Interhalogen Compounds and Pseudohalogens

The general pattern

Interhalogens take the form XY_n with n equal to 1, 3, 5 or 7. The central atom X is the larger and less electronegative halogen; the surrounding Y atoms are smaller and more electronegative.

Shapes from VSEPR

Each shape follows from counting electron pairs and placing lone pairs where repulsion is minimised. The T-shape of XY₃ is the one most often asked, and it arises because two lone pairs occupy equatorial positions of a trigonal bipyramidal arrangement, leaving the three bonds in a T.

Reactivity

Interhalogens are more reactive than the halogens themselves, with the exception of fluorine. The reason is that the X–Y bond is weaker than either X–X or Y–Y, because the two atoms differ in size and electronegativity so the overlap is poorer.

Being asked why an interhalogen is more reactive than the corresponding halogens is a standard question, and the bond-strength argument is the expected answer.

Hydrolysis

Interhalogens hydrolyse to give a halide from the more electronegative halogen and an oxoacid or oxohalide from the less electronegative one. The reasoning is that the more electronegative atom takes the negative charge:

Predicting the hydrolysis products therefore requires only deciding which halogen is more electronegative, and that determines which becomes the halide.

Polyhalide ions

Halogens also form polyhalide ions, the best known being the triiodide ion formed when iodine dissolves in iodide solution. This is why iodine, otherwise poorly soluble in water, dissolves readily in potassium iodide solution — a practical point worth knowing.

Triiodide is linear, with three lone pairs on the central atom occupying equatorial positions, exactly as VSEPR predicts.

Pseudohalogens

Certain polyatomic groups behave chemically like halogens. They exist as dimers analogous to the halogen molecules, form anions analogous to halides, and form acids analogous to the hydrogen halides.

The analogy extends usefully: pseudohalides form insoluble silver salts, they can be oxidised to the dimer, and they undergo comparable substitution chemistry. Recognising the parallel is what makes the concept worth having, since it lets halogen chemistry be transferred to a new set of species.

Frequently asked questions

Why is the larger halogen always central?

Because the central atom must accommodate several others around it, which requires size, and because the less electronegative atom bears the positive polarisation more comfortably.

Why are interhalogens more reactive than halogens?

Because the bond between two different halogens is weaker than either homonuclear bond, so it dissociates more readily.

Why does iodine dissolve in potassium iodide solution?

Because it forms the soluble triiodide ion with iodide. Molecular iodine alone is only sparingly soluble in water.

What makes a group a pseudohalogen?

That it forms a dimer analogous to a halogen molecule, an anion analogous to a halide, and an acid analogous to a hydrogen halide, and behaves comparably in reactions.