pseudohalides

Pseudohalides are a class of anions and neutral molecules that imitate the chemical behavior of halide ions, such as chloride or iodide, in a range of reactions, even though they are not halogens themselves. They commonly form salts with alkali and alkaline earth metals and participate in substitution and precipitation reactions in a manner reminiscent of halides. In coordination chemistry, pseudohalides often act as ligands, binding to transition metals and forming stable complexes.

Common pseudohalide species include cyanide (CN−), azide (N3−), thiocyanate (SCN−), and cyanate (OCN−). Cyanide is a strong-field ligand and forms well-known complexes such as ferrocyanide and ferricyanide, [Fe(CN)6]n−. Thiocyanate and cyanate are ambidentate ligands that can bind through different atoms (SCN− through S or N; OCN− through O or N). The azide ion N3− is linear and typically coordinates through terminal nitrogen. These ligands enable a variety of coordination modes and bridge structures in metal complexes.

In salts, pseudohalides often display halide-like solubility and reactivity, with classic insoluble silver salts such as AgCN, AgN3, AgSCN, and AgOCN illustrating their halide-like behavior. In inorganic and organometallic synthesis, pseudohalides facilitate ligand exchange and the construction of complex structures, including cyanometalate and related species, as well as serving as counterions in various coordination compounds.

Safety and applications: cyanide compounds are highly toxic and require careful handling. Pseudohalide chemistry is central to electroplating, inorganic and coordination chemistry, and several areas of materials science and catalysis.