What is Half sandwich compound?

Half sandwich compounds, also known as piano stool complexes, are organometallic complexes that feature a cyclic polyhapto ligand bound to an MLn center, where L is a unidentate ligand. Thousands of such complexes are known. Well-known examples include cyclobutadieneiron tricarbonyl and TiCl3. Commercially useful examples include Co2, which is used in the synthesis of substituted pyridines, and methylcyclopentadienyl manganese tricarbonyl, an antiknock agent in petrol.

MMT is a commercially useful antiknock compound.

CpCo2 is a catalyst for the synthesis of pyridines.

Fe3.