Triiodide
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2008-05-11T04:57:03Z
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[[Image:Triiodide-anion-3D-vdW.png|thumb|right|200px|[[Space-filling model]] of I<sub>3</sub><sup>−</sup>]]
In chemistry, '''triiodide''' (sometimes written tri-iodide) can have several meanings. Triiodide primarily refers to the triiodide ion, I<sub>3</sub><sup>−</sup>, a [[polyatomic ion|polyatomic]] [[anion]] composed of three [[iodine]] atoms. For some chemical compounds, triiodide indicates a [[salt (chemistry)|salt]] of the named [[cation]] with the triiodide anion. Examples include sodium triiodide, [[thallium triiodide]], and [[ammonium triiodide]], [NH<sub>4</sub>][I<sub>3</sub>]. Each of these compounds has a (mono)[[iodide]] counterpart. For other chemical compounds, triiodide indicates that each molecule contains three iodine atoms that are not bonded to each other, not forming the triiodide ion. Examples include [[nitrogen triiodide]] (NI<sub>3</sub>), [[phosphorus triiodide]], [[antimony triiodide]], and [[gallium(III) iodide|gallium triiodide]] (Ga<sub>2</sub>I<sub>6</sub>). Some anions have the theoretical possibility to form either kind of triiodide. [[Thallium triiodide]] is described as thallium(I) triiodide; thallium(III) iodide is unknown.
==Triiodide ion==
The triiodide ion is the simplest [[polyiodide]]; several higher polyiodides exist. In solution, it appears yellow in low concentration, and brown at higher concentration. The triiodide ion is responsible for the well-known blue-black color which arises when iodine solutions react with [[Starch#Tests|starch]]. Iodide does not react with starch; nor do solutions of iodine in nonpolar [[solvent]]s. [[Lugol's iodine]] contains potassium iodide as well, so that significant amounts of triiodide ion can exist in solution. [[Tincture of iodine]] contains significant amounts of triiodide.
==Formation and structure==
[[Image:Triiodide-2D-dimensions.png|thumb|right|200px|Approximate dimensions of the triiodide anion. These dimensions vary slightly depending on the compound in which I<sub>3</sub><sup>−</sup> is found.]]
The following endergonic equilibrium gives rise to the triiodide [[ion]]:
:I<sub>2</sub> + I<sup>−</sup> {{unicode|⇌}} I<sub>3</sub><sup>−</sup>
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 [[sulfur|S<sub>8</sub>]] with [[sodium sulfide]], except that the higher polyiodides have branched structures.<ref>Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.</ref>
The ion is linear, as predicted by [[VSEPR theory]]. A common explanation for the [[hypervalent]] bonding on the central atom involves a [[three-center four-electron bond]]. The bond lengths and angles of triiodide vary, depending on the compound. The dimensions of the tri-iodide I<sub>a</sub><nowiki>−</nowiki>I<sub>b</sub><nowiki>−</nowiki>I<sub>c</sub> bonds in a few sample compounds are shown below:
{| class="wikitable"
|-
! compound
! I<sub>a</sub><nowiki>−</nowiki>I<sub>b</sub> (pm)
! I<sub>b</sub> - I<sub>c</sub> (pm)
! angle (°)
|-
| TlI<sub>3</sub>
| 306.3
| 282.6
|177.9
|-
| RbI<sub>3</sub>
| 305.1
| 283.3
| 178.11
|-
| CsI<sub>3</sub>
| 303.8
| 284.2
| 178.00
|-
| NH<sub>4</sub>I<sub>3</sub>
| 311.4
| 279.7
| 178.55
|}
==References==
<references/>
== External links ==
*[http://www.chemistry.mtu.edu/~kmsmith/PChem/Experiments/3521/kinetic/kinetic.html Kinetic Study of the iodine-persulfate reaction]
*[http://www.solvaychemicals.us/pdf/Hydrogen_Peroxide/XX-122.pdf Determination of Hydrogen Peroxide Concentration]
{{inorganic-compound-stub}}
[[Category:Iodine compounds]]
[[Category:Anions]]
[[ja:三ヨウ化物]]