Onium ion
From MS Terms
Positively charged higher-valency (higher-coordination) ion of the nonmetallic elements. More common types are the carbonium, oxonium, sulfonium, nitronium, diazonium, phosphonium, and halonium ions. Classical examples are the methonium ion CH5+, the hydrogenonium ion H3+ and and the hydronium ion H3O+. Onium ions are not limited to monopositive ions, and multiply-charged onium ions exist such as the gitonic (proximal) oxonium dication 4O2+ and the distonic (distant) oxonium dication H2O+-CH2-CH2-OH2+.