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 [ W.W. Porterfield, "Inorganic Chemistry", Addison Wesley, Reading, Mass, USA (1984), pp 223-224 ] , and the bond lengths of approximately 2.8 Å (0.28 nm) for the terminal bonds and 3.17 Å (0.317 nm) for the centre bonds [ P.W. Atkins, D.W. Shriver, "Inorganic Chemistry", 3rd Edn., OUP, Oxford, UK (2001)] 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. [Per H. Svensson, Lars Kloo, "Synthesis, Structure, and Bonding in Polyiodide and Metal Iodide-Iodine Systems", Chem. Rev., (2003), 103(5), pp 1649-1684.]

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