Uranocene, U(C8H8)2, is an organouranium compound composed of a uranium atom sandwiched between two cyclooctatetraenide rings. It was one of the first organoactinide compounds to be synthesized. It is a green air-sensitive solid that dissolves in organic solvents. Uranocene, a member of the "actinocenes," a group of incorporating Chemical element from the actinide series. It is the most studied bis[Cyclooctatetraene|[8annulene]]-metal system, although it has no known practical applications.
Uranocene is highly reactive toward oxygen, being pyrophoricity in air but stable to hydrolysis. The x-ray crystal structure of uranocene was first elucidated by the group of Ken Raymond. Considering the molecule to be U4+(C8H82−)2, the hapticity-cyclooctatetraenide groups are planar, as expected for a ring containing 10 π-electrons, and are mutually parallel, forming a sandwich containing the uranium atom. In the solid state, the rings are eclipsed, conferring D8h symmetry on the molecule. In solution the rings rotate with a low activation energy.
The uranium-cyclooctatetraenyl chemical bond was shown by photoelectron spectroscopy to be primarily due to mixing of uranium 6d orbitals into ligand pi orbitals and therefore donation of electronic charge to the uranium, with a smaller such interaction involving the uranium ( 5f)2 orbitals. Electronic theory calculations agree with this result and point out that the weaker interaction of the open-shell 5f orbitals with the ligand orbitals determines | MJ|, the magnitude of the angular momentum quantum number along the 8-fold symmetry axis of the ground state.
The green color of uranocene is due to three strong transitions in its visible spectrum. In addition to finding vibrational frequencies, Raman spectra indicate the presence of a low-lying ( E2g) excited electronic state. On the basis of calculations, the visible transitions are assigned to transitions primarily of 5f-to- 6d nature, giving rise to E2u and E3u states.
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