Definition of onium compounds

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1. Cations (with their counterions) derived by addition of a hydron to a mononuclear parent hydride of the nitrogen, chalcogen and halogen families. NOC Rules C-82.1, D-5.33.

H4N+ammoniumH3O+oxoniumH2F+fluoroniumH4P+phosphoniumH3S+sulfoniumH2 Cl+chloroniumH4As+arsoniumH3Se+selenoniumH2Br+bromoniumH4Sb+stiboniumH3Te+telluroniumH2I+iodoniumH4Bi+bismuthonium

2. Derivatives formed by substitution of the above parent ions by univalent groups. The number of substituted hydrogen atoms is, especially in the case of hydrocarbyl substituents, indicated by the adjectives primary, secondary, tertiary or quaternary. RNRI Rule RC-82.1.1.1. E.g. Cl2F+ dichlorofluoronium, (CH3)2S+H dimethylsulfonium (a secondary sulfonium ion), ClCH3)3P+ chlorotrimethylphosphonium, (CH3CH2)4N+ tetraethylammonium (a quaternary ammonium ion). See also quaternary ammonium compounds, phosphonium compounds, arsonium compounds, stibonium compounds, oxonium ions, sulfonium compounds, halonium ions.

3. Derivatives formed by substitution of the above parent ions by groups having two or three free valencies on the same atom. Such derivatives are, where possible, designated by a specific class name. E.g. RCO+ hydrocarbylidyne oxonium ions, R2C=N+H2 X - iminium compounds, RCNH+ nitrilium ions.