pentamethylcyclopentadienide

Pentamethylcyclopentadienide refers to the anion derived from pentamethylcyclopentadiene, C5Me5−, and is most commonly encountered as the pentamethylcyclopentadienyl ligand (Cp*−) in organometallic chemistry. The five methyl groups on the cyclopentadienyl ring make the ligand highly electron-donating and sterically bulky compared with unsubstituted cyclopentadienyl. As an η5-bound ligand, Cp*− delivers a six-electron donor set to a metal center and stabilizes high oxidation states and crowded coordination environments.

Preparation generally involves deprotonation of pentamethylcyclopentadiene (H-C5Me5) with a strong base, giving salts such as Li[C5Me5], Na[C5Me5], or K[C5Me5], which can then participate in salt metathesis to form metal Cp* complexes. The pentamethylcyclopentadienide ligand is also generated in situ when Cp* ligands are introduced to metal precursors.

In coordination chemistry, Cp*− is one of the most widely used cyclopentadienyl ligands. It supports a broad range of complexes with transition metals and is central to many homogeneous catalytic systems, including hydrogenation, hydrofunctionalization, and various cross-coupling reactions. The bulky methyl substituents often impart enhanced stability to reactive intermediates and influence catalytic selectivity.

See also: cyclopentadienyl ligands, Cp−, Cp*, organometallic chemistry.