Inter halogen compounds

Each halogen can combine with a different halogen to form a series of binary compounds called interhalogen compounds, generally written AB, AB3, AB5 or AB7, in which the less electronegative (generally larger) halogen A is the central atom and the more electronegative (generally smaller) halogen B occupies the terminal positions. Representative AB-type compounds include ClF, BrF, BrCl, ICl and IBr; AB3-type compounds include ClF3, BrF3, IF3 and ICl3; AB5-type compounds include BrF5 and IF5; and the sole AB7-type compound is IF7.

General properties of interhalogen compounds: (i) the central atom is always the larger of the two halogens present; (ii) an interhalogen compound can only be formed between exactly two different halogens, never more than two; (iii) fluorine, being the smallest halogen, can never act as the central atom; (iv) because fluorine is both highly electronegative and very small, its presence allows the central (larger) halogen atom to reach unusually high coordination numbers, which is why the AB5 and AB7 interhalogens are exclusively fluorides; (v) interhalogen compounds can undergo autoionisation in the liquid state, for example 2ICl <=> I+ + ICl2-, and 2ICl3 <=> ICl2+ + ICl4-; and (vi) they are all strong oxidising agents, generally more reactive than the parent halogens themselves (except fluorine) because the A-B bond in an interhalogen is weaker than the A-A bond in the corresponding pure halogen.

Reaction with alkali: when heated with alkali, the larger halogen of an interhalogen compound is oxidised to an oxyhalogen anion while the smaller halogen is reduced to a simple halide ion — for example, BrF5 with hydroxide gives fluoride and bromate ion (BrF5 + 6OH- -> 5F- + BrO3- + 3H2O), while ICl with hydroxide gives chloride and hypoiodite ion (ICl + 2OH- -> Cl- + OI- + H2O). …