Interhalogen Compounds, Pseudohalogens and Polyhalides

Interhalogen compounds are formed by the direct combination of two different halogens, and

fall into four general structural families, written $\text{XX}'$, $\text{XX}'_3$,

$\text{XX}'_5$ and $\text{XX}'_7$, where $X$ is the larger, less electronegative halogen and

$X'$ (almost always fluorine, occasionally chlorine) is the smaller, more electronegative one.

This asymmetry reflects a simple size argument: the smaller $X'$ atoms are the only ones able to

pack closely enough around the larger central $X$ atom to reach the higher coordination numbers

of the $\text{XX}'_5$ and $\text{XX}'_7$ families.

$\text{ClF}$, $\text{BrF}$, $\text{ICl}$, $\text{IBr}$ (the $\text{XX}'$ type) are simple linear

diatomic molecules. $\text{ClF}_3$, $\text{BrF}_3$ and $\text{ICl}_3$ (the $\text{XX}'_3$ type)

have a T-shaped structure: the central halogen is $sp^3d$ hybridised, with five electron

domains arranged in an underlying trigonal bipyramid -- three fluorine atoms and two lone pairs

-- and, because the two lone pairs occupy the two (lower-repulsion) equatorial positions

alongside one fluorine, the three actual F-X-F connections trace out a T shape rather than a

simple trigonal-planar one. $\text{IF}_5$ and $\text{BrF}_5$ (the $\text{XX}'_5$ type) are

square pyramidal: central iodine (or bromine) is $sp^3d^2$ hybridised, with six electron

domains -- five fluorine atoms and one lone pair -- arranged in an underlying octahedron, and the

single lone pair pushes the five fluorine atoms into a square-based pyramid, with the apex

fluorine closer to being directly opposite the lone pair. $\text{IF}_7$ (the $\text{XX}'_7$ type,

the only interhalogen known at this level of this stoichiometry, since only iodine is large

enough to accommodate seven fluorines) is pentagonal bipyramidal: iodine is $sp^3d^3$

hybridised, with seven fluorine atoms and no lone pair at all, five in an equatorial pentagon and

two axial.

Interhalogen compounds are, almost without exception, more chemically reactive than the parent halogens themselves (with the single exception of fluorine, which remains more reactive than

any interhalogen). The reason is a bond-strength argument: the $\text{X-X}'$ bond in an

interhalogen compound is generally weaker than the corresponding $\text{X-X}$ bond in the

heavier parent halogen (though still typically stronger than the anomalously weak $\text{F-F}$

bond), because the two atoms differ enough in size/electronegativity that their orbital overlap

is somewhat less efficient than a true homonuclear bond; a weaker bond breaks more readily,

supplying reactive halogen atoms/fragments more easily and so making the interhalogen compound

more kinetically reactive overall.

Two related classes of species extend this same logic. Pseudohalogens are dimeric species

that behave chemically much like the halogens themselves, each corresponding to a pseudohalide

ion analogous to a halide ion $X^-$: cyanogen, $(\text{CN})_2$, corresponds to the cyanide

ion $\text{CN}^-$, and thiocyanogen, $(\text{SCN})_2$, corresponds to the thiocyanate ion

$\text{SCN}^-$ (a third common example is $(\text{OCN})_2$, corresponding to cyanate,

$\text{OCN}^-$). Polyhalide ions are formed when a halogen molecule adds to a halide ion; the …