Benzannulated homologues of cyclopentadienide as ligands in organometallic chemistry

• NICS are introduced as a means to evaluate the ligand properties of polycyclic Cp-anions.
• Synthetic methods to access various benzannulated Cp-derivatives are discussed.
• Transition metals complexes of benzannulated Cp-derivatives are reviewed.

Organometallic coordination chemistry is dominated by the pervasive cyclopentadienide (Cp−) ligand and its benzannulated derivatives indenide (Ind−) and fluorenide (Flu−). However, in the past decades a diverse coordination chemistry involving higher benzannulated homologues has developed. In this review, the synthesis and electronic structure of π-extended Cp-ligands will be discussed, as well as the factors that affect complex stability. An overview of known transition metal complexes containing both ‘classic’ benzannulated Cp-ligands, and ‘non-classic’ ligands derived from polycyclic arenes will be given.