text: A pair of cyclopentadienyl ligands can be covalently linked giving rise to so-call ansa metallocenes.  The angle between the two Cp rings is fixed.  Rotation of the rings about the metal-centroid axis is stopped as well.  A related class of derivatives give rise to the constrained geometry complexes.  In these cases, a Cp ligand as linked to a non-Cp ligand.  Such complexes have been commercialized for the production of polypropylene.