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ionic compounds of the general formula (Rn + 1M)+X –, where R is an organic radical or hydrogen; M represents an atom of nitrogen, phosphorus, oxygen, and so on (ammonium, phosphonium, oxonium, and other compounds); X is an anion; and n denotes the lowest valence number of M in organic compounds. An onium atom of M may be part of a heterocyclic compound—for example, a salt of pyridinium.
The stability of onium compounds depends essentially on the nature of the M atom, the X– anion, and the organic radical; most onium compounds are therefore stable in the presence of complex anions (BF4–, SbCl6–, FeCl4–, and so on). Stable onium compounds are solid saltlike substances, which are soluble in water and polar organic solvents because of their ionic nature. The well-known reaction of amines and alkyl halides is the most general example of formation of onium compounds:
R3N: + ICH3→R3(CH3)N+I–
In this reaction, the donor atom (N:) gains one positive charge and one additional covalent bond because of collectivization of the free electron pair.
Onium compounds are intermediate products in many organic reactions and are also constituents of various pharmaceuticals, neutral soaps, and fixatives (in the textile industry).