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What are inter halogen compounds? How are they formed?

Compounds formed with bonds between different Halogen atoms are known as interhalogen compounds. The binary compounds have formulae XY, XY₃, XY₅ and XY₇ (where X is the heavier halogen atom). Ternary species are also formed, and the species can be neutral, cations or anions.

All the interhalogens are thermodynamically stable with respect to decomposition to the elements, though the low valency compounds are unstable with respect to disproportionation.

Most of the interhalogen compounds contain fluorine: the non-fluoride compounds are BrCl, BrI, ICl and I₂Cl₆.

The highest oxidation states are exhibited with iodine: in chlorine interhalogens the highest iodochloride is ICl₃ whereas the highest bromochloride is BrCl, and in fluorine interhalogens the highest iodofluoride is IF₇ whereas the highest bromofluoride is BrF₅.

There are two reasons for this:Firstly iodine is the least electronegative halogen, and so its higher oxidation states are more stable than those of the lighter members of the group.

Secondly, iodine is the largest halogen, and so the steric demands of packing other halogen atoms around it are lessened.

All the interhalogens are violently hydrolyzed by water.

Some interhalogens undergo self ionization (in non-aqueous ionizing solvents). The reaction of bromotrifluoride is an example of fluoride ion transfer.

The self ionization reaction results in the formation of interhalogen cations and anions. Interhalogen anions are also formed by reaction with halide ion donors, whereas interhalogen cations are formed by reaction with halide ion acceptors.

Interhalogen anion (IF6-) formation
Interhalogen cation (ClF2+) formation

The formation of interhalogens with elements in high oxidation states, eg. Cl^VII, requires the use of strong oxidizing agents (which are, in effect, F⁺ donors).

KrF+ is the F+ donor.

In the reaction above, it should be seen that the fluorinating agent is KrF+. One of the aspects of the chemistry of fluorine is that it is so reactive that it can form compounds with the Noble Gases. Many XeF_n compounds are formed which are isoelectronic with the interhalogen compounds (an X^- ion has the same electronic configuration as the adjacent noble gas element).