Editing Iodine (section)

===Polyiodine compounds===
When iodine dissolves in strong acids, such as fuming sulfuric acid, a bright blue [[Paramagnetism|paramagnetic]] solution including {{chem|I|2|+}} cations is formed. A solid salt of the diiodine cation may be obtained by oxidising iodine with [[antimony pentafluoride]]:<ref name="King" />

{{block indent|2 I<sub>2</sub> + 5 SbF<sub>5</sub> {{overunderset|{{big|⟶}}|SO<sub>2</sub>|20&nbsp;°C}} 2 I<sub>2</sub>Sb<sub>2</sub>F<sub>11</sub> + SbF<sub>3</sub>}}

The salt I<sub>2</sub>Sb<sub>2</sub>F<sub>11</sub> is dark blue, and the blue [[tantalum]] analogue I<sub>2</sub>Ta<sub>2</sub>F<sub>11</sub> is also known. Whereas the I–I bond length in I<sub>2</sub> is 267&nbsp;pm, that in {{chem|I|2|+}} is only 256&nbsp;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 {{chem|I|2|+}} reversibly dimerises below −60&nbsp;°C, forming red rectangular diamagnetic {{chem|I|4|2+}}. Other polyiodine cations are not as well-characterised, including bent dark-brown or black {{chem|I|3|+}} and centrosymmetric ''C''<sub>2''h''</sub> green or black {{chem|I|5|+}}, known in the {{chem|AsF|6|-}} and {{chem|AlCl|4|-}} salts among others.<ref name="King" /><ref name="Greenwood842">Greenwood and Earnshaw, pp. 842–4</ref>

The only important polyiodide anion in aqueous solution is linear [[triiodide]], {{chem|I|3|-}}. Its formation explains why the solubility of iodine in water may be increased by the addition of potassium iodide solution:<ref name="King" />

{{block indent|I<sub>2</sub> + I<sup>−</sup> {{eqm}} {{chem|I|3|-}} (''K''<sub>eq</sub> {{=}} c. 700 at 20&nbsp;°C)}}

Many other polyiodides may be found when solutions containing iodine and iodide crystallise, such as {{chem|I|5|-}}, {{chem|I|9|-}}, {{chem|I|4|2-}}, and {{chem|I|8|2-}}, whose salts with large, weakly polarising cations such as [[caesium|Cs<sup>+</sup>]] may be isolated.<ref name="King" /><ref name="Greenwood835">Greenwood and Earnshaw, pp. 835–9</ref>