biscyclopentadienylcobaltII

Biscyclopentadienyl refers to complexes in which two cyclopentadienyl ligands coordinate to a single metal center. Each cyclopentadienyl (Cp) is a five-membered aromatic anion, C5H5−, that binds in an η5 fashion and donates six electrons to the metal. The resulting motifs are collectively called bis(cyclopentadienyl) complexes or Cp2M.

The most famous examples are metallocenes, in which the metal is sandwiched between two Cp rings, as in ferrocene, Fe(Cp)2, and nickelocene, Ni(Cp)2. Similar Cp2M units occur with many transition metals, including ruthenium, cobalt, chromium, and zirconium, sometimes with additional ligands such as chlorides (e.g., Cp2ZrCl2). Substituted Cp ligands (e.g., Cp*, Cp^tBu, etc.) are common and modify steric and electronic properties.

Synthesis typically proceeds by forming the cyclopentadienide salt (e.g., NaCp) and reacting with a suitable metal precursor, such as a dichloride or halide, e.g., 2 NaCp + FeCl2 -> FeCp2 + 2 NaCl. Alternative routes involve metathesis or direct metalation of Cp− ligands.

Electronic considerations: Cp ligands, each counted as a 6-electron donor, often give Cp2M complexes an 18-electron configuration, contributing to stability in many cases, though electron counts vary with oxidation state and ligands.

Applications: Cp2M compounds serve as precursors to catalysts for olefin polymerization and as building blocks in organometallic chemistry. The field of metallocene chemistry expanded after the discovery of ferrocene in the 1950s.