Actinocene

Actinocenes are a family of organoactinide compounds consisting of metallocenes containing elements from the actinide series. They typically have a sandwich structure with two dianionic cyclooctatetraenyl ligands (COT^2-, which is C
₈H²⁻
₈) bound to an actinide-metal center (An) in the oxidation state IV, resulting in the general formula An(C₈H₈)₂.^[1][2]

Skeletal formula

Ball-and-stick

Space-filling

Uranocene molecular representations showing the U atom sandwiched between 2 COT^2- ligands

Characterised actinocenes

Name	Formula	AnIV centre	First synthesis	Crystal colour	An–COT distance (Å)	Space group
Thorocene	Th(C8H8)2	Th	1969	bright yellow	2.004	P21/n
Protactinocene	Pa(C8H8)2	Pa	1974	yellowish	–	P21/n
Uranocene	U(C8H8)2	U	1968	deep green	1.926	P21/n
Neptunocene	Np(C8H8)2	Np	1970	yellow-brown	1.909	P21/n
Plutonocene	Pu(C8H8)2	Pu	1970	dark red	1.898	I2/m

The most studied actinocene is uranocene, U(C₈H₈)₂, which in 1968 was the first member of this family to be synthesised and is still viewed as the archetypal example.^[2][3] Other actinocenes that have been synthesised are protactinocene^[4] (Pa(C₈H₈)₂), thorocene^[5] (Th(C₈H₈)₂), neptunocene^[6] (Np(C₈H₈)₂), and plutonocene^[7][8] (Pu(C₈H₈)₂). Especially the latter two, neptunocene and plutonocene, have not been extensively studied experimentally since the 1980s because of the radiation hazard they pose.^[7][8]

Bonding

The actinide-cyclooctatetraenyl bonding has been of interest for multiple theoretical studies.^[8][9] Computational chemistry methods indicate bonding with a large covalent character resulting mainly from the mixing of actinide 6d orbitals with ligand π-orbitals, with a smaller interaction involving the actinide 5f and ligand π-orbitals.^[9] The covalent component is characterised by donation of electron density to the actinide.

Analogous sandwiched M(C₈H₈)₂ compounds also exist for lanthanides M = Nd, Tb, and Yb, but therein the bonding is mostly ionic rather than covalent (see lanthanocenes).^[3]

See also

• Organoactinide chemistry
• f-block metallocene

References

1. Minasian, Stefan G.; Keith, Jason M. (2014). "New evidence for 5f covalency in actinocenes determined from carbon K-edge XAS and electronic structure theory". Chem. Sci. 5 (1): 351–359. doi:10.1039/C3SC52030G.
2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements. Butterworth-Heinemann. pp. 1278–1280. ISBN 978-0-08-037941-8.
3. 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.
4. Goffart, J.; Fuger, J.; Brown, D.; Duyckaerts, G. (1974-05-01). "On the cyclooctatetraenyl compounds of actinide elements part II. Bis-(cyclooctatetraenyl) protactinium(IV)". Inorganic and Nuclear Chemistry Letters. 10 (5): 413–419. doi:10.1016/0020-1650(74)80119-4. ISSN 0020-1650.
5. Avdeef, Alex; Raymond, Kenneth N.; Hodgson, Keith O.; Zalkin, Allan (1972-05-01). "Two isostructural actinide .pi. complexes. Crystal and molecular structure of bis(cyclooctatetraenyl)uranium(IV), U(C8H8)2, and bis(cyclooctatetraenyl)thorium(IV), Th(C8H8)2". Inorganic Chemistry. 11 (5): 1083–1088. doi:10.1021/ic50111a034. ISSN 0020-1669.
6. De Ridder, D. J. A.; Rebizant, J.; Apostolidis, C.; Kanellakopulos, B.; Dornberger, E. (1996-03-15). "Bis(cyclooctatetraenyl)neptunium(IV)". Acta Crystallographica Section C: Crystal Structure Communications. 52 (3): 597–600. doi:10.1107/S0108270195013047. ISSN 0108-2701.
7. Windorff, Cory J.; Sperling, Joseph M.; Albrecht-Schönzart, Thomas E.; Bai, Zhuanling; Evans, William J.; Gaiser, Alyssa N.; Gaunt, Andrew J.; Goodwin, Conrad A. P.; Hobart, David E.; Huffman, Zachary K.; Huh, Daniel N. (2020-09-21). "A Single Small-Scale Plutonium Redox Reaction System Yields Three Crystallographically-Characterizable Organoplutonium Complexes". Inorganic Chemistry. 59 (18): 13301–13314. doi:10.1021/acs.inorgchem.0c01671. ISSN 0020-1669. OSTI 1680020. PMID 32910649. S2CID 221623763.
8. Apostolidis, Christos; Walter, Olaf; Vogt, Jochen; Liebing, Phil; Maron, Laurent; Edelmann, Frank T. (2017-04-24). "A Structurally Characterized Organometallic Plutonium(IV) Complex". Angewandte Chemie International Edition. 56 (18): 5066–5070. doi:10.1002/anie.201701858. PMC 5485009. PMID 28371148.
9. Kerridge, Andrew (2014). "f-Orbital covalency in the actinocenes (An = Th–Cm): multiconfigurational studies and topological analysis" (PDF). RSC Advances. 4 (24): 12078–12086. Bibcode:2014RSCAd...412078K. doi:10.1039/C3RA47088A.