Uranocene

{{chembox | Verifiedfields = changed | Watchedfields = changed | verifiedrevid = 426984115 | ImageFileL1 = Uranocene-2D-skeletal.svg | ImageFileR1 = Uranocene-2D-dimensions.svg | ImageFileL2 = Uranocene-3D-vdW.png | ImageFileR2 = Uranocene-3D-balls.png | IUPACName = Bis(η⁸-cyclooctatetraene)uranium<ref>{{Cite web | title=Nomenclature of inorganic chemistry - IUPAC Recommendations 2005 | url=https://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf | archive-url=https://web.archive.org/web/20081031111951/http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf | archive-date=2008-10-31 | access-date=2025-03-29 | url-status=live }}</ref> | OtherNames = Uranium cyclooctatetraenide<br>U(COT)₂ | Section1 = {{Chembox Identifiers |CASNo_Ref = {{cascite|correct|??}} |CASNo = 11079-26-8 |ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}} |ChemSpiderID = 57575622 |PubChem = 12865934 |SMILES = C1=CC=CC=CC=C1.C1=CC=CC=CC=C1.[U] |InChI = 1/2C8H8.U/c2*1-2-4-6-8-7-5-3-1;/h2*1-8H;/q2*-2;/b2*2-1-,5-3-,8-6-; |InChIKey = RHDYKSUWBHNFEJ-GIKYMASMBM |StdInChI_Ref = {{stdinchicite|changed|chemspider}} |StdInChI = 1S/2C8H8.U/c2*1-2-4-6-8-7-5-3-1;/h2*1-8H;/q2*-2;/b2*2-1-,5-3-,8-6-; |StdInChIKey_Ref = {{stdinchicite|changed|chemspider}} |StdInChIKey = RHDYKSUWBHNFEJ-GIKYMASMSA-N }} | Section2 = {{Chembox Properties |Formula = C₁₆H₁₆U |MolarMass = 446.33 g/mol |Appearance = green crystals<ref name=synth>{{cite journal |last1= Streitwieser |first1= A. |last2= Mueller-Westerhoff |first2= U. |title= Bis(cyclooctatetraenyl)uranium (uranocene). A new class of sandwich complexes that utilize atomic f orbitals |year= 1968 |journal= J. Am. Chem. Soc. |volume= 90 |issue= 26 |pages= 7364 |doi= 10.1021/ja01028a044}}</ref> }} | Section3 = {{Chembox Hazards |MainHazards = pyrophoric, radioactive, and toxic }} }} '''Uranocene''', U(C₈H₈)₂, is an organouranium compound composed of a uranium atom sandwiched between two cyclooctatetraenide rings. It was one of the first organoactinide 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[[Cyclooctatetraene|[8]annulene]]-metal system, although it has no known practical applications.<ref name=Seyferth>{{cite journal |last= Seyferth |first= D. |title= Uranocene. The First Member of a New Class of Organometallic Derivatives of the f Elements |year= 2004 |journal= Organometallics |volume= 23 |issue= 15 |pages= 3562–3583 |doi= 10.1021/om0400705|doi-access=free }}</ref>

==Synthesis, structure and bonding== 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:<ref name=synth />

:443x443px|Synthesis of uranocene

Uranocene is highly reactive toward oxygen, being pyrophoric in air but stable to hydrolysis. The x-ray crystal structure of uranocene was first elucidated by the group of Ken Raymond.<ref>{{Cite journal|last1=Zalkin|first1=Allan|last2=Raymond|first2=Kenneth N.|date=1969|title=Structure of di-.pi.-cyclooctatetraeneuranium (uranocene)|url=https://pubs.acs.org/doi/abs/10.1021/ja01048a055|journal=Journal of the American Chemical Society|language=en|volume=91|issue=20|pages=5667–5668|doi=10.1021/ja01048a055|issn=0002-7863|url-access=subscription}}</ref> 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 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.<ref>{{cite journal |last1= Clark |first1= J. P. |last2= Green |first2= J. C. |title= An Investigation of the Electronic Structure of Bis(''eta''-cyclo-octatetraene)-actinoids by Helium-(I) and -(II) Photoelectron Spectroscopy |year= 1977 |journal= J. Chem. Soc., Dalton Trans. |issue= 5 |pages= 505–508 |doi= 10.1039/DT9770000505}}</ref> Electronic theory calculations agree with this result<ref>{{cite journal |last1= Roesch |first1= N. |last2= Streitwieser |first2= A. |title= Quasirelativistic SCF-X''alpha'' Scattered-Wave Study of Uranocene, Thorocene, and Cerocene |year= 1983 |journal= J. Am. Chem. Soc. |volume= 105 |issue= 25 |pages= 7237–7240 |doi= 10.1021/ja00363a004}}</ref><ref name=chang>{{cite journal |last1= Chang |first1= A. H. H. |last2= Pitzer |first2= R. M. |title= Electronic Structure and Spectra of Uranocene |year= 1989 |journal= J. Am. Chem. Soc. |volume= 111 |issue= 7 |pages= 2500–2507 |doi= 10.1021/ja00189a022}}</ref> 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.<ref name=chang/>

===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.<ref>{{cite journal |last1= Karraker |first1= D. G. |last2= Stone |first2= J. A. |last3= Jones |first3= E. R. |last4= Edelstein |first4= N. |title= Bis(cyclooctatetraenyl)neptunium(IV) and Bis(cyclooctatetraenyl)plutonium(IV) |year= 1970 |journal= J. Chem. Phys. |volume= 92 |issue= 16 |pages= 4841–4845 |doi= 10.1021/ja00719a014}}</ref> Its NMR spectrum is consistent with an |''M_J''| value of 3.<ref>{{cite book |last= Fischer |first= R. D. |editor1-last= Marks |editor1-first= T. J. |editor2-last= Fischer |editor2-first= R. D. |title= Volume 44 – Organometallics of the f-Elements |series= NATO Advanced Study Institutes Series: Series C – Mathematical and Physical Sciences |publisher= Reidel |location= Dordrecht, Holland |year= 1979 |pages= 337–377 |chapter= NMR Spectroscopy of Organometallic Compounds of the f-Elements: Practical Applications |isbn= 90-277-0990-4}}</ref> Electronic theory calculations from the simplest<ref>{{cite journal |last1= Hayes |first1= R. G. |last2= Edelstein |first2= N. |title= 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 |year= 1972 |journal= J. Am. Chem. Soc. |volume= 94 |issue= 25 |pages= 8688–8691 |doi= 10.1021/ja00780a008}}</ref> to the most accurate<ref>{{cite journal |last1= Liu |first1= W. |last2= Dolg |first2= M. |last3= Fulde |first3= P. |year= 1997 |title= Low-lying electronic states of lanthanocenes and actinocenes M(C₈H₈)₂ (M=Nd, Tb, Yb, U) |journal= J. Chem. Phys. |volume= 107 |issue= 9 |pages= 3584–3591 |doi= 10.1063/1.474698|bibcode= 1997JChPh.107.3584L }}</ref> also give |''M_J''| values of 3 for the ground state and 2 for the first excited state, corresponding to double-group symmetry designations<ref>{{cite book |last= Herzberg |first= G. |title= Molecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules |year= 1966 |publisher= D. Van Nostrand |location= Princeton, New Jersey |page= 566}}</ref> of ''E_3g'' and ''E_2g'' for these states.

The green color of uranocene is due to three strong transitions in its visible spectrum.<ref name=synth /><ref name=dall>{{cite journal |last1= Dallinger |first1= R. F. |last2= Stein |first2= P. |last3= Spiro |first3= T. G. |title= Resonance Raman Spectroscopy of Uranocene: Observation of an Anomalously Polarized Electronic Band and Assignment of Energy Levels |year= 1978 |journal= J. Am. Chem. Soc. |volume= 100 |issue= 25 |pages= 7865–7870 |doi= 10.1021/ja00493a013}}</ref> In addition to finding vibrational frequencies, Raman spectra indicate the presence of a low-lying (''E_2g'') excited electronic state.<ref name=dall /><ref>{{cite journal |last1= Hager |first1= J. S. |last2= Zahardis |first2= J. |last3= Pagni |first3= R. M. |last4= Compton |first4= R. N. |last5= Li |first5= J. |display-authors= 3 |title= Raman under nitrogen. The high-resolution Raman spectroscopy of crystalline uranocene, thorocene, and ferrocene |year= 2004 |journal= J. Chem. Phys. |volume= 120 |issue= 6 |pages= 2708–2718 |doi= 10.1063/1.1637586 |pmid= 15268415|bibcode= 2004JChPh.120.2708H }}</ref> On the basis of calculations,<ref name=chang/> 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, Yb, Th, Pa, Np, and Pu). Extensions include the air-stable derivative U(C₈H₄Ph₄)₂ and the cycloheptatrienyl species [U(C₇H₇)₂]⁻.<ref name=Seyferth/> In contrast, bis(cyclooctatetraene)iron has a very different structure, with one each of a η⁶- and η⁴-C₈H₈ ligands.

==References== {{reflist|30em}}

{{commons category|Uranocene|lcfirst=yes}}

==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}} *[http://www.radiochemistry.org/periodictable/la_series/A12.html Lanthanides & Actinides: Organoactinides] {{Webarchive|url=https://web.archive.org/web/20200220123255/http://www.radiochemistry.org/periodictable/la_series/A12.html |date=2020-02-20 }} {{Uranium compounds}}

{{Authority control}} Category:Organouranium compounds Category:Metallocenes Category:Uranium(IV) compounds Category:Substances discovered in the 1960s