Polyiodide

The polyiodides are a class of polyhalogen anions composed entirely of iodine atoms. The most common member is the triiodide ion,. Other known larger polyiodides include [I4]2−, [I5]−, [I6]2−, [I7]−, [I8]2−, [I9]−, [I10]2−, [I10]4−, [I11]3−, [I12]2−, [I13]3−, [I14]4-, [I16]2−, [I22]4−, [I26]3−, [I26]4−, [I28]4− and [I29]3−. All these can be considered as formed from the interaction of the I–, I2, and building blocks.

Preparation
The polyiodides can be made by addition of stoichiometric amounts of I2 to solutions containing I− and, with the presence of large countercations to stabilize them. For example, KI3·H2O can be crystallized from a saturated solution of KI when a stoichiometric amount of I2 is added and cooled.

Structure
Polyiodides adopt diverse structures. Most can be considered as associations of I2, I−, and 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.

Reactivity
Polyiodide compounds are generally sensitive to light.

Triiodide,, 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.