In organometallic chemistry, (η-CH) piano stool compounds are half-sandwich compounds with (η-CH)ML structure (M = Cr, Mo, W, Mn(I), Re(I) and L = typically CO). (η-CH) piano stool complexes are stable 18-electron coordination compounds with a variety of chemical and material applications. Early studies on (η-CH)Cr(CO) were carried out by Natta, Ercoli and Calderazzo, and Fischer and Ofele, and the crystal structure was determined by Corradini and Allegra in 1959.   The X-ray data indicate that the plane of the benzene ring is nearly parallel to the plane defined by the oxygen atoms of the carbonyl ligands, and so the structure resembles a benzene seat mounted on three carbonyl legs tethered by the metal atom.