Cyclopentadienyl complexes were among the first organo— rare earth(III) species to be produced, and their study has burgeoned over the past half‐century due, in part, to improved handling techniques excluding oxygen and moisture. Their synthesis is commonly by means of direct replacement of smaller counterions using agents such as alkali metal cyclopentadienides. This reaction being sterically limiting, the introduction of cyclopentadienide ligands is generally sequential. The interactions between metal and ligand are dominated by electrostatics rather than covalent factors, and thus the predominant mode of bonding is with the metal ion sitting approximately on a planar ligand face, with all five C atoms interacting to approximately the same degree. Since rare earth ionic chemistry is generally electrostatically driven, other ligands may participate in the coordination sphere, according to the available remaining space. Coordination saturation seems to occur with three cyclopentadienyl ligands and two other commonly sized donor atoms such as S, even for the larger, early rare earths. Such ancillary ligands can frequently be accommodated flexibly and reversibly, the cyclopentadienyl ligands adjusting distance and orientation to compensate.
For complexes containing three cyclopentadienyl ligands, reactivity is generally limited to such reversible accommodation of ancillary ligands, or removal of ring ligands by protic reagents. For complexes containing two cyclopentadienyl ligands, increased steric space opens up a rich chemistry. Binuclear bridging species become common, and reactions involving insertion or activation of small ligand species become more common: several catalytic cycles are mediated by biscyclopentadienyl rare earth species. Single cyclopentadienyl rare earth complexes are yet more complex in their behavior because of their increased steric openness: however, they do number among them some interesting compounds of hypervalent oxygen and chlorine.
