In chemistry, triiodide (sometimes written tri-iodide) can have several meanings. Triiodide primarily refers to the triiodide ion, I3−, a polyatomic anion composed of three iodine atoms. For some chemical compounds, triiodide indicates a salt of the named cation with the triiodide anion. Examples include sodium triiodide, thallium triiodide, and ammonium triiodide (NH4I3). Each of these compounds has a [mono]iodide counterpart. For other chemical compounds, triiodide indicates that each molecule contains three iodine atoms that are not bonded to each other, not forming the triiodide ion. Examples include nitrogen triiodide (NI3), phosphorus triiodide, antimony triiodide, and gallium triiodide (Ga2I6). Some anions have the theoretical possibility to form either kind of triiodide. Thallium triiodide is described as thallium(I) triiodide; thallium(III) iodide is unknown.
The triiodide ion, is the simplest polyiodide; several higher polyiodides exist. In solution, it appears yellow in low concentration, and brown at higher concentration. The triiodide ion is responsible for the well-known blue-black color which arises when iodine solutions react with starch. Iodide does not react with starch; nor do solutions of iodine in nonpolar solvents. Lugol's iodine contains potassium iodide as well, so that significant amounts of triiodide ion can exist in solution. Tincture of iodine contains significant amounts of triiodide.
Formation and structure
The following endergonic 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, except that the higher polyiodides have branched structures.
The ion is linear, as predicted by VSEPR theory. A common explanation for the hypervalent bonding on the central atom involves a three-center four-electron bond. The bond lengths and angles of triiodide vary, depending on the compound. The dimensions of the tri-iodide Ia−Ib−Ic bonds in a few sample compounds are shown below:
|compound||Ia−Ib (pm)||Ib - Ic (pm)||angle (°)|
- Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.