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Onium compounds (or onium salts) are saltlike compounds containing a complex cation, in which the central atom (the term "central atom" is not entirely precise, but is often used) is of an element with nonmetallic character (nitrogen, phosphorus, oxygen, sulfur, chlorine, bromine, iodine, etc.). The onium cations in these cases are named "ammonium", "phosphonium", "oxonium", "sulfonium", 'chloronium", "bromonium" and "iodonium", respectively. Different definitions for onium compounds are given in the literature. According to the nomenclature rules of IUPAC (1),
polyatomic cations formed by adding more protons to monatomic anions than are required to give a neutral unit have the ending -onium. Substituted derivatives may be formed from the names of the basic cations, e. g. hydroxylammonium, tetramethylstibonium, dimethyloxonium (CH^sub 3^)^sub 2^OH+.
In the complex onium cation the central atom bears a positive charge. The basic onium cations (not all of them can exist independently, but organic derivatives for all of them are well known) and their systematic names are given in Table 1.
Historical Review
Ammonium salts were known in the oldest times. Some of these salts are met in the nature. Some were described and used by the alchemists. For example, "Basil Valentine" used ammonium chloride (which he called "salt-armoniac", "because it comes from Armenia") for the synthesis of antimony chloride (2).
Diazonium salts are also from the big class of the onium compounds. They contain the diazonium cation (only aryl derivatives of the basic cation, ArN^sub 2^+, shown in Table 1, are stable). Diazonium compounds were discovered by J. P. Griess by passing nitrous fumes into a solution of picramic acid (2-amino-4,6-dinitrophenol). Later Griess prepared several solid diazo compounds, including the very explosive phenyldiazonium nitrate. The name "diazogroup" was given by Griess in his first paper. The first satisfactory structural formula for the diazonium compounds was proposed by A. Kekule (1866), R*N:N*X, where X is an acid radical (3). The diazonium cation structurally is not a typical onium ion, but as A. Nesmeyanov et al. (4) propose, it can be considered as an onium ion of the nitrogen molecule.
In 1863 A. von Oefele found that ethyl iodide combines with diethylsulfide and forms a compound similar to an ammonium salt, which reacts with silver(I) oxide, giving the corresponding hydroxide:
Structure and Properties of...