Iodine
Polyiodine compounds

When iodine dissolves in strong acids, such as fuming sulfuric acid, a bright blue paramagnetic solution including I cations is formed. A solid salt of the diiodine cation may be obtained by oxidising iodine with antimony pentafluoride:

The salt I Sb F is dark blue, and the blue tantalum analogue I Ta F is also known. Whereas the I–I bond length in I is 267 pm, that in I is only 256 pm as the missing electron in the latter has been removed from an antibonding orbital, making the bond stronger and hence shorter. In fluorosulfuric acid solution, deep-blue I reversibly dimerises below −60 °C, forming red rectangular diamagnetic I. Other polyiodine cations are not as well-characterised, including bent dark-brown or black I and centrosymmetric C green or black I, known in the AsF and AlCl salts among others.

The only important polyiodide anion in aqueous solution is linear triiodide, I. Its formation explains why the solubility of iodine in water may be increased by the addition of potassium iodide solution:

Many other polyiodides may be found when solutions containing iodine and iodide crystallise, such as I, I, I, and I, whose salts with large, weakly polarising cations such as Cs may be isolated.
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