Editing Iodine (section)

===Other binary iodine compounds===
With the exception of the [[Noble gas|noble gases]], nearly all elements on the periodic table up to einsteinium ([[Einsteinium(III) iodide|EsI<sub>3</sub>]] is known) are known to form binary compounds with iodine. Until 1990, [[nitrogen triiodide]]<ref>The ammonia adduct NI<sub>3</sub>•NH<sub>3</sub> is more stable and can be isolated at room temperature as a notoriously shock-sensitive black solid.</ref> was only known as an ammonia adduct. Ammonia-free NI<sub>3</sub> was found to be isolable at –196 °C but spontaneously decomposes at 0 °C.<ref>{{cite journal |last1=Tornieporth-Oetting |first1=Inis |last2=Klapötke |first2=Thomas |date=June 1990 |title=Nitrogen Triiodide |url=https://onlinelibrary.wiley.com/doi/10.1002/anie.199006771 |journal=Angewandte Chemie |edition=international |language=en |volume=29 |issue=6 |pages=677–679 |doi=10.1002/anie.199006771 |issn=0570-0833 |access-date=5 March 2023 |archive-date=5 March 2023 |archive-url=https://web.archive.org/web/20230305194218/https://onlinelibrary.wiley.com/doi/10.1002/anie.199006771 |url-status=live }}</ref> For thermodynamic reasons related to electronegativity of the elements, neutral sulfur and selenium iodides that are stable at room temperature are also nonexistent, although S<sub>2</sub>I<sub>2</sub> and SI<sub>2</sub> are stable up to 183 and 9 K, respectively.  As of 2022, no neutral binary selenium iodide has been unambiguously identified (at any temperature).<ref>{{cite journal |last=Vilarrubias |first=Pere |date=17 November 2022 |title=The elusive diiodosulphanes and diiodoselenanes |url=https://doi.org/10.1080/00268976.2022.2129106 |journal=Molecular Physics |volume=120 |issue=22 |pages=e2129106 |doi=10.1080/00268976.2022.2129106 |bibcode=2022MolPh.12029106V |s2cid=252744393 |issn=0026-8976 |access-date=5 March 2023 |archive-date=19 March 2024 |archive-url=https://web.archive.org/web/20240319070247/https://www.tandfonline.com/pb/css/t1709911000430-v1707891316000/head_4_698_en.css |url-status=live }}</ref> Sulfur-iodine and selenium-iodine polyatomic cations (e.g., [S<sub>2</sub>I<sub>4</sub><sup>2+</sup>][AsF<sub>6</sub><sup>–</sup>]<sub>2</sub> and [Se<sub>2</sub>I<sub>4</sub><sup>2+</sup>][Sb<sub>2</sub>F<sub>11</sub><sup>–</sup>]<sub>2</sub>) have been prepared and characterised crystallographically.<ref>{{cite journal |last1=Klapoetke |first1=T. |last2=Passmore |first2=J. |date=1 July 1989 |title=Sulfur and selenium iodine compounds: from non-existence to significance |url=https://pubs.acs.org/doi/abs/10.1021/ar00163a002 |journal=Accounts of Chemical Research |language=en |volume=22 |issue=7 |pages=234–240 |doi=10.1021/ar00163a002 |issn=0001-4842 |access-date=15 January 2023 |archive-date=15 January 2023 |archive-url=https://web.archive.org/web/20230115160630/https://pubs.acs.org/doi/abs/10.1021/ar00163a002 |url-status=live }}</ref>

Given the large size of the iodide anion and iodine's weak oxidising power, high oxidation states are difficult to achieve in binary iodides, the maximum known being in the pentaiodides of [[niobium]], [[tantalum]], and [[protactinium]]. Iodides can be made by reaction of an element or its oxide, hydroxide, or carbonate with hydroiodic acid, and then dehydrated by mildly high temperatures combined with either low pressure or anhydrous hydrogen iodide gas. These methods work best when the iodide product is stable to hydrolysis. Other syntheses include high-temperature oxidative iodination of the element with iodine or hydrogen iodide, high-temperature iodination of a metal oxide or other halide by iodine, a volatile metal halide, [[carbon tetraiodide]], or an organic iodide. For example, [[Molybdenum dioxide|molybdenum(IV) oxide]] reacts with [[Aluminium iodide|aluminium(III) iodide]] at 230&nbsp;°C to give [[molybdenum(II) iodide]]. An example involving halogen exchange is given below, involving the reaction of [[tantalum(V) chloride]] with excess aluminium(III) iodide at 400&nbsp;°C to give [[tantalum(V) iodide]]:<ref name="Greenwood821">Greenwood and Earnshaw, pp. 821–4</ref>

<chem display="block">3TaCl5 + \underset{(excess)}{5AlI3} -> 3TaI5 + 5AlCl3</chem>

Lower iodides may be produced either through thermal decomposition or disproportionation, or by reducing the higher iodide with hydrogen or a metal, for example:<ref name="Greenwood821" />

<chem display="block">TaI5{} + Ta ->[\text{thermal gradient}] [\ce{630^\circ C\ ->\ 575^\circ C}] Ta6I14</chem>

Most metal iodides with the metal in low oxidation states (+1 to +3) are ionic. Nonmetals tend to form covalent molecular iodides, as do metals in high oxidation states from +3 and above. Both ionic and covalent iodides are known for metals in oxidation state +3 (e.g. [[Scandium triiodide|scandium iodide]] is mostly ionic, but [[aluminium iodide]] is not). Ionic iodides MI<sub>''n''</sub> tend to have the lowest melting and boiling points among the halides MX<sub>''n''</sub> of the same element, because the electrostatic forces of attraction between the cations and anions are weakest for the large iodide anion. In contrast, covalent iodides tend to instead have the highest melting and boiling points among the halides of the same element, since iodine is the most polarisable of the halogens and, having the most electrons among them, can contribute the most to van der Waals forces. Naturally, exceptions abound in intermediate iodides where one trend gives way to the other. Similarly, solubilities in water of predominantly ionic iodides (e.g. [[potassium]] and [[calcium]]) are the greatest among ionic halides of that element, while those of covalent iodides (e.g. [[silver]]) are the lowest of that element. In particular, [[silver iodide]] is very insoluble in water and its formation is often used as a qualitative test for iodine.<ref name="Greenwood821" />