This element is mixed in its +2 and +3 oxidation states in magnetite.
It's not magnesium, but the triÂ­aceto∙acetonate compound of this element can be used to catalyze the Kumada coupling, where it forms an "inorganic Grignard" ("greenÂ­YARD") species. After binding oxygen, a peroxide bridge forms between this element and tetrahydro∙biopterin, which is transformed into an activated oxo compound where this element is at its +4 oxidation state; that occurs in the mechanism for phenylalanine hydroxylase. When reacted with CO, this element forms a pentacarbonyl compound where it has oxidation state zero. It's not (*) aluminum, but this element's trihalide salts are used as Lewis acid catalysts in the halogenation and FriedelÂ–Crafts reactions of arenes. The powdered oxide of this element is a catalyst in the Haber process. Austenite is phase on the binary phase diagram between this element and carbon. This element is mixed in its +2 and +3 oxidation states in magnetite. For 10 points, name this element complexed by the heme in hemoglobin. 
It's not magnesium, but the triÂ­aceto∙acetonate compound of this element can be used to catalyze the Kumada coupling, where it forms an "inorganic Grignard" ("greenÂ­YARD") species.
iron