Pentamethylcyclopentadiene gives rise to pentamethylcyclopentadienyl (Cp*) complexes.  These ligands are more basic and more lipophilic.  Replacing methyl groups with larger substituents results in cyclopentadienes that are so encumbered that pentaalkyl derivatives are no longer possible. Well-studied ligands of this type include CRH (R = iso-Pr) and 1,2,4-CRH (R = tert-Bu).