| |||
| Names | |||
|---|---|---|---|
| IUPAC name Triiodide anion | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChEMBL | |||
| ChemSpider |
| ||
| UNII | |||
| |||
| |||
| Properties | |||
| I− 3 | |||
| Molar mass | 380.71341 g·mol−1 | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
In chemistry,triiodide usually refers to the triiodide ion,I−
3. This anion, one of thepolyhalogen ions, is composed of threeiodine atoms. It is formed by combiningaqueous solutions ofiodide salts andiodine. Some salts of the anion have been isolated, includingthallium(I) triiodide (Tl+[I3]−) andammonium triiodide ([NH4]+[I3]−). Triiodide is observed to be a red colour in solution.[1]
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 foriodide—a 3:1stoichiometric ratio[2]—rather than part of the name of the unified three-iodine structure. Examples inclludenitrogen triiodide (NI3) andphosphorus 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 usetris as an alternative prefix, such as the chemical thallium tris(iodide), where there three "iodide" units,[3] as distinct from the chemical thallium(I) triiodide, where thallium is oxidation state +1 and therefore "triiodide" is the single monovalent unit.
The followingexergonic equilibrium gives rise to the triiodideion:
In this reaction, iodide is viewed as aLewis base, and theiodine is aLewis acid. The process is analogous to the reaction ofS8 withsodium sulfide (which formspolysulfides) except that the higher polyiodides have branched structures.[4]
The ion is linear and symmetrical. According tovalence 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 themolecular orbital model, a common explanation for thehypervalent bonding on the central iodine involves athree-center four-electron bond. The I−I bond is longer than in diatomic iodine,I2.
Inionic compounds, the bond lengths and angles of triiodide vary depending on the nature of thecation. 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. aquaternary ammonium such as [N(CH3)4]+, may the triiodide remain roughly symmetrical.[5]
Insolution phase, the bond lengths and angles of triiodide vary depending on the nature ofsolvent. The protic solvents tend to localize the triiodide anion's excess charge, resulting in the triiodide anion's asymmetric structure.[6][7] For example, the triiodide anion in methanol has an asymmetric bent structure with a charge localized on the longer end of the anion.[8]
The dimensions of the triiodide [Ia−Ib−Ic]− bonds in a few sample compounds are shown below:
| compound | Ia−Ib (pm) | Ib−Ic (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 |
| I−3 (in methanol)[8] | 309.0 | 296.0 | 152.0 |
The triiodide ion is the simplestpolyiodide; 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 withstarch. Iodide does not react with starch; nor do solutions of iodine in nonpolarsolvents.
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.
Triiodide is a model system inphotochemistry. Its reaction mechanism has been studied ingas phase,solution and the solid state. In gas phase, the reaction proceeds in multiplepathways that includeiodine molecule,metastable ions and iodine radicals as photoproducts, which are formed by two-body and three-bodydissociation.[9][10] In condensed phases, due to confinement,geminate recombination is more common. In solution, only two-body dissociation of triiodide has been observed.[11][12] In the protic solvents, an iodine atom at the shorter end of the triiodide anion dissociates uponphotoexcitation showing two-body dissociation.[8] In the solid state, the triiodide photochemistry has been studied in compounds involvingquaternary ammonium cations, such astetrabutylammonium triiodide.[13] It has been shown that the solid state photoreaction mechanism depends on the light wavelength, yielding fast recovery in a fewpicoseconds[14] or going through a two-stage process that involves the formation and break-up of a tetraiodideintermediate on longer timescales.[15] Besides, triiodide photochemistry is an important contributor in the environmental cycle ofiodine.[16] Because of the presence of heavyiodine atoms and the well-calibrated chemical pathways, triiodide has also become a computational benchmark system forrelativistic quantum chemistry.[17]
Theredox reactions of triiodide andiodide has been proposed as critical steps indye-sensitized solar cells.[18] andrechargeable batteries.[19]
