Polyiodides

Polyiodides are chemical species that contain chains or clusters of iodine atoms beyond the simple diatomic I2, typically in the form of polyiodide anions I_n− with n greater than 2. The most common examples are I3−, I5−, and I7−. They arise when iodine dissolves in iodide-containing solutions or when iodine is incorporated into solid salts, often stabilised by surrounding cations in the crystal lattice.

In solution, polyiodides are formed through sequential reactions of iodine with iodide: I2 + I− ⇌ I3−, followed by further association with additional I2 to give I5−, I7−, and so on (I3− + I2 ⇌ I5−; I5− + I2 ⇌ I7−). In solids, polyiodide ions can assemble into extended linear or zigzag chains and networks within a crystal, with the lattice cations stabilising the charge and arrangement of the chains. The iodine–iodine bonds in these species are typically longer and weaker than in I2, reflecting their polyionic character and delocalized electron distribution.

Properties and applications vary with composition. Polyiodides are highly polarizable and can endow materials with interesting optical and electronic properties; some polyiodide salts exhibit electrical conductivity along the polyiodide chains, making them of interest in solid-state chemistry and materials science. They also serve as reagents in organic synthesis and iodometric titrations, where iodine release or transfer is involved.

Stability is sensitive to light, temperature, and redox conditions; polyiodides can decompose to I2 and iodide under suitable stimuli. They are typically handled under controlled conditions to maintain sample integrity.