Uranocene

Uranocene U(C8H8)2 is the most notable cyclooctatetraenide of the f elements and one of the first organouranium compounds to be synthesized. Uranocene is a member of the actinocenes, a group of metallocenes incorporating elements from the actinide series. It is the most studied bis[8]annulene-metal system.

Synthesis
Uranocene was first prepared by the reaction of uranium tetrachloride and dipotassium cyclooctatetraene, viz.
 * 2K + C8H8 → K2(C8H8)
 * 2K2(C8H8) + UCl4 → U(C8H8)2 + 4KCl.

Physical and chemical properties
Uranocene is paramagnetic, pyrophoric, and stable to hydrolysis. The η8-cyclooctatetraenyl 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 uranocene molecule. In solution the rings rotate with a low energy barrier.

Uranium-COT bond
The nature of the uranium-cyclooctatetraenyl bond is the subject of continuing research and debate. UV-PES indicates the bonding in uranocene has contributions from 5f and 6d orbitals.

Analogous compounds
Some examples of analogous compounds of the form M(C8H8)2 exist for M = (Nd, Tb, Pu, Pa, Np, Th, and Yb). Extensions include the air-stable derivative U(C8H4Ph4)2 and the cycloheptatrienyl species [U(C7H7)2]−.