Polyiodide

The polyiodides are a class of polyatomic halide anions composed entirely of iodine atoms.

The triiodide ion, I3-, is the simplest polyiodide. Larger polyiodides are known, with single or multiple negative charges.

The basic 'building blocks' of polyiodides can be considered as I2, I-, and I3-. The more complex polyiodides can be made by addition of I2 to solutions containing I- and I3-, with the condition of presence of large cations to stabilise them. With cations much smaller than NMe4+, it is either not possible to precipitate the salt (for example with Na+ or K+ cations), or the crystal structure will show asymmetric iodide anions, like in caesium triiodide, CsI3.

The shapes of the polyiodides depend on their associated cations quite strongly, however some of the more simple ions have roughly constant shapes:
 * I5- normally adopts a V-shaped structure and can be regarded as two I2 molecules attached to an I- ion. Bonding in this species suggests p-orbital interactions, and the bond lengths of approximately 2.8Å for the terminal bonds and 3.17Å for the centre bonds supports the I2 and I-bonding model.
 * I42- is notionally composed of either two I2 species, or is almost equally bonded, depending on environment of the ion. Both models are linear.