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Halogen Chemistry 207

oxidation state (smallest charge) forms the most ionic Studies to determine the relative stabilities of complex
halide. In any series of halides of the same metal, the halides with respect to dissociation into their constituent
smaller fluorides are the most ionic, while the larger io- ions in aqueous solution have shown that two types of be-
dides have the most covalent character. havior exist. Small, highly charged metal cations, called
3+
4+
2+
type A or hard Lewis acids (Be , Fe , Ce , etc.),
tend to form their most stable complexes with fluoride
C. Structure
−
ion (F > Cl > Br > I ), while larger metal cations of
−
−
−
2+
+
In the solid state, the binary ionic halides exist as crystals lower charge, called type B or soft Lewis acids (Ag , Pt ,
made up of an ordered array of halide anions and metal etc.), tend to form their most stable complexes with iodide
−
−
−
−
n +
cations (M ). The arrangement of the crystalline array ion (F < Cl < Br < I ).
is determined by the lattice energy of the crystal, and the Complexes formed by a transition-metal ion and halide
relative numbers (stoichiometry) and sizes of the anions ligands are typically highly colored as a result of ligand-
and cations which are present. Because of the small size induced splitting or differentiation of the metal d-orbital
−
of the F anion, fluorides often differ in structure from energies. Comparison of the ultraviolet–visible spectra of
other halides of the same metal. these complexes indicates that the extent of splitting de-
−
−
−
−
The actual arrangement of ions in the solid state can creases according to the sequence F > Cl > Br > I .
frequently be predicted by a rather simple procedure. The The degree of differentiation between the metal d-orbitals
−
larger ions (usually the X anion) are stacked in a three- has been related to the charge-to-radius ratio of the ligand,
dimensional, closest-packed array like billiard balls, and and the strength and type (σ or π) of the metal–ligand
n +
the smaller ions (usually the M cation) are evenly dis- bond.
tributed in the holes left in this structure. If the cations
and anions are approximately the same size, the most ef-
ficient lattice for maximizing cation–anion interaction is IV. INTERHALOGEN COMPOUNDS
one in which eight ions of one type surround one ion of
the second type. If the halide has a fair amount of covalent A. General Survey
character, a chain or layer, structure may result in which
The halogens combine with each other to form four types
the internuclear distance between layers is greater than
of binary, neutral, interhalogen compounds: XY (all possi-
within a layer.
ble combinations), XY 3 (Y = F only), XY 5 (Y = F only),
Covalent halides exist as discrete molecules that per-
and XY 7 (X = I and Y = F only), where X is the heav-
sist throughout the solid, liquid, and gaseous phases. The
ier halogen (Table VI). The greater the electronegativity
forces between two or more molecular units in the crys-
difference (Table III) between the central atom X and the
talline state are weak, thus accounting for high volatilities
terminal atoms Y, the greater the total number of Y atoms
and low melting and boiling points. Intermolecular forces
thatcanbeboundinthemolecule.Thus,iodinecanbindup
are greatest for iodides and weakest for fluorides. Conse-
to seven F atoms, but only a maximum of three Cl atoms
quently, for the same nonmetallic element M, boiling and
or one Br atom. The structures of these molecules can
melting points of the halides generally increase down the
generally be predicted by simple valence-shell electron-
group: fluoride < chloride < bromide < iodide. The crys-
pair repulsion (VSEPR) theory, in which the bonded and
tal structures of the covalent halides are primarily deter-
mined by the arrangements of the atoms which make up
the individual molecules. TABLE VI Binary Interhalogen Molecules
General formula a
D. Halide Complexes XY XY 3 XY 5 XY 7
All of the halide ions can act as ligands (Lewis bases or ClF ClF 3 CIF 5
electron pair donors) toward the majority of metal ions and BrF BrF 3 BrF 5
a number of molecular halides (Lewis acids or electron- IF b IF 3 IF 5 IF 7
pair acceptors), forming complex species such as AIF , BrCl
3−
6
−
AgCl , SbBr , and HgI 2− [See Eq. (2)]. Halide ligands ICl I 2 Cl c
−
2 4 4 6
may also be present with ligands of many other types: IBr
−
NbOCl , [Co(NH 3 ) 4 Br 2 ] , Mn(CO) 5 I, and so on. The
+
4 a The oxidation state of Y is −1. The oxidation
bonds formed by this attachment are called coordinate
state of X is equal to the number of Y atoms present.
covalent, indicating that the ligand has contributed both b Observed spectroscopically.
electrons to the shared pair. c Dimer of ICl 3 .