Half sandwich compound

Half sandwich compounds are organometallic complexes that feature a cyclic polyhapto ligand bound to an ML_n center, where L is a unidentate ligand. Thousands of such complexes are known. Well known examples include Cyclobutadieneiron tricarbonyl and (C₅H₅)TiCl₃. Commercially useful examples include (C₅H₅)Co(CO)₂, which is used in the synthesis of substituted pyridines, and methylcyclopentadienyl manganese tricarbonyl, an antiknock agent in petrol.