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| Names | |||
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| IUPAC name Bis(η8-cyclooctatetraenyl)uranium(IV) | |||
| Other names Uranium cyclooctatetraenide U(COT)2 | |||
| Identifiers | |||
3D model (JSmol) | |||
| ChemSpider |
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| Properties | |||
| C16H16U | |||
| Molar mass | 446.33 g/mol | ||
| Appearance | green crystals[1] | ||
| Hazards | |||
| Occupational safety and health (OHS/OSH): | |||
Main hazards | pyrophoric, radioactive, and toxic | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Uranocene, U(C8H8)2, is anorganouranium compound composed of a uranium atom sandwiched between twocyclooctatetraenide rings. It was one of the firstorganoactinide compounds to be synthesized. It is a green air-sensitive solid that dissolves in organic solvents. Uranocene, a member of the "actinocenes," a group ofmetallocenes incorporatingelements from theactinide series. It is the most studied bis[8]annulene-metal system, although it has no known practical applications.[2]
Uranocene was first described in 1968 by the group ofAndrew Streitwieser, when it was prepared by the reaction ofdipotassium cyclooctatetraenide anduranium tetrachloride in THF at 0°C:[1]
Uranocene is highly reactive toward oxygen, beingpyrophoric in air but stable tohydrolysis. The x-ray crystal structure of uranocene was first elucidated by the group ofKen Raymond.[3] Considering the molecule to be U4+(C8H82−)2, theη8-cyclooctatetraenide groups are planar, as expected for a ring containing 10π-electrons, and are mutually parallel, forming asandwich containing theuranium atom. In the solid state, the rings are eclipsed, conferringD8h symmetry on the molecule. In solution the rings rotate with a lowactivation energy.
The uranium-cyclooctatetraenylbonding was shown byphotoelectron spectroscopy to be primarily due to mixing of uranium 6d orbitals into ligand piorbitals and therefore donation of electronic charge to the uranium, with a smaller such interaction involving the uranium (5f)2 orbitals.[4] Electronic theory calculations agree with this result[5][6] and point out that the weaker interaction of the open-shell5f 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.[6]
Uranocene isparamagnetic. Itsmagnetic susceptibility is consistent with values of 3 or 4 for |MJ|, with the accompanying magnetic moment being affected by thespin-orbit coupling.[7] ItsNMR spectrum is consistent with an |MJ| value of 3.[8] Electronic theory calculations from the simplest[9] to the most accurate[10] also give |MJ| values of 3 for the ground state and 2 for the first excited state, corresponding to double-group symmetry designations[11] ofE3g andE2g for these states.
The green color of uranocene is due to three strong transitions in itsvisible spectrum.[1][12] In addition to finding vibrational frequencies,Raman spectra indicate the presence of a low-lying (E2g) excited electronic state.[12][13] On the basis of calculations,[6] the visible transitions are assigned to transitions primarily of5f-to-6d nature, giving rise toE2u andE3u states.
Analogous compounds of the form M(C8H8)2 exist for M = (Nd,Tb,Yb,Th,Pa,Np, andPu). Extensions include the air-stable derivative U(C8H4Ph4)2 and thecycloheptatrienyl species [U(C7H7)2]−.[2] In contrast,bis(cyclooctatetraene)iron has a very different structure, with one each of a η6- and η4-C8H8 ligands.
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