Across the periodic table the trans-influence operates, whereby tightly bonded ligands selectively lengthen mutually trans metal–ligand bonds. Conversely, in high oxidation state actinide complexes the inverse-trans-influence operates, where normally cis strongly donating ligands instead reside trans and actually reinforce each other. However, because the inverse-trans-influence is restricted to high-valent actinyls and a few uranium(V/VI) complexes, it has had limited scope in an area with few unifying rules. Here we report tetravalent cerium, uranium and thorium bis(carbene) complexes with trans C=M=C cores where experimental and theoretical data suggest the presence of an inverse-trans-influence. Studies of hypothetical praseodymium(IV) and terbium(IV) analogues suggest the inverse-trans-influence may extend to these ions but it also diminishes significantly as the 4f orbitals are populated. This work suggests that the inverse-trans-influence may occur beyond high oxidation state 5f metals and hence could encompass mid-range oxidation state actinides and lanthanides. Thus, the inverse-trans-influence might be a more general f-block principle.

The electronic structure of f-element compounds is complex due to a combination of relativistic effects, strong electron correlation and weak crystal field environments. However, a quantitative understanding of bonding in these compounds is becoming increasingly technologically relevant. Recently, bonding interpretations based on analyses of the physically observable electronic density have gained popularity and, in this Feature Article, the utility of such density-based approaches is demonstrated. Application of Bader's Quantum Theory of Atoms in Molecules (QTAIM) is shown to elucidate many properties including bonding trends, orbital overlap and energy degeneracy-driven covalency, oxidation state identification and bond stability, demonstrating the increasingly important role that simulation and analysis play in the area of f-element bond characterisation.

We report comparable levels of covalency in cerium– and uranium–carbon multiple bonds in the iso-structural carbene complexes [M(BIPM^TMS)(ODipp)₂] [M = Ce (1), U (2), Th (3); BIPM^TMS = C(PPh₂NSiMe₃)₂; Dipp = C₆H₃-2,6-ⁱPr₂] whereas for M = Th the MC bond interaction is much more ionic. On the basis of single crystal X-ray diffraction, NMR, IR, EPR, and XANES spectroscopies, and SQUID magnetometry complexes 1–3 are confirmed formally as bona fide metal(IV) complexes. In order to avoid the deficiencies of orbital-based theoretical analysis approaches we probed the bonding of 1–3via analysis of RASSCF- and CASSCF-derived densities that explicitly treats the orbital energy near-degeneracy and overlap contributions to covalency. For these complexes similar levels of covalency are found for cerium(IV) and uranium(IV), whereas thorium(IV) is found to be more ionic, and this trend is independently found in all computational methods employed. The computationally determined trends in covalency of these systems of Ce ∼ U > Th are also reproduced in experimental exchange reactions of 1–3 with MCl₄ salts where 1 and 2 do not exchange with ThCl₄, but 3 does exchange with MCl₄ (M = Ce, U) and 1 and 2 react with UCl₄ and CeCl₄, respectively, to establish equilibria. This study therefore provides complementary theoretical and experimental evidence that contrasts to the accepted description that generally lanthanide–ligand bonding in non-zero oxidation state complexes is overwhelmingly ionic but that of uranium is more covalent.

CASSCF calculated wavefunctions are presented for three f-element metallocenes, MCOT₂ (M = Ce, Th, Pu; COT = η⁸-C₈H₈). The configurational admixture of these systems is investigated and, where the ThCOT₂ ground state is well-defined as a monodeterminantal Th(IV) state, the cerocene ground state is found to be strong multiconfigurational and to bear strong similarities to that of plutonocene. Associated electronic densities are studied using QTAIM topological analysis and compared to CASSCF-derived densities of the aromatic systems benzene and the COT dianion. This analysis provides evidence of enhanced covalent character in plutonocene, supporting structural data calculated previously. Evidence of charge localisation in found in cerocene, this being most pronounced in its excited state of A_g symmetry. QTAIM analysis reveals that the ligand electronic structure is very similar in all metallocenes, and density differences show little variation in the ligand between the cerocene ground and excited state. Orbital contributions to integrated QTAIM properties are considered, and excellent agreement with experimentally determined f-orbital occupation is obtained. All methods of analysis support a Ce(IV) or mixed valence assignment of the cerocene ground state, whereas the A_g excited state is best described as a Ce(III) state.