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.
• I₄^2- is notionally composed of either two I₂ species, or is almost equally bonded, depending on environment of the ion. Both models are linear.