Uranocene was first described in 1968 by the group of
Andrew Streitwieser, when it was prepared by the reaction of
dipotassium cyclooctatetraenide and
uranium tetrachloride in THF at 0°C: Considering the molecule to be U4+(C8H82−)2, the
η8-
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
bonding 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. Its
NMR spectrum is consistent with an |
MJ| value of 3. Electronic theory calculations from the simplest to the most accurate also give |
MJ| values of 3 for the ground state and 2 for the first excited state, corresponding to double-group symmetry designations of
E3g and
E2g for these states. 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. ==Analogous compounds==