 Ferrocene and methylcyclopentadienyl manganese tricarbonyl have been used as antiknock agents.Certain bent metallocenes of zirconium and hafnium are effective precatalysts for the polymerization of propylene.Many half sandwich complexes of ruthenium, such as those derived from (cymene)ruthenium dichloride dimer catalyse transfer hydrogenation, a useful reaction in organic synthesis.