Aquo ions of divalent metal ions are less acidic than those of trivalent cations. Although usually trivalent, europium readily forms divalent compounds. Although usually trivalent, ytterbium readily forms divalent compounds. Metal cations that bind RNA can be monovalent, divalent or trivalent. This is typical of all redox reactions where hexavalent chromium is reduced to trivalent chromium. The trivalent lanthanides mostly form ionic salts. The most common mixed chromates include hexavalent iridescent, trivalent or black trivalent chromate. With trivalent metal ions, alums such as ferric ammonium sulfate are formed. This is an issue because hexavalent chromium can cause cancer, whereas trivalent chromium does not. Samarium forms trivalent sulfide, selenide and telluride. Europium compounds tend to exist trivalent oxidation state under most conditions. Most coordination and organometallic complexes contain terbium in the trivalent oxidation state. In some cases, solid solutions of alums with different monovalent and trivalent cations may occur. An oxonium ion is any ion with a trivalent oxygen cation. Bismuth forms trivalent and pentavalent compounds, the trivalent ones being more common.