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Concept
Triiodide
Summary
In chemistry, triiodide usually refers to the triiodide ion, I3−. This anion, one of the polyhalogen ions, is composed of three iodine atoms. It is formed by combining aqueous solutions of iodide salts and iodine. Some salts of the anion have been isolated, including thallium(I) triiodide (Tl+[I3]−) and ammonium triiodide ([NH4]+[I3]−). Triiodide is observed to be a red colour in solution.
Other chemical compounds with "triiodide" in their name may contain three iodide centers that are not bonded to each other as the triiodide ion, but exist instead as separate iodine atoms or iodide ions. Examples include nitrogen triiodide (NI3) and phosphorus triiodide (PI3), where individual iodine atoms are covalently bonded to a central atom. As some cations have the theoretical possibility to form compounds with both triiodide and iodide ions, such as ammonium, compounds containing iodide anions in a 3:1 stoichiometric ratio should only be referred to as triiodides in cases where the triiodide anion is present. It may also be helpful to indicate the oxidation number of a metal cation, where appropriate. For example, the covalent molecule gallium triiodide (Ga2I6) is better referred to as gallium(III) iodide to emphasise that it is iodide anions that are present, and not triiodide.
The following exergonic equilibrium gives rise to the triiodide ion:
I2 + I− ⇌ I3−
In this reaction, iodide is viewed as a Lewis base, and the iodine is a Lewis acid. The process is analogous to the reaction of S8 with sodium sulfide (which forms polysulfides) except that the higher polyiodides have branched structures.
The ion is linear and symmetrical. According to valence shell electron pair repulsion theory, the central iodine atom has three equatorial lone pairs, and the terminal iodine atoms are bonded axially in a linear fashion, due to the three lone pairs bonding to the central iodine-atom. In the molecular orbital model, a common explanation for the hypervalent bonding on the central iodine involves a three-center four-electron bond.
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