Most interhalogen compounds known are binary (composed of only two distinct elements). Their formulae are generally XYn, where n = 1, 3, 5 or 7, and X is the less electronegative of the two halogens. The value of n in interhalogens is always odd, because of the odd valence of halogens. They are all prone to hydrolysis, and ionize to give rise to polyhalogen ions. Those formed with astatine have a very short half-life due to astatine being intensely radioactive.
No interhalogen compounds containing three or more different halogens are definitely known, although a few books claim that and have been obtained, and theoretical studies seem to indicate that some compounds in the series are barely stable.
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 halogen, X, being oxidised and having a partial positive charge. All combinations of fluorine, chlorine, bromine, and iodine that have the above-mentioned general formula are known, but not all are stable. Some combinations of astatine with other halogens are not even known, and those that are known are highly unstable.
Bromine monofluoride dissociates like this:
No astatine fluorides have been discovered yet. Their absence has been speculatively attributed to the extreme reactivity of such compounds, including the reaction of an initially formed fluoride with the walls of the glass container to form a non-volatile product.[a] Thus, although the synthesis of an astatine fluoride is thought to be possible, it may require a liquid halogen fluoride solvent, as has already been used for the characterization of radon fluorides.
All stable hexatomic and octatomic interhalogens involve a heavier halogen combined with five or seven fluorine atoms. Unlike the other halogens, fluorine atoms have high electronegativity and small size which is able to stabilize them.
Typically, interhalogen bonds are more reactive than diatomic halogen bonds--because interhalogen bonds are weaker than diatomic halogen bonds, except for F2. If interhalogens are exposed to water, they convert to halide and oxyhalide ions. With BrF5, this reaction can be explosive. If interhalogens are exposed to silicon dioxide, or metal oxides, then silicon or metal respectively bond with one of the types of halogen, leaving free diatomic halogens and diatomic oxygen. Most interhalogens are halogen fluorides, and all but three (IBr, AtBr, and AtI) of the remainder are halogen chlorides. Chlorine and bromine can each bond to five fluorine atoms, and iodine can bond to seven. AX and AX3 interhalogens can form between two halogens whose electronegativities are relatively close to one another. When interhalogens are exposed to metals, they react to form metal halides of the constituent halogens. The oxidation power of an interhalogen increases with the number of halogens attached to the central atom of the interhalogen, as well as with the decreasing size of the central atom of the compound. Interhalogens containing fluorine are more likely to be volatile than interhalogens containing heavier halogens.
Interhalogens with one or three halogens bonded to a central atom are formed by two elements whose electronegativities are not far apart. Interhalogens with five or seven halogens bonded to a central atom are formed by two elements whose sizes are very different. The number of smaller halogens that can bond to a large central halogen is guided by the ratio of the atomic radius of the larger halogen over the atomic radius of the smaller halogen. A number of interhalogens, such as IF7, react with all metals except for those in the platinum group. IF7, unlike interhalogens in the XY5 series, does not react with the fluorides of the alkali metals.
ClF3 is the most reactive of the XY3 interhalogens. ICl3 is the least reactive. BrF3 has the highest thermal stability of the interhalogens with four atoms. ICl3 has the lowest. Chlorine trifluoride has a boiling point of -12 °C. Bromine trifluoride has a boiling point of 127 °C and is a liquid at room temperature. Iodine trichloride melts at 101 °C.
Most interhalogens are covalent gases. Some interhalogens, especially those containing bromine, are liquids, and most iodine-containing interhalogens are solids. Most of the interhalogens composed of lighter halogens are fairly colorless, but the interhalogens containing heavier halogens are deeper in color due to their higher molecular weight. In this respect, the interhalogens are similar to the halogens. The greater the difference between the electronegativities of the two halogens in an interhalogen, the higher the boiling point of the interhalogen. All interhalogens are diamagnetic. The bond length of interhalogens in the XY series increases with the size of the constituent halogens. For instance, ClF has a bond length of 1.628 Å, and IBr has a bond length of 2.47 Å.
It is possible to produce larger interhalogens, such as ClF3, by exposing smaller interhalogens, such as ClF, to pure diatomic halogens, such as F2. This method of production is especially useful for generating halogen fluorides. At temperatures of 250 to 300 °C, this type of production method can also convert larger interhalogens into smaller ones. It is also possible to produce interhalogens by combining two pure halogens at various conditions. This method can generate any interhalogen save for IF7.
Smaller interhalogens, such as ClF, can form by direct reaction with pure halogens. For instance, F2 reacts with Cl2 at 250 °C to form two molecules of ClF. Br2 reacts with diatomic fluorine in the same way, but at 60 °C. I2 reacts with diatomic fluorine at only 35 °C. ClF and BrF can both be produced by the reaction of a larger interhalogen, such as ClF3 or BrF3 and a diatomic molecule of the element lower in the periodic table. Among the hexatomic interhalogens, IF5 has a higher boiling point (97 °C) than BrF5 (40.5 °C), although both compounds are liquids at room temperature. The interhalogen IF7 can be formed by reacting palladium iodide with fluorine.
Some interhalogens, such as BrF3, IF5, and ICl, are good halogenating agents. BrF5 is too reactive to generate fluorine. Beyond that, iodine monochloride has several applications, including helping to measure the saturation of fats and oils, and as a catalyst for some reactions. A number of interhalogens, including IF7, are used to form polyhalides.
A few ternary compounds, such as and , are also known [no source given].
The only two interhalogen compounds are and [no source given].
Only a few ternary interhalogen compounds such as and have been prepared [no source given].