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| The '''polyiodides''' are a class of [[polyhalogen anions]] composed of entirely iodine atoms. The most common and simplest member is the [[triiodide]] ion, I<sub>3</sub><sup>−</sup>. Other known, larger polyiodides include [I<sub>4</sub>]<sup>2-</sup>, [I<sub>5</sub>]<sup>-</sup>, [I<sub>7</sub>]<sup>-</sup>, [I<sub>8</sub>]<sup>2-</sup>, [I<sub>9</sub>]<sup>-</sup>, [I<sub>10</sub>]<sup>2-</sup>, [I<sub>10</sub>]<sup>4-</sup>, [I<sub>11</sub>]<sup>-</sup>, [I<sub>12</sub>]<sup>2-</sup>, [I<sub>13</sub>]<sup>3-</sup>, [I<sub>16</sub>]<sup>2-</sup>, [I<sub>22</sub>]<sup>4-</sup>, [I<sub>26</sub>]<sup>3-</sup>, [I<sub>26</sub>]<sup>4-</sup>, [I<sub>28</sub>]<sup>4-</sup> and [I<sub>29</sub>]<sup>3-</sup>.<ref name="InorgChem">{{cite book
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| | title = Inorganic Chemistry, 3rd Edition
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| | chapter = Chapter 17: The group 17 elements
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| | author1 = Catherine E. Housecroft
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| | author2 = Alan G. Sharpe
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| | publisher = Pearson
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| | year = 2008
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| | isbn = 978-0-13-175553-6
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| | page = 547
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| }}</ref>
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| ==Preparation==
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| The polyiodides can be made by addition of stoichiometric amounts of I<sub>2</sub> to solutions containing I<sup>−</sup> and I<sub>3</sub><sup>−</sup>, with the presence of large [[cation|counter-cations]] to stabilize them. For example, KI<sub>3</sub>•H<sub>2</sub>O can be crystallized from a saturated solution of [[potassium iodide|KI]] when a stoichiometric amount of I<sub>2</sub> is added and cooled.<ref>"Potassium triiodide" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 294.</ref>
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| ==Structure==
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| [[File:Solid state structure of the (I11)3- ion in (((16)aneS4)PdIPd((16)aneS4))(I11).png|right|thumb|300px|The 14-membered ring array of iodine atoms in [([16]aneS<sub>4</sub>)PdIPd([16]aneS<sub>4</sub>)][I<sub>11</sub>]]]
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| [[File:Solid state structure of (I26)4- in (DMFc)4(I26).png|right|thumb|300px|The primitive cubic lattice of iodide ions bridge by I<sub>2</sub> molecules, present in [DMFc]<sub>4</sub>[I<sub>26</sub>]]]
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| Polyiodides are characterized by their highly complex and variable structures, and can be considered as associations of I<sub>2</sub>, I<sup>−</sup>, and I<sub>3</sub><sup>−</sup> units. Discrete polyiodides are usually linear, reflecting the origin of the ion. The more complex two- or three-dimensional network structures of chains and cages are formed as the ions interact with each other, with their shapes depend on their associated cations quite strongly. The table below lists the polyiodide salts which have been structurally characterized, along with their counter-cation.<ref name="Encyclo">{{cite book
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| | title = Encyclopedia of Inorganic Chemistry, 2nd edition
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| | chapter = Chlorine, Bromine, Iodine, & Astatine: Inorganic Chemistry
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| | author1 = R. Bruce King
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| | author2 =
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| | publisher = Wiley
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| | year = 2005
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| | isbn = 9780470862100
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| | page = 747
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| }}</ref>
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| {| class="wikitable"
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| ! colspan=3|Structure of higher polyiodides
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| |-
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| ! Anion !! Counter-cation !! Structural description
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| | [I<sub>3</sub>]<sup>-</sup> || [[Cesium|Cs]]<sup>+</sup> || linear
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| | [I<sub>4</sub>]<sup>2-</sup> || [Cu(NH<sub>3</sub>)<sub>4</sub>]<sup>2+</sup> || symmetric linear array of iodine atoms<ref>Per H. Svensson, Lars Kloo, "Synthesis, Structure, and Bonding in Polyiodide and Metal Iodide-Iodine Systems", Chem. Rev., (2003), 103(5), pp 1649-1684.</ref>
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| | [I<sub>5</sub>]<sup>-</sup> || [EtMe<sub>3</sub>N]<sup>+</sup> || V-shaped with polymeric layers
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| | || [EtMePh<sub>2</sub>N]<sup>+</sup> || V-shaped with isolated [I<sub>5</sub>]<sup>-</sup> ions
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| | [I<sub>7</sub>]<sup>-</sup> || [Ag([[1,4,7,10,13,16-hexathiacyclo-octadecane|18aneS<sub>6</sub>]])]<sup>+</sup> || an anionic network derived from a primitive rhombohedral lattice of iodide ions bridged by I<sub>2</sub> molecules
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| | [I<sub>8</sub>]<sup>2-</sup> || [Ni([[phenanthroline|phen]])<sub>3</sub>]<sup>2+</sup> || regular anionic shapes, can be described as [I<sub>3</sub><sup>-</sup>•I<sub>2</sub>•I<sub>8</sub><sup>-</sup>] or [I<sub>3</sub><sup>-</sup>•I<sub>5</sub><sup>-</sup>]
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| | [I<sub>9</sub>]<sup>-</sup> || [Me<sub>2</sub>[[isopropyl|<sup>''i''</sup>Pr]][[phenyl|Ph]]N]<sup>+</sup> || 14-membered ring tied by two I<sub>2</sub> bridges to give 10-membered rings
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| | || [Me<sub>4</sub>N]<sup>+</sup> || non-octahedral, but a twisted "h"-like arrangement of I<sub>3</sub><sup>-</sup> and I<sub>2</sub> units
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| | [I<sub>10</sub>]<sup>2-</sup> || [Cd([[12-crown-4]])<sub>2</sub>]<sup>2+</sup> || twisted ring configuration with two [I<sub>3</sub>]<sup>-</sup> units linked by two I<sub>2</sub> molecules
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| | [I<sub>11</sub>]<sup>3-</sup> || [([[1,5,9,13-tetrathiacyclohexadecane|16aneS<sub>4</sub>]])[[palladium|Pd]]IPd(16aneS<sub>4</sub>)]<sup>3+</sup> || 14-membered ring (9.66 × 12.64 Å) around the complex cation, with the rings interlink further to give an infinite 2D sheet
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| | [I<sub>12</sub>]<sup>2-</sup> || [Ag<sub>2</sub>([[1,4,7,10,13-pentathiacyclopentadecane|15aneS<sub>5</sub>]])<sub>2</sub>]<sup>2+</sup> || extended 3D spiral superstructure supported by Ag–I bonds and weak I•••S interactions
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| | || [Cu([[4,5-Diazafluoren-9-one|Dafone]])<sub>3</sub>]<sup>2+</sup> || planar configuration
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| | [I<sub>13</sub>]<sup>3-</sup> || [Me<sub>2</sub>Ph<sub>2</sub>N]<sup>+</sup> || consists of zigzag chains of I<sup>-</sup> and I<sub>2</sub>
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| | [I<sub>16</sub>]<sup>2-</sup> || [Me<sub>2</sub>Ph<sub>2</sub>N]<sup>+</sup> || centrosymmetric arrangement of [I<sub>7</sub><sup>-</sup>•I<sub>2</sub>•I<sub>7</sub><sup>-</sup>]
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| | || [<sup>''i''</sup>PrMe<sub>2</sub>PhN]<sup>+</sup> || the anion forms 14-membered rings catenated by I<sub>2</sub> molecules, which further link into layers with 10- and 14-membered rings
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| | [I<sub>22</sub>]<sup>4-</sup> || [MePh<sub>3</sub>P]<sup>+</sup> || two L-shaped [I<sub>5</sub>]<sup>-</sup> units linked by an I<sub>2</sub> molecule and completed by two end-on [I<sub>5</sub>]<sup>-</sup> groups
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| | [I<sub>26</sub>]<sup>3-</sup> || [Me<sub>3</sub>S]<sup>+</sup> || consists of [I<sub>5</sub>]<sup>-</sup> and [I<sub>7</sub>]<sup>-</sup> ions with intercalated I<sub>2</sub> molecules
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| | [I<sub>26</sub>]<sup>4-</sup> || [[Decamethylferrocene|DMFc]]<sup>+</sup> || an anionic network derived from a primitive cubic lattice built from I<sup>-</sup> ions, with I<sub>2</sub> bridges on all edges and systematically removing <math>\tfrac{1}{12}</math> of the I<sub>2</sub> molecules
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| | [I<sub>29</sub>]<sup>3-</sup> || [[ferrocenium|Cp<sub>2</sub>Fe]] || an anionic 3D network with a cage-like structure of [{(I<sub>5</sub><sup>-</sup>)<sub>0.5</sub>•I<sub>2</sub>}•{(I<sub>12</sub><sup>2-</sup>)<sub>0.5</sub>•I<sub>2</sub>}•I<sub>2</sub>], with [Cp<sub>2</sub>Fe]<sup>+</sup> ions interacting with the anion in the cavities
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| |}
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| [[File:Structures of some polyiodide ions.png|center|thumb|650px|Structures of some polyiodide ions.]]
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| ==References==
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| <references/>
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| [[Category:Anions]]
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| [[Category:Iodides]]
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