Triiodide
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Triiodide is an anion composed of three iodine atoms.
The following equilibrium :
I2 + I− ⇌ I3−
gives rise to the triiodide ion in solution. The Lewis base is the iodide ion and the Lewis acid is iodine. The reaction is endothermic. The central atom of I3− is sp3d hybridised, and the ion thus has linear shape according to VSEPR theory. It is the triiodide ion which is responsible for the well-known blue-black color which arises when iodine solutions react with starch. Elemental iodine does not react with starch in either the massive state or in a finely divided state, and solutions of iodine in nonpolar solvents do not react with starch either. The solution called tincture of iodine actually contains potassium iodide as well, so that significant amounts of triiodide ion can exist in solution (similarly to how elemental sulfur will dissolve in aqueous solutions of sodium sulfide). Triiodide appears yellow in low concentration, and brown at higher concentration.
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