polyiodide

Polyiodide refers to chemical species containing multiple iodine atoms connected to form polyatomic ions or neutral/polyiodide-containing solids. The most familiar example is the triiodide ion, I3−, but higher polyiodide ions such as I5−, I7−, and beyond also occur. Polyiodides are common in iodine-rich solutions and in solid salts where iodide is present in excess of iodine.

In solution, polyiodides form by successive association of iodine with iodide:

I2 + I− ⇌ I3−

I3− + I2 ⇌ I5−

I5− + I2 ⇌ I7−

These equilibria shift with concentration, solvent, and temperature. The resulting species are stabilized by the surrounding counterions, which can be inorganic cations (for example, alkali metals) or organic cations in crystalline salts. Structurally, polyiodides feature I−I contacts that can arrange into linear chains, cyclic motifs, or more extended networks, with charge delocalized along the iodine framework. Bond lengths and geometries differ from those in diatomic I2, reflecting the polynuclear character of the anions.

Properties and applications of polyiodides depend on size and environment. They are typically colored and highly polarizable, and they influence the optical and electrical characteristics of the materials in which they reside. In solids, polyiodide chains are stabilized by lattice interactions and can contribute to ionic conductivity in certain electrolytes. Polyiodides also appear in charge-transfer complexes and iodine-rich coordination compounds, where they play a role in the material’s electronic structure.