Uranocene

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Uranocene
Uranocene-2D-skeletal.png Uranocene-2D-dimensions.png
Uranocene-3D-vdW.png Uranocene-3D-balls.png
Identifiers
CAS number 11079-26-8 YesY
PubChem 139204
Jmol-3D images Image 1
Properties
Molecular formula C16H16U
Molar mass 446.33 g/mol
Appearance green crystals[1]
Hazards
Main hazards ignites in air
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Uranocene U(C8H8)2 is the most notable cyclooctatetraenide of the f-block 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[edit]

Uranocene was first prepared by the reaction of uranium tetrachloride and dipotassium cyclooctatetraenide, via.

2 K + C8H8 → K2(C8H8)
2 K2(C8H8) + UCl4 → U(C8H8)2 + 4 KCl.[1]

Structure and chemical properties[edit]

Uranocene is pyrophoric, and 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 D8h symmetry on the uranocene molecule. In solution the rings rotate with a low energy barrier.

Magnetic properties and angular momentum[edit]

Uranocene is paramagnetic. Its measured magnetic susceptibility was found to be consistent with values of 3 or 4 for the magnitude of the (approximate) quantum number for angular momentum around the 8-fold axis.[2] The NMR spectrum of uranocene was analyzed and found to be consistent with an angular momentum quantum number of 3.[3] Electronic theory calculations from the simplest[4] to the most accurate[5] have always given angular momentum quantum numbers of 3 for the ground state and 2 for the first excited state, corresponding to symmetry designations[6] of E3g and E2g for these states.

Visible spectrum[edit]

The green color of uranocene is due to three strong transitions in its visible spectrum.[1][7] Raman spectra, in addition to finding vibrational frequencies, found the low-lying excited electronic state.[7][8] It was noted that one of the visible bands originated in this excited state rather than the ground state.[7] Electronic theory calculations showed that the visible transitions are to two excited states, with symmetry labels E2u and E3u, both of which have major components with 5f-to-6d orbital excitation with smaller components of ligand-pi-to-metal-5f excitation.[9]

Uranium-COT bond[edit]

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 orbitals.[10] Electronic theory calculations agree with this result[11] and point out that the weaker interaction of the open-shell 5f orbitals with the ligand orbitals determines the angular momentum quantum number of the ground state.[9]

Analogous compounds[edit]

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].[12]

References[edit]

  1. ^ a b c Streitwieser, A.; Mueller-Westerhoff, U. (1968). "Bis(cyclooctatetraenyl)uranium (uranocene). A new class of sandwich complexes that utilize atomic f orbitals". J. Am. Chem. Soc. 90 (26): 7364–7364. doi:10.1021/ja01028a044. 
  2. ^ Karraker, D. G.; Stone, J. A.; Jones, E. R.; Edelstein, N. (1970). "Bis(cyclooctatetraenyl)neptunium(IV) and Bis(cyclooctatetraenyl)plutonium(IV)". J. Chem. Phys. 92 (16): 4841–4845. doi:10.1021/ja00719a014. 
  3. ^ Fischer, R. D. (1979). "NMR Spectroscopy of Organometallic Compounds of the f-Elements: Practical Applications". In Marks, T. J.; Fischer, R. D. Volume 44 – Organometallics of the f-Elements. NATO Advanced Study Institutes Series: Series C – Mathematical and Physical Sciences. Dordrecht, Holland: Reidel. pp. 337–377. ISBN 90-277-0990-4. 
  4. ^ Hayes, R. G.; Edelstein, N. (1972). "An Elementary Molecular Orbital Calculation on U(C8H8)2 and Its Application to the Electronic Structure of U(C8H8)2, Np(C8H8)2. and Pu(C8H8)2". J. Am. Chem. Soc. 94 (25): 8688–8691. doi:10.1021/ja00780a008. 
  5. ^ Liu, W.; Dolg, M.; Fulde, P. (1997). "Low-lying electronic states of lanthanocenes and actinocenes M(C8H8)2 (M=Nd, Tb, Yb, U)". J. Chem. Phys. 107 (9): 3584–3591. doi:10.1063/1.474698. 
  6. ^ Herzberg, G. (1966). Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules. Princeton, New Jersey: D. Van Nostrand. p. 566. 
  7. ^ a b c Dallinger, R. F.; Stein, P.; Spiro, T. G. (1978). "Resonance Raman Spectroscopy of Uranocene: Observation of an Anomalously Polarized Electronic Band and Assignment of Energy Levels". J. Am. Chem. Soc. 100 (25): 7865–7870. doi:10.1021/ja00493a013. 
  8. ^ Hager, J. S.; Zahardis, J.; Pagni, R. M.; Compton, R. N.; Li, J. (2004). "Raman under nitrogen. The high-resolution Raman spectroscopy of crystalline uranocene, thorocene, and ferrocene". J. Chem. Phys. 120 (6): 2708–2718. doi:10.1063/1.1637586. PMID 15268415. 
  9. ^ a b Chang, A. H. H.; Pitzer, R. M. (1989). "Electronic Structure and Spectra of Uranocene". J. Am. Chem. Soc. 111 (7): 2500–2507. doi:10.1021/ja00189a022. 
  10. ^ Clark, J. P.; Green, J. C. (1977). "An Investigation of the Electronic Structure of Bis(eta-cyclo-octatetraene)-actinoids by Helium-(I) and -(II) Photoelectron Spectroscopy". J. Chem. Soc., Dalton Trans. (5): 505–508. doi:10.1039/DT9770000505. 
  11. ^ Roesch, N.; Streitwieser, A. (1983). "Quasirelativistic SCF-Xalpha Scattered-Wave Study of Uranocene, Thorocene, and Cerocene". J. Am. Chem. Soc. 105 (25): 7237–7240. doi:10.1021/ja00363a004. 
  12. ^ Seyferth, D. (2004). "Uranocene. The First Member of a New Class of Organometallic Derivatives of the f Elements". Organometallics 23 (15): 3562–3583. doi:10.1021/om0400705. 

Further reading[edit]