In 2005, Gagliardi and Roos published calculations supporting the presence of a quintuple phi bond between two atoms of this element, which was later shown to only be a quadruple bond. For 10 points each:
[10h] Name this element found at the center of an air-sensitive green sandwich compound notable for its E-sub-3g ground state and D8h symmetry.
ANSWER: uranium [or U]
[10m] The aforementioned D8h symmetric uranium sandwich compound is an analog of this other metallocene, in which two cyclo·penta·dienyl ligands coordinate to a central iron atom.
ANSWER: ferrocene
[10e] While uranocene does not exhibit phi bonding, it has been experimentally shown to exist between a COT ligand and thorium, another element from this radioactive series below the lanthanides.
ANSWER: actinides [or actinoids; prompt on f-block elements]
<Munir Siddiqui, Chemistry>