Changing the geometry of a metallocene complex from linear to bent or vice versa is of importance for the control of physicochemical properties and/or reactivity and for the evaluation of the covalency and steric interactions in the metal-cyclopentadienyl bonding.
Whereas the biscyclopentadienyl compounds of the f-elements (lanthanides and actinides) form a widespread series of compounds, these metallocenes are found exclusively in a bent-sandwich configuration, whatever the 4f or 5f ion, the oxidation state, the electronic charge, and the nature and number of auxiliary ligands. In contrast to the trivalent lanthanides, we found that linear “(C5Me5)2Mf” species can be obtained with uranium and their formation is strongly related to the nature of the counter-anion, the solvent, the number of 5f electrons in the available 5f-orbitals.1 Such linear “(C5Me5)2U” complexes give rise to a new class of metallocenes by the full filling of the equatorial girdle with appropriate neutral or anionic ligands and offer interest for their reactivity.2
In the opposite, the linear sandwich [(C8H8)2Mf]q− complexes (Mf = actinides and lanthanides; q = 0, 1) viewed as the ferrocene analogues of the f-elements, are always found in the D8h symmetry and the numerous studies mainly carried out on the mythical uranocene, eg (C8H8)2U, suggested these compound to be chemically relatively inert and devoid of any coordination chemistry due to the steric saturation imposed by the two C8H82− ligands around the metal center. We now have broken this mind by reporting that not only bending of (C8H8)2U, the most stable of the actinocenes, is possible by choosing appropriate ligand,3 but also that the nature of the actinide center is crucial.
Actinocenes may also display unexpected reactivity as exemplified with the reductive formation of polyoxo U(IV) aggregates from uranocene and the robust uranyl(VI) ion UO22+.4
 An Unprecedented Type of Linear Metallocene with an f-Element,
J. Maynadié, J.-C. Berthet, P. Thuéry, M. Ephritikhine, J. Am. Chem. Soc., 2006, 128, 1082;
Bent and linear uranium(IV) metallocenes with terminal and bridging cyanide ligands,
J. Maynadié, J.-C. Berthet, P. Thuéry, M. Ephritikhine, Organometallics, 2007, 26, 4585;
Cyanide metallocenes of trivalent f-elements,
J. Maynadié, J.- C. Berthet, P. Thuéry, M. Ephritikhine, Organometallics, 2007, 26, 2623;
Linear uranium metallocenes with polydentate aromatic nitrogen ligands,
J.- C. Berthet, J. Maynadié, P. Thuéry, M. Ephritikhine, Dalton Trans., 2010 39, 6801.
 The first cyclopentadienyl complex of uranyl,
J. Maynadié, J. -C. Berthet, P. Thuéry , M. Ephritikhine, Chem. Commun., 2007, 486.
 Bending of “Uranocene” ((η8-C8H8)2U): Synthesis and Crystal Structure of the Cyanido Complex [(η8-C8H8)2U(CN)][NEt4],
J.-C. Berthet, P. Thuéry, M. Ephritikhine, Organometallics, 2008, 27, 1664.
 Formation of uranium(IV) oxide clusters from uranocene [U(η8-C8H8)2] and uranyl [UO2X2] compounds,
J.- C. Berthet, P. Thuéry, M. Ephritikhine, Inorg. Chem., 2010, 49, 8173.
Contact : J.C. Berthet.