The halogens react with each other to form interhalogen compounds.
The general formula of most interhalogen compounds is XYn, where n = 1, 3, 5 or 7 (X is the less electronegative of the two halogens). ClF3 is used in the manufacture of uranium hexafluoride. They are all prone to hydrolysis, and ionise to give rise to polyatomic cations and anions.
The interhalogens of form XY have physical properties intermediate between those of the two parent halogens. The covalent bond between the two atoms has some ionic character, the less electronegative element, X, being oxidised and having a partial positive charge. Most combinations of F, Cl, Br and I are known, but not all are stable.
- Chlorine monofluoride (ClF), the lightest interhalogen compound, is a colorless gas with a normal boiling point of −100 °C.
- Bromine monofluoride (BrF) has not been obtained pure — it dissociates into the trifluoride and free bromine. Similarly, iodine monofluoride is unknown — iodine reacts with fluorine to form a pentafluoride.
- Iodine monofluoride (IF) is unstable and disproportionates rapidly and irreversibly at room temperature: 5IF → 2I2 + IF5. However, its molecular properties can still be precisely determined by spectroscopy: the iodine-fluorine distance is 190.9 pm and the I−F bond dissociation energy is around 277 kJ mol−1. ΔHf° = −95.4 kJ mol−1 and ΔGf° = −117.6 kJ mol−1, both at 298 K. IF can be generated, albeit fleetingly, by the following reactions: I2 + F2 →2IF at −45 °C in CCl3F; I2 + IF3 → 3IF at −78 °C in CCl3F; I2 + AgF → IF + AgI at 0 °C.
- Bromine monochloride (BrCl) is a red-brown gas with a boiling point of 5 °C.
- Iodine monochloride (ICl) consists of red transparent crystals which melt at 27.2° C to form a choking brownish liquid (similar in appearance and weight to bromine). It reacts with HCl to form the strong acid HICl2. The crystal structure of iodine monochloride consists of puckered zig-zag chains, with strong interactions between the chains.
- Iodine monobromide (IBr) is made by direct combination of the elements to form a dark red crystalline solid. It melts at 42° C and boils at 116° C to form a partially dissociated vapour.
- Chlorine trifluoride (ClF3) is a colourless gas which condenses to a green liquid, and freezes to a white solid. It is made by reacting chlorine with an excess of fluorine at 250° C in a nickel tube. It reacts more violently than fluorine, often explosively. The molecule is planar and T-shaped.
- Bromine trifluoride (BrF3) is a yellow green liquid which conducts electricity — it ionises to form [BrF2+] + [BrF4−]. It reacts with many metals and metal oxides to form similar ionised entities; with some others it forms the metal fluoride plus free bromine and oxygen. It is used in organic chemistry as a fluorinating agent. It has the same molecular shape as chlorine trifluoride.
- Iodine trifluoride (IF3) is a yellow solid which decomposes above −28 °C. It can be synthesised from the elements, but care must be taken to avoid the formation of IF5. F2 attacks I2 to yield IF3 at −45 °C in CCl3F. Alternatively, at low temperatures, the fluorination reaction I2 + 3XeF2 → 2IF3 + 3Xe can be used. Not much is known about iodine trifluoride as it is so unstable.
- Iodine trichloride (ICl3) forms lemon yellow crystals which can be melted under pressure to a brown liquid. It can be made from the elements at low temperature, or from iodine pentoxide and hydrogen chloride. It reacts with many metal chlorides to form tetrachloriodides, and hydrolyses in water. The molecule is a planar dimer, with each iodine atom surrounded by four chlorine atoms.
- Bromine pentafluoride (BrF5) is a colourless fuming liquid, made by reacting bromine trifluoride with fluorine at 200° C. It is physically stable, but reacts violently with water and most metals and nonmetals.
- Iodine pentafluoride (IF5) is a colourless liquid, made by reacting iodine pentoxide with fluorine, or iodine with silver fluoride. It is highly reactive, even slowly with glass. It reacts with elements, oxides and carbon halides. The molecule has the form of a tetragonal pyramid.
- Iodine heptafluoride (IF7) is a colourless gas. It is made by reacting the pentafluoride with fluorine. IF7 is chemically inert, having no lone pair of electrons in the valency shell; in this it resembles sulfur hexafluoride. The molecule is a pentagonal bipyramid. This compound is the only interhalogen compound possible where the larger atom is carrying seven of the smaller atoms.
- All attempts to form bromine heptafluoride have met with failure; instead, bromine pentafluoride and fluorine gas are produced.
Summary of known interhalogens
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