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 (COT²⁻, 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 COT²⁻ 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	1.933 (calculated)[3]	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
Berkelocene[note 1]	Bk(C14H16)2	Bk	2025	indigo	1.88	P1

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][5] Other actinocenes that have been synthesised are protactinocene^[6] (Pa(C₈H₈)₂), thorocene^[7] (Th(C₈H₈)₂), neptunocene^[8] (Np(C₈H₈)₂), and plutonocene^[9][10] (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.^[9][10] Berkelocene (with a modified COT ligand) was synthesised in 2025, the first actinocene with a new actinide in over 50 years.^[11]

Bonding

The actinide-cyclooctatetraenyl bonding has been of interest for multiple theoretical studies.^[10][12] 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.^[12] The covalent component is characterised by donation of electron density to the actinide. This donation is notably reduced in Berkelocene relative to other characterized actinocenes, due to the stable 5f⁷ electron configuration of Bk⁴⁺.^[11]

Related compounds

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).^[5]

See also

• Organoactinide chemistry
• f-block metallocene

Notes

1. Berkelocene was prepared with an annulated COT ligand, hexahydrodicyclopenta[8]annulene (hdcCOT); its properties are not strictly comparable to the other COT-ligated Actinocenes in this table, but rather those also prepared with hdcCOT.^[4]