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 (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. MMT-2D-skeletal.png|MMT is a commercially useful antiknock compound. Cpco(CO)2.png|CpCo(CO)2 is a catalyst for the synthesis of pyridines. Cyclobutadienyl-iron-tricarbonyl-from-xtal-3D-balls.png|(C4H4)Fe(CO)3. Cp2Fe(CO)2I-2D-skeletal.png|[[Cyclopentadienyliron dicarbonyl iodide|CpFe(CO)2I]] is an example of a piano stool complex with two different monodentate ligands. RuCymCl2.png|The diruthenium of [[cymene]] is readily cleaved by ligands to give monoRu half-sandwich derivatives. CHTMo(CO)3.png|[[Cycloheptatrienemolybdenum tricarbonyl|Cycloheptatriene molybdenum tricarbonyl]] CPPCDV01.png|Cp2V2(CO)5 featuring a pair of semi-bridging CO ligands.{{cite journal|last1=Huffman |first1=J. C. |last2=Lewis |first2=L. N. |last3=Caulton |first3=K. G.|title=A donor semibridge? Molecular structures of dicyclopentadienyldivanadiumtetracarbonyltriphenylphosphine and dicyclopentadienyldivanadiumpentacarbonyl|journal=Inorganic Chemistry|year=1980|volume=19|issue=9|pages=2755–2762|doi=10.1021/ic50211a052}} Half sandwich complexes containing cyclopentadienyl ligands are common. Well studied examples include (η5-C5H5)V(CO)4, (η5-C5H5)Cr(CO)3H, (η5-CH3C5H4)Mn(CO)3, (η5-C5H5)Cr(CO)3H, [(η5-C5H5)Fe(CO)3]+, (η5-C5H5)V(CO)4I, and (η5-C5H5)Ru(NCMe). (η5-C5H5)Co(CO)2 is a two-legged piano stool complex. Bulky cyclopentadienyl ligands such as 1,2,4-C5H2(tert-Bu)3− form unusual half-sandwich complexes.