polyhalide ions

Pure iodine barely dissolves in water, yet add a little potassium iodide and the iodine vanishes into a deep brown solution. What happened? An iodide ion reached out and grabbed a neutral I2 molecule, joining them into the triiodide ion, I3-. This is the simplest example of a polyhalide ion — a charged species built from three or more halogen atoms, where a halide ion acts as a donor to one or more neutral halogen (or interhalogen) molecules.

Take the triiodide ion as the model. Three iodine atoms line up in a row; counting electrons around the central iodine gives two bonding pairs and three lone pairs, five domains in a trigonal-bipyramidal arrangement, so VSEPR predicts a linear ion with the lone pairs equatorial. The central iodine carries the negative charge in the bookkeeping. Larger and mixed versions exist too — I5-, I7-, ICl2-, BrCl2-, and the big iodine cages — and as a rule the bigger, more polarizable halogens (iodine above all) form the richest family of polyhalides, while fluorine, too small and too electronegative, forms almost none.

Polyhalides matter in everyday and analytical chemistry: triiodide is what dissolves iodine for the tincture and for the starch-iodine test (the blue-black colour is starch wrapping around long polyiodide chains), and it is the active species in iodometric titrations. They also reinforce two themes of halogen chemistry — that a halide can behave as a Lewis base toward a halogen molecule, and that the ability to do so grows with size and polarizability down the group. The honest note: writing the central atom as simply 'hypervalent' hides the same subtlety as the interhalogens — the bonding is best seen as a three-centre, four-electron interaction along the linear axis, not a classical expanded octet.