Triiodide 2505643 211600015 2008-05-11T04:57:03Z Thijs!bot 1392310 robot Adding: [[ja:三ヨウ化物]] [[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>&minus;</nowiki>I<sub>b</sub><nowiki>&minus;</nowiki>I<sub>c</sub> bonds in a few sample compounds are shown below: {| class="wikitable" |- ! compound ! I<sub>a</sub><nowiki>&minus;</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]]