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208 Halogen Chemistry

nonbonded valence electrons are arranged on X so as to production of incendiary bombs and experimental rocket
minimize electrostatic repulsions between electron pairs. motors. It is an exceptionally good fluorinating agent and
A few ternary compounds, such as IFCl 2 and IF 2 Cl, are is manufactured on a moderately large scale for the prepa-
also known. As can be seen from the formulas, the neutral ration of UF 6 in nuclear fuel processing.
interhalogen molecules always contain an even number Bromine trifluoride is somewhat less reactive than ClF 3 ,
of halogen atoms. A variety of polyhalide ions have also but is also a useful fluorinating and brominating agent.
been identified. Many contain an odd number of halogen Liquid BrF 3 has a high specific conductance and has been
atoms. exploited as a nonaqueous, ionizing solvent.
Chemically, all of the interhalogens are rather reactive.
They are good oxidizing agents and attack almost all el-
D. Hexa-atomic, XY 5 , and Octa-atomic,
ements to produce mixtures of halides. Most are readily
XY 7 , Interhalogens
hydrolyzed in water, yielding hypohalic acids (HOX) and
halide ions under appropriate conditions. Only four interhalogens with the formula XY 5 or XY 7 are
known(TableVI).Synthesisinvolvesdirectfluorinationof
the element or a lower interhalogen fluoride. Fluorination
B. Diatomic Interhalogens, XY
of appropriate metal salts may also be employed.
All six of the possible diatomic compounds between F, Cl,
KBr + 3F 2 → KF + BrF 5 ; (31)
Br, and I are known (Table VI) and, except for BrF and IF,
which are too unstable with respect to disproportionation PdI 2 + 8F 2 → PdF 2 + 2IF 7 . (32)
to permit isolation at room temperature, they can be pre-
All of these molecules are excellent fluorinating agents,
pared by direct combination of the elements X 2 and Y 2 .
with the general sequence of reactivity for the halogen
The properties of the compounds tend to be intermediate fluorides being ClF 3 > BrF 5 > IF 7 > ClF > BrF 3 > IF 5 >
between those of the pure, parent halogens. Most add to
BrF > IF 3 .
carbon–carbon double bonds (Section IX. C), and some
are useful as nonaqueous solvents. Liquid ICl, in partic-
ular, dissolves the chlorides of Group IA to give highly E. Polyhalide, Polyhalonium, and Halogen Ions
conducting solutions.
A large class of anions is formed by the reaction of a
The stabilities of the diatomic interhalogens are vari-
halogen or interhalogen molecule and a halide ion:
able. The diatomic interhalogen ClF is the most stable
−
and IF the least. Like their pure halogen parents, inter- Y 2 + X → Y 2 X ; (33)
−
halogen molecules dissociate into free atoms at elevated −
−
X Y n + X → XX Y . (34)

n
temperatures. The diatomic interhalogen ICl resembles I 2
by dissolving in a variety of organic solvents and reacts The halogens involved may or may not be of the same
with molecules having unshared electrons to form stable, kind. Although compounds formed from these anions tend
charge-transfer complexes. to be unstable, decomposing to a metal halide and neutral
halogen or interhalogen molecule, stabilities may be en-
hanced by the presence of large cations, such as Cs or
+
C. Tetra-atomic Interhalogens, XY 3
+
tetraalkylammonium ions (R 4 N ).
The four known interhalogens of the XY 3 type (Table VI) Iodine forms the most extensive group of polyhalide
−
can also be prepared by direct reaction of the elements, but anions, with triiodide, I , being the best-known example.
3
conditions must be carefully chosen to avoid the formation The formation of this anion is responsible for the increased
of mixtures of interhalogens with different stoichiome- solubility of I 2 in water on addition of potassium iodide
tries. Another synthetic approach involves reaction of a (KI). Triiodide is used extensively in analytical chem-
lower interhalogen with a halogen: istry, many analytical procedures being based on the re-
lease or uptake of iodine and its subsequent titration with
ClF + F 2 → ClF 3 . (30)
sodium thiosulfate (iodometry). The largest polyhalide an-
4−
The two iodine-containing compounds have not been stud- ion known, I , consists of an alternate arrangement of
16
iedextensively,butBrF 3 andespeciallyClF 3 areextremely triiodide anions and neutral iodine molecules.
useful chemical reagents. Fluorine shows the least tendency for polyhalide for-
Chlorine trifluoride is a very powerful oxidizing agent mation. The F ions does not exist under normal condi-
−
3
and one of the most reactive chemicals known. It attacks tions. Species such as IBrF have been identified, but the
−
many materials normally considered inert (e.g., asbestos, fluorine atom never occupies a center position in these
xenon gas) and for this reason has been employed in the structures.