature== Other chemical compounds with "triiodide" in their name contain three separate or independent iodide centers rather than a single unit that consists of three iodine atoms. The prefix "tri" in that situation is a multiplicative prefix for ''iodide''—a 3:1 stoichiometric ratio<ref>{{RedBookRef|page=76}}<!-- IR-5.4.2.1 --></ref>—rather than part of the name of the unified three-iodine structure. Examples include nitrogen triiodide (NI₃) and phosphorus triiodide (PI₃), each of which have a central atom to which each of the three iodine atoms is bonded. To clarify the structural nature of a chemical with three iodides, other naming schemes can be used. Typical options are to use ''tris'' as an alternative prefix, such as the chemical thallium tris(iodide), where there three "iodide" units,<ref>{{RedBookRef|page=37}}</ref> as distinct from the chemical thallium(I) triiodide, where thallium is oxidation state +1 and therefore "triiodide" is the single monovalent unit.

==Preparation== The following exergonic equilibrium gives rise to the triiodide ion: :I₂ + I⁻ ⇌ {{chem|I|3|−}} In this reaction, iodide is viewed as a Lewis base, and the iodine is a Lewis acid. The process is analogous to the reaction of S₈ with sodium sulfide (which forms polysulfides) except that the higher polyiodides have branched structures.<ref>{{cite book|last=Wells|first=A. F.|date=1984|title=Structural Inorganic Chemistry|location=Oxford|publisher=Clarendon Press|isbn=0-19-855370-6}}</ref>

==Structure and bonding== The ion is linear and symmetrical. According to valence shell electron pair repulsion theory, the central iodine atom has three equatorial lone pairs, and the terminal iodine atoms are bonded axially in a linear fashion, due to the three lone pairs bonding to the central iodine-atom. In the molecular orbital model, a common explanation for the hypervalent bonding on the central iodine involves a three-center four-electron bond. The I−I bond is longer than in diatomic iodine, I₂.

In ionic compounds, the bond lengths and angles of triiodide vary depending on the nature of the cation. The triiodide anion is easily polarised and in many salts, one I−I bond becomes shorter than the other. Only in combination with large cations, e.g. a quaternary ammonium such as [N(CH₃)₄]⁺, may the triiodide remain roughly symmetrical.<ref>{{cite book |title = Inorganic Chemistry |edition = 5th |last = Atkins|year = 2010 |publisher = Oxford University Press |isbn = 978-0-19-923617-6 |page = 431|display-authors=etal}}</ref>

In solution phase, the bond lengths and angles of triiodide vary depending on the nature of solvent. The protic solvents tend to localize the triiodide anion's excess charge, resulting in the triiodide anion's asymmetric structure.<ref>{{cite journal |last1=Johnson |first1=Alan E. |last2=Myers |first2=Anne B. |title=Solvent Effects in the Raman Spectra of the Triiodide Ion: Observation of Dynamic Symmetry Breaking and Solvent Degrees of Freedom |journal=The Journal of Physical Chemistry |date=1 January 1996 |volume=100 |issue=19 |pages=7778–7788 |doi=10.1021/jp953052x |url=https://pubs.acs.org/doi/10.1021/jp953052x |language=en |issn=0022-3654|url-access=subscription }}</ref><ref>{{cite journal |last1=Lynden-Bell |first1=R. M. |last2=Kosloff |first2=R. |last3=Ruhman |first3=S. |last4=Danovich |first4=D. |last5=Vala |first5=J. |title=Does solvation cause symmetry breaking in the I3− ion in aqueous solution? |journal=The Journal of Chemical Physics |date=8 December 1998 |volume=109 |issue=22 |pages=9928–9937 |doi=10.1063/1.477659 |url=https://doi.org/10.1063/1.477659 |issn=0021-9606|url-access=subscription }}</ref> For example, the triiodide anion in methanol has an asymmetric bent structure with a charge localized on the longer end of the anion.<ref name="nat">{{cite journal |last1=Heo |first1=Jun |last2=Kim |first2=Jong Goo |last3=Choi |first3=Eun Hyuk |last4=Ki |first4=Hosung |last5=Ahn |first5=Doo-Sik |last6=Kim |first6=Jungmin |last7=Lee |first7=Seonggon |last8=Ihee |first8=Hyotcherl |title=Determining the charge distribution and the direction of bond cleavage with femtosecond anisotropic x-ray liquidography |journal=Nature Communications |date=26 January 2022 |volume=13 |issue=1 |pages=522 |doi=10.1038/s41467-022-28168-0 |pmid=35082327 |pmc=8792042 |bibcode=2022NatCo..13..522H |language=en |issn=2041-1723}}</ref>

The dimensions of the triiodide [I_a−I_b−I_c]⁻ bonds in a few sample compounds are shown below: :{| class="wikitable" |- ! compound ! I_a−I_b (pm) ! I_b−I_c (pm) ! angle (°) |- | TlI₃ | 306.3 | 282.6 | 177.9 |- | RbI₃ | 305.1 | 283.3 | 178.11 |- | CsI₃ | 303.8 | 284.2 | 178.00 |- | NH₄I₃ | 311.4 | 279.7 | 178.55 |- | {{chem2|I3(-)}} (in methanol)<ref name="nat" /> | 309.0 | 296.0 | 152.0 |}

==Properties== The triiodide ion is the simplest polyiodide; several higher polyiodides exist. In solution, it appears yellow in low concentrations, and brown at higher concentrations. The triiodide ion is responsible for the well-known blue-black color which arises when iodine solutions interact with starch. Iodide does not react with starch; nor do solutions of iodine in nonpolar solvents.

Lugol's iodine contains potassium iodide and a stoichiometric amount of elemental iodine, so that significant amounts of triiodide ion exist in this solution. Tincture of iodine, although nominally a solution of elemental iodine in ethanol, also contains significant amounts of triiodide, due to its content of both iodide and water.

===Photochemistry=== Triiodide is a model system in photochemistry. Its reaction mechanism has been studied in gas phase, solution and the solid state. In gas phase, the reaction proceeds in multiple pathways that include iodine molecule, metastable ions and iodine radicals as photoproducts, which are formed by two-body and three-body dissociation.<ref>{{Cite journal |last1=Hoops |first1=Alexandra A. |last2=Gascooke |first2=Jason R. |last3=Faulhaber |first3=Ann Elise |last4=Kautzman |first4=Kathryn E. |last5=Neumark |first5=Daniel M. |date=2004-04-03 |title=Two- and three-body photodissociation of gas phase I3− |url=http://aip.scitation.org/doi/10.1063/1.1691017 |journal=The Journal of Chemical Physics |language=en |volume=120 |issue=17 |pages=7901–7909 |doi=10.1063/1.1691017 |pmid=15267705 |bibcode=2004JChPh.120.7901H |hdl=2440/34955 |s2cid=94632820 |issn=0021-9606|hdl-access=free}}</ref><ref>{{Cite journal |last1=Nakanishi |first1=Ryuzo |last2=Saitou |first2=Naoya |last3=Ohno |first3=Tomoyo |last4=Kowashi |first4=Satomi |last5=Yabushita |first5=Satoshi |last6=Nagata |first6=Takashi |date=2007-05-28 |title=Photodissociation of gas-phase I3−: Comprehensive understanding of nonadiabatic dissociation dynamics |url=http://aip.scitation.org/doi/10.1063/1.2736691 |journal=The Journal of Chemical Physics |language=en |volume=126 |issue=20 |pages=204311 |doi=10.1063/1.2736691 |pmid=17552766 |bibcode=2007JChPh.126t4311N |issn=0021-9606|url-access=subscription }}</ref> In condensed phases, due to confinement, geminate recombination is more common. In solution, only two-body dissociation of triiodide has been observed.<ref>{{Cite journal |last1=Banin |first1=Uri |last2=Ruhman |first2=Sanford |date=1993-03-15 |title=Ultrafast photodissociation of I 3 . Coherent photochemistry in solution |url=http://aip.scitation.org/doi/10.1063/1.465066 |journal=The Journal of Chemical Physics |language=en |volume=98 |issue=6 |pages=4391–4403 |doi=10.1063/1.465066 |bibcode=1993JChPh..98.4391B |issn=0021-9606|url-access=subscription }}</ref><ref>{{Cite journal |last1=Kühne |first1=Thomas |last2=Vöhringer |first2=Peter |date=1996-12-22 |title=Vibrational relaxation and geminate recombination in the femtosecond-photodissociation of triiodide in solution |url=http://aip.scitation.org/doi/10.1063/1.472887 |journal=The Journal of Chemical Physics |language=en |volume=105 |issue=24 |pages=10788–10802 |doi=10.1063/1.472887 |bibcode=1996JChPh.10510788K |issn=0021-9606|url-access=subscription }}</ref> In the protic solvents, an iodine atom at the shorter end of the triiodide anion dissociates upon photoexcitation showing two-body dissociation.<ref name="nat" /> In the solid state, the triiodide photochemistry has been studied in compounds involving quaternary ammonium cations, such as tetrabutylammonium triiodide.<ref>{{Cite journal |date=1981-01-01 |editor-last=Herbstein |editor-first=F. H. |editor2-last=Kaftory |editor2-first=M. |editor3-last=Kapon |editor3-first=M. |editor4-last=Saenger |editor4-first=W. |title=Structures of three crystals containing approximately — linear chains of triiodide ions |url=https://www.degruyter.com/document/doi/10.1524/zkri.1981.154.1-2.11/html |journal=Zeitschrift für Kristallographie - Crystalline Materials |language=en |volume=154 |issue=1–2 |pages=11–30 |doi=10.1524/zkri.1981.154.1-2.11 |bibcode=1981ZK....154...11H |issn=2194-4946|last1=Herbstein |first1=F. H. |last2=Kaftory |first2=M. |last3=Kapon |first3=M. |last4=Saenger |first4=W. |url-access=subscription }}</ref> It has been shown that the solid state photoreaction mechanism depends on the light wavelength, yielding fast recovery in a few picoseconds<ref>{{Cite journal |last1=Poulin |first1=Peter R. |last2=Nelson |first2=Keith A. |date=2006-09-22 |title=Irreversible Organic Crystalline Chemistry Monitored in Real Time |journal=Science |language=en |volume=313 |issue=5794 |pages=1756–1760 |doi=10.1126/science.1127826 |pmid=16946037 |bibcode=2006Sci...313.1756P |s2cid=35002522 |issn=0036-8075|doi-access=free }}</ref> or going through a two-stage process that involves the formation and break-up of a tetraiodide intermediate on longer timescales.<ref>{{Cite journal |last1=Xian |first1=Rui |last2=Corthey |first2=Gastón |last3=Rogers |first3=David M. |last4=Morrison |first4=Carole A. |last5=Prokhorenko |first5=Valentyn I. |last6=Hayes |first6=Stuart A. |last7=Miller |first7=R. J. Dwayne |date=2017-03-27 |title=Coherent ultrafast lattice-directed reaction dynamics of triiodide anion photodissociation |url=http://www.nature.com/articles/nchem.2751 |journal=Nature Chemistry |language=en |volume=9 |issue=6 |pages=516–522 |doi=10.1038/nchem.2751 |pmid=28537597 |bibcode=2017NatCh...9..516X |issn=1755-4330|hdl=20.500.11820/52dbea74-99b4-454b-aac2-56c7be20947b |hdl-access=free |url-access=subscription }}</ref> Besides, triiodide photochemistry is an important contributor in the environmental cycle of iodine.<ref>{{Cite journal |last1=Raso |first1=Angela R. W. |last2=Custard |first2=Kyle D. |last3=May |first3=Nathaniel W. |last4=Tanner |first4=David |last5=Newburn |first5=Matt K. |last6=Walker |first6=Lawrence |last7=Moore |first7=Ronald J. |last8=Huey |first8=L. G. |last9=Alexander |first9=Liz |last10=Shepson |first10=Paul B. |last11=Pratt |first11=Kerri A. |date=2017-09-19 |title=Active molecular iodine photochemistry in the Arctic |journal=Proceedings of the National Academy of Sciences |language=en |volume=114 |issue=38 |pages=10053–10058 |doi=10.1073/pnas.1702803114 |issn=0027-8424 |pmc=5617258 |pmid=28874585|bibcode=2017PNAS..11410053R |doi-access=free }}</ref> Because of the presence of heavy iodine atoms and the well-calibrated chemical pathways, triiodide has also become a computational benchmark system for relativistic quantum chemistry.<ref>{{Cite journal |last1=Gomes |first1=André Severo Pereira |last2=Visscher |first2=Lucas |last3=Bolvin |first3=Hélène |last4=Saue |first4=Trond |last5=Knecht |first5=Stefan |last6=Fleig |first6=Timo |last7=Eliav |first7=Ephraim |date=2010-08-14 |title=The electronic structure of the triiodide ion from relativistic correlated calculations: A comparison of different methodologies |url=https://aip.scitation.org/doi/10.1063/1.3474571 |journal=The Journal of Chemical Physics |volume=133 |issue=6 |pages=064305 |doi=10.1063/1.3474571 |pmid=20707568 |bibcode=2010JChPh.133f4305G |s2cid=8849684 |issn=0021-9606|hdl=20.500.12210/35342 |hdl-access=free }}</ref>

===Electrochemistry=== The redox reactions of triiodide and iodide has been proposed as critical steps in dye-sensitized solar cells.<ref>{{Cite journal |last1=Gibson |first1=Elizabeth A. |last2=Le Pleux |first2=Loïc |last3=Fortage |first3=Jérôme |last4=Pellegrin |first4=Yann |last5=Blart |first5=Errol |last6=Odobel |first6=Fabrice |last7=Hagfeldt |first7=Anders |last8=Boschloo |first8=Gerrit |date=2012-04-17 |title=Role of the Triiodide/Iodide Redox Couple in Dye Regeneration in p-Type Dye-Sensitized Solar Cells |url=https://pubs.acs.org/doi/10.1021/la300215q |journal=Langmuir |language=en |volume=28 |issue=15 |pages=6485–6493 |doi=10.1021/la300215q |pmid=22432412 |issn=0743-7463|url-access=subscription }}</ref> and rechargeable batteries.<ref>{{Cite journal |last1=Ma |first1=Jizhen |last2=Liu |first2=Miaomiao |last3=He |first3=Yulong |last4=Zhang |first4=Jintao |date=2021-02-17 |title=Iodine Redox Chemistry in Rechargeable Batteries |url=https://doi.org/10.1002/anie.202009871 |journal=Angewandte Chemie International Edition |volume=60 |issue=23 |pages=12636–12647 |doi=10.1002/anie.202009871 |pmid=32939916 |s2cid=221769817 |issn=1433-7851|url-access=subscription }}</ref>

==See also== * Polyiodide * Tribromide * Polyhalogen ions * Three-center four-electron bond * Iodine–starch test * Iodometry * Povidone-iodine * Lugol's iodine * Dye-sensitized solar cell * Organic superconductor

==References== <references/>

== External links == *[http://www.chemistry.mtu.edu/~kmsmith/PChem/Experiments/3521/kinetic/kinetic.html Kinetic study of the iodine–persulfate reaction] {{Webarchive|url=https://web.archive.org/web/20110716112950/http://www.chemistry.mtu.edu/~kmsmith/PChem/Experiments/3521/kinetic/kinetic.html |date=2011-07-16 }}

Category:Anions Category:Iodides Category:Hypervalent molecules Category:Homonuclear triatomic molecules Category:Polyhalides Category:Triiodides

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