oxygen-containing onium compounds of the general formula R3O+X-, for example, [(C2H5)3O]+[BF4]-. Aliphatic oxonium compounds are stable only in the presence of such complex anions as BF4- and SbCl6-, while aromatic oxonium compounds can also exhibit stability with simple anions, for example, I-.
Stable oxonium compounds are saltlike solids with relatively high melting points and are soluble in polar solvents. They are strong alkylating agents and may be used to synthesize many products, including other onium compounds, for example, sulfonium compounds from sulfides. Onium compounds that contain one oxygen atom in their aromatic ring are also classed as oxonium compounds; they include salts of pyrylium, xanthylium, and flavylium.