# Polyiodide

The polyiodides are a class of polyhalogen anions composed entirely of iodine atoms. The most common member is the triiodide ion, I$^−$ $_3$. Other known larger polyiodides include [I$_4$]$^{2−}$, [I$_5$]$^−$, [I$_6$]$^{2−}$, [I$_7$]$^−$, [I$_8$]$^{2−}$, [I$_9$]$^−$, [I$_{10}$]$^{2−}$, [I$_{10}$]$^{4−}$, [I$_{11}$]$^{3−}$, [I$_{12}$]$^{2−}$, [I$_{13}$]$^{3−}$, [I$_{14}$]$^{4-}$, [I$_{16}$]$^{2−}$, [I$_{22}$]$^{4−}$, [I$_{26}$]$^{3−}$, [I$_{26}$]$^{4−}$, [I$_{28}$]$^{4−}$ and [I$_{29}$]$^{3−}$. All these can be considered as formed from the interaction of the I$^–$, I$_2$, and I$^−$ $_3$ building blocks.

## Preparation

The polyiodides can be made by addition of stoichiometric amounts of I$_2$ to solutions containing I$^−$ and I$^−$ $_3$, with the presence of large countercations to stabilize them. For example, KI$_3$·H$_2$O can be crystallized from a saturated solution of KI when a stoichiometric amount of I$_2$ is added and cooled.

## Structure

Polyiodides adopt diverse structures. Most can be considered as associations of I$_2$, I$^−$, and I$^−$ $_3$ units. Discrete polyiodides are usually linear. 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 depending on their associated cations quite strongly, a phenomenon named dimensional caging. The table below lists the polyiodide salts which have been structurally characterized, along with their counter-cation.
Structure of higher polyiodides :
| Anion | Counter-cation | Structural description |
| [I$_2$]$^−$ | Na(C$_3$H$_6$O)$^+$ $_3$ | linear |
| [I$_3$]$^−$ | Cs$^+$, (C$_4$H$_9$)$_4$N$^+$ | linear |
| [I$_4$]$^{2−}$ | [Cu(NH$_3$)$_4$]$^{2+}$ | symmetric linear array of iodine atoms |
| [I$_5$]$^−$ | [EtMe$_3$N]$^+$ | V-shaped with polymeric layers |
| [I$_5$]$^−$ | [EtMePh$_2$N]$^+$ | V-shaped with isolated [I$_5$]$^−$ ions |
| [I$_6$]$^{2−}$ | [NH$_3$(CH$_2$)$_8$NH$_3$]$^{2+}$ | almost linear |
| [I$_7$]$^−$ | [Ag(18aneS$_6$)]$^+$ | an anionic network derived from a primitive rhombohedral lattice of iodide ions bridged by I$_2$ molecules |
| [I$_8$]$^{2−}$ | [Ni(phen)$_3$]$^{2+}$ | regular anionic shapes, can be described as [I$^−$ $_3$·I$_2$·I$^−$ $_8$] or [I$^−$ $_3$·I$^−$ $_5$] |
| [I$_9$]$^−$ | [Me$_2$$^i$PrPhN]$^+$ | 14-membered ring tied by two I$_2$ bridges to give 10-membered rings |
| [I$_9$]$^−$ | [Me$_4$N]$^+$ | non-octahedral, but a twisted "h"-like arrangement of I$^−$ $_3$ and I$_2$ units |
| [I$_{10}$]$^{2−}$ | [Cd(12-crown-4)$_2$]$^{2+;}$ Theophyllinium | twisted ring configuration with two I$^−$ $_3$ units linked by two I$_2$ molecules |
| [I$_{11}$]$^{3−}$ | [(16aneS$_4$)PdIPd(16aneS$_4$)]$^{3+}$ | 14-membered ring (9.66 × 12.64 Å) around the complex cation, with the rings interlink further to give an infinite 2D sheet |
| [I$_{12}$]$^{2−}$ | [Ag$_2$(15aneS$_5$)$_2$]$^{2+}$ | extended 3D spiral superstructure supported by Ag–I bonds and weak I···S interactions |
| [I$_{12}$]$^{2−}$ | [Cu(Dafone)$_3$]$^{2+}$ | planar configuration |
| [I$_{13}$]$^{3−}$ | [Me$_2$Ph$_2$N]$^+$ | consists of zigzag chains of I$^−$ and I$_2$ |
| [I$_{14}$]$^{4−}$ | 4,4′-bipyridinium | double hook (I$^−$ $_3$·I$_2$·I$^−$·I$_2$·I$^−$·I$_2$·I$^−$ $_3$) |
| [I$_{16}$]$^{2−}$ | [Me$_2$Ph$_2$N]$^+$ | centrosymmetric arrangement of [I$^−$ $_7$·I$_2$·I$^−$ $_7$] |
| [I$_{16}$]$^{2−}$ | [$^i$PrMe$_2$PhN]$^+$ | the anion forms 14-membered rings catenated by I$_2$ molecules, which further link into layers with 10- and 14-membered rings |
| [I$_{22}$]$^{4−}$ | [MePh$_3$P]$^+$ | two L-shaped [I$_5$]$^−$ units linked by an I$_2$ molecule and completed by two end-on [I$_5$]$^−$ groups |
| [I$_{26}$]$^{3−}$ | [Me$_3$S]$^+$ | consists of [I$_5$]$^−$ and [I$_7$]$^−$ ions with intercalated I$_2$ molecules |
| [I$_{26}$]$^{4−}$ | Cp*$_2$Fe$^+$ | an anionic network derived from a primitive cubic lattice built from I$^−$ ions, with I$_2$ bridges on all edges and systematically removing 1⁄12 of the I$_2$ molecules |
| [I$_{29}$]$^{3−}$ | Cp$_2$Fe$^+$ | an anionic 3D network with a cage-like structure of [{(I$^−$ $_5$)$_{1⁄2}$·I$_2$}·{(I$^{2−}$ $_{12}$)$_{1⁄2}$·I$_2$}·I$_2$], with [Cp$_2$Fe]$^+$ ions interacting with the anion in the cavities |
| [I$_∞$]$^{δ−}$ | Pyrroloperylene$^{+•}$ | Infinite polyiodide homopolymer. |

## Reactivity

Polyiodide compounds are generally sensitive to light.
Triiodide, I$^−$ $_3$, undergoes unimolecular photodissociation. Polyiodide has been used to improve the scalability in the synthesis of halide perovskite photovoltaic materials.

## Conductivity

Solid state compounds containing linear-chain polyiodide ions exhibit enhanced conductivity than their simple iodide counterparts. The conductivity can be drastically modified by external pressure, which changes the interatomic distances between iodine moieties and the charge distribution.