For 10 points, name these complexes of a transition metal ion with a cyclic organic system.
Examples of these compounds include the Petasis reagent and Tebbe's reagent, both of which are based on titanium, and the variety with intramolecular bridging among multiple centers, known as the ansa- types. They do not require aromaticity in their structures, as is evident by the compound formed by (*) cyclooctatetraene and uranium.  A special variant are the "piano stool" compounds, which include the manganese-based gasoline additive MMT.   Their ligands are described in terms of their hapticity with the central ion, with the canonical example being a cyclopentadienyl ligand with a hapticity of five.  For 10 points, name these complexes of a transition metal ion with a cyclic organic system.
Examples of these compounds include the Petasis reagent and Tebbe's reagent, both of which are based on titanium, and the variety with intramolecular bridging among multiple centers, known as the ansa- types.
{metallocene}s (4) [AU]