Uranocene

Uranocene

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Names
IUPAC name Bis(η8-cyclooctatetraenyl)uranium(IV)
Other names Uranium cyclooctatetraenide U(COT)2
Identifiers
CAS Number	11079-26-8 Y
ChemSpider	122783 N
Jmol 3D model	Interactive image
PubChem	139204
SMILES [U].[CH-]1[CH-][CH-][CH-][CH-][CH-][CH-][CH-]1.[CH-]1[CH-][CH-][CH-][CH-][CH-][CH-][CH-]1
Properties
Chemical formula	C16H16U
Molar mass	446.33 g/mol
Appearance	green crystals
Vapor pressure	{{{value}}}
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Uranocene, U(C₈H₈)₂, is an organouranium compound composed of a uranium atom sandwiched between two cyclooctatetraenide rings. It was one of the first organouranium 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 metallocenes incorporating elements from the actinide series. It is the most studied bis[8]annulene-metal system, although it has no practical applications.

Synthesis, structure and bonding

Uranocene was first described in 1968, when it was prepared by the reaction of dipotassium cyclooctatetraenide and uranium tetrachloride in THF at 0°C:

Synthesis of uranocene

Uranocene is highly reactive toward oxygen, being pyrophoric in air but stable to hydrolysis. 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 D_8h symmetry on the molecule. In solution the rings rotate with a low energy barrier.

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 |M_J|, the magnitude of the angular momentum quantum number along the 8-fold symmetry axis of the ground state.

Spectroscopic properties

Uranocene is paramagnetic. Its magnetic susceptibility is consistent with values of 3 or 4 for |M_J|, with the accompanying magnetic moment being affected by the spin-orbit coupling. Its NMR spectrum is consistent with an |M_J| value of 3. Electronic theory calculations from the simplest to the most accurate also give |M_J| values of 3 for the ground state and 2 for the first excited state, corresponding to double-group symmetry designations of E_3g and E_2g 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 (E_2g) excited electronic state. On the basis of calculations, the visible transitions are assigned to transitions primarily of 5f-to-6d nature, giving rise to E_2u and E_3u states.

Analogous compounds

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]⁻. In contrast, bis(cyclooctatetraene)iron has a very different structure, with one each of a η⁶- and η⁴-C₈H₈ ligands.

Further reading

• The f elements, Nikolas Kaltsoyannis and Peter Scott. ISBN 0-19-850467-5
• Chemistry of the Elements, N. N. Greenwood and A. Earnshaw. ISBN 0-08-022057-6
• Lanthanides & Actinides: Organoactinides