Pseudohalogen
Pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds. Pseudohalogens occur in pseudohalogen molecules, inorganic molecules of the general forms "Ps"–"Ps" or "Ps"–X (where "Ps" is a pseudohalogen group), such as cyanogen; pseudohalide anions, such as cyanide ion; inorganic acids, such as hydrogen cyanide; as ligands in coordination complexes, such as ferricyanide; and as functional groups in organic molecules, such as the nitrile group. Well-known pseudohalogen functional groups include cyanide, cyanate, thiocyanate, and azide. Common pseudohalogens and their nomenclature. Many pseudohalogens  are known by specialized common names according to where they occur in a compound. Well-known ones include (the true halogen chlorine is listed for comparison): Au is considered to be a pseudohalogen ion due to its disproportionation reaction with alkali and the ability to form covalent bonds with hydrogen. Examples of pseudohalogen molecules. Examples of symmetrical pseudohalogens ("Ps"–"Ps") include cyanogen (CN), thiocyanogen (SCN), hydrogen peroxide HO. Another complex symmetrical pseudohalogen is dicobalt octacarbonyl, Co(CO). This substance can be considered as a dimer of the hypothetical cobalt tetracarbonyl, Co(CO). Examples of non-symmetrical pseudohalogens ("Ps"–X), analogous to the binary interhalogen  compounds, are cyanogen halides like ClCN or BrCN; nitryl fluoride, and other compounds. Sometimes nitrosyl chloride NOCl also is considered as pseudohalogen. Not all combinations are known to be stable. Pseudohalides. Pseudohalides are the univalent anions (or functional groups) which form hydracids with hydrogen and form insoluble salts with silver such as cyanide, cyanates, fulminate, thiocyanate and azide. A common complex pseudohalide is a tetracarbonylcobaltate . The acid HCo(CO) is in fact quite a strong acid, though its low solubility renders it not as strong as the true hydrohalic acids. The behavior and chemical properties of the above pseudohalides are identical  to that of the true halide ions. The presence of the internal multiple bonds does not appear to affect their chemical behavior. For example, they can form strong acids of the type HX (compare HCl to HCo(CO)), and they can react with metals to form compounds like MX (compare NaCl to NaN). Nanoclusters of aluminium (often referred to as superatoms) are sometimes considered to be pseudohalides since they, too, behave chemically as halide ions, forming AlI (analogous to I) and similar compounds. This is due to the effects of metallic bonding on small scales.