Definitions for onium compounds
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U.S. National Library of Medicine
Ions with the suffix -onium, indicating cations with coordination number 4 of the type RxA+ which are analogous to QUATERNARY AMMONIUM COMPOUNDS (H4N+). Ions include phosphonium R4P+, oxonium R3O+, sulfonium R3S+, chloronium R2Cl+
In chemistry, an onium compound, or onium ion, is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen, chalcogen, or halogen. The oldest-known onium ion, and the namesake for the class, is ammonium, NH+ 4, the protonated derivative of ammonia, NH 3. The name onium is also used for cations that would result from the substitution of hydrogen atoms in those ions by other groups, such as organic radicals, or halogens; such as tetraphenylphosphonium, 4P+ . The substituent groups may be di- or trivalent, yielding ions such as iminium and nitrilium. Onium ions have a charge of +1. A molecule with two or more onium ion subgroups is said to be a double onium ion, triple onium ion, and so on. Compounds of an onium cation and some other negative ion are known as onium salts. Onium compounds are inversely analogous to ate complexes. Lewis bases form onium ions when the central atom gains one more bond and becomes a positive cation. Lewis acids form -ate ions when the central atom gains one more bond and becomes a negative anion.
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"onium compounds." Definitions.net. STANDS4 LLC, 2015. Web. 30 Nov. 2015. <http://www.definitions.net/definition/onium compounds>.