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
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
Secondly, iodine is the largest halogen, and so
the steric demands of packing other halogen atoms around it
All the interhalogens are violently hydrolyzed
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-)
|Interhalogen cation (ClF2+)
The formation of interhalogens with elements in
high oxidation states, eg. ClVII, requires the use
of strong oxidizing agents (which are, in effect, F+
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
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
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.
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.
The bridging Cl atoms act as Lewis acids to the I atoms,
much like in the formation of the Al2Cl6