Interhalogen Compounds
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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. ClVII, 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 XeFn compounds are formed which are isoelectronic with the interhalogen compounds (an X- ion has the same electronic configuration as the adjacent noble gas element).

The structure of interhalogen compounds

The structures of the interhalogen compounds can largely be predicted by Valence Shell Electron Pair Repulsion (VSEPR) Theory.

The XY species are linear.

The XY3 species have 10 electrons around the central X atom (7 from X and 1 from each of the three Y atoms), which makes five pairs, 3 bonding and 2 lone. This means that the basic structure is a trigonal bipyramid, and the two lone pairs occupy equatorial positions, where the inter-electron pair repulsion is minimized. The resulting structure is therefore of C3v symmetry.

The XY5 species have 12 electrons around the central X atom (7 from X and 1 from each of the five Y atoms), which makes six pairs, 5 bonding and 1 lone. This means that the basic structure is an octahedron, and the lone pair occupies one of the vertices. The resulting structure is therefore of C4v symmetry.

The XY7 species have 14 electrons around the central X atom (7 from X and 1 from each of the seven Y atoms), which makes seven pairs, all of them bonding. This means that the structure is a pentagonal bipyramid, with D5h symmetry.

XY3:

ClF3 (C3v)

XY5:

BrF5 (C4v)

XY7:

IF7 (D5h)

In VSEPR, the order of repulsion is lp-lp>lp-bp>bp-bp (where lp refers to a lone pair and bp refers to a bonding pair), and hence in ClF3, the Fa-Cl-Fa system (where Fa is an axial fluorine atom) is not linear. Similarly, in BrF5, the four equatorial fluorine atoms are not in the same plane as the bromine atom (though they are in the same plane as each other) due to the extra repulsion from the lone pair compared to the axial bond pair.

There are exceptions to the VSEPR: one is ICl3 which adopts a bridged structure as it forms a dimer, I2Cl6.

I2Cl6: The bridging Cl atoms act as Lewis acids to the I atoms, much like in the formation of the Al2Cl6 dimer.
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