Some Chemistry of Pentakis(methoxycarbonyl)cyclopentadiene, HC5(CO2Me)5, and Related Molecules

This article summarizes the results of investigations into the chemistry of HC5(CO2Me)5 and, in particular, of metal complexes containing the C5(CO2Me)5 ligand . As an anion, the ligand is very stable, forming air-stable, water-soluble salts with many cations with coordination to the metal atom in the solid state generally occurring through the ester carbonyl groups. Second- and third-row transition metals form complexes which retain the covalent ligand-metal bond in solution, 'harder' metals coordinating by the ester carbonyl groups, while 'softer' metals are bound to the ring carbons; a variety of behaviour is shown by the Group 11 metals. Even when the ligand is η5-bonded to the metal, ready displacement by other ligands may occur, as found with Ru (η-C5H5){η5-C5(CO2Me)5}, for example. In the rhodium system, formal replacement of CO2Me groups by hydrogen is found, as with the formation of [ Rh {η5-C5H2(CO2Me)3}2][C5(CO2Me)5]. Brief mention is made of other polysubstituted cyclopentadienyls with electron-withdrawing ligands and some related compounds, and their metal derivatives where known.