A pseudohalogen or halogenoid is a chemical species that acts like the halogen elements: a pseudohalogen molecule is similar to the diatomic halogen molecules, a pseudohalide anion is similar to halides, and a pseudohalogen radical is similar to single halogen atoms. There is no single definition for what makes a chemical a pseudohalogen, and different chemists use different lists depending on what properties they want to study.
The word was invented in 1925 by the German chemists Lothar Birckenbach and Karl Kellermann, who were studying several nitrogen-containing ions: azide (N−3), cyanide (CN−), and the chalcogen derivatives of cyanate (OCN−, SCN−, SeCN−, TeCN−).
To look at example definitions of pseudohalogens, consider the following table of properties and reactions of halogens, taken from lists of properties in two different inorganic chemistry textbooks.
| Properties of Px2 | Cotton & Wilkinson (1968) | Downs & Adams (1973) |
|---|---|---|
| Pure compound is a symmetric dimer Px2 | Yes | Yes |
| Px2 is volatile | Yes | |
| Px2 reacts with metals to make salts MPxn | Yes | Yes |
| Px2 adds to alkenes like a halogen: H2C=CH2 + Px2 → H2PxC−CPxH2 |
Yes | |
| Px2 reacts with alkali like a halogen: 2 OH− + Px2 → Px− + PxO− + H2O |
Yes | |
| Properties of HPx | ||
| HPx is an acid | Yes | Yes |
| HPx is weaker than hydrohalic acids | Yes | |
| Properties of Px− | ||
| AgPx, HgPx, and PbPx2 are insoluble in water | Yes | Yes |
| Oxidation of Px− makes Px2 | Yes | Yes |
| Structure of coordination complexes is similar to chlorido complexes | Yes | Yes |
| Properties of other compounds | ||
| Makes compounds PxX with halogens | Yes | Yes |
| Makes compounds PxPx' with other pseudohalogens | Yes | Yes |
| Other covalent compounds are similar to covalent halides | Yes | Yes |
| Covalent compounds have structural isomers where different atoms in the pseudohalogen are part of the R−Px bond | Some | |
| Makes covalent polypseudohalide ions similar to triiodide, with self or other pseudohalogens | Some |
Not all of these properties are always required for a chemical to be considered a pseudohalogen.