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The duality of electron localization and covalency in lanthanide and actinide metallocenes

  • Author(s): Smiles, DE
  • Batista, ER
  • Booth, CH
  • Clark, DL
  • Keith, JM
  • Kozimor, SA
  • Martin, RL
  • Minasian, SG
  • Shuh, DK
  • Stieber, SCE
  • Tyliszczak, T
  • et al.
Abstract

Previous magnetic, spectroscopic, and theoretical studies of cerocene, Ce(C H ) , have provided evidence for non-negligible 4f-electron density on Ce and implied that charge transfer from the ligands occurs as a result of covalent bonding. Strong correlations of the localized 4f-electrons to the delocalized ligand π-system result in emergence of Kondo-like behavior and other quantum chemical phenomena that are rarely observed in molecular systems. In this study, Ce(C H ) is analyzed experimentally using carbon K-edge and cerium M -edge X-ray absorption spectroscopies (XAS), and computationally using configuration interaction (CI) calculations and density functional theory (DFT) as well as time-dependent DFT (TDDFT). Both spectroscopic approaches provide strong evidence for ligand → metal electron transfer as a result of Ce 4f and 5d mixing with the occupied C 2p orbitals of the C H ligands. Specifically, the Ce M -edge XAS and CI calculations show that the contribution of the 4f , or Ce , configuration to the ground state of Ce(C H ) is similar to strongly correlated materials such as CeRh and significantly larger than observed for other formally Ce compounds including CeO and CeCl . Pre-edge features in the experimental and TDDFT-simulated C K-edge XAS provide unequivocal evidence for C 2p and Ce 4f covalent orbital mixing in the δ-antibonding orbitals of e symmetry, which are the unoccupied counterparts to the occupied, ligand-based δ-bonding e orbitals. The C K-edge peak intensities, which can be compared directly to the C 2p and Ce 4f orbital mixing coefficients determined by DFT, show that covalency in Ce(C H ) is comparable in magnitude to values reported previously for U(C H ) . An intuitive model is presented to show how similar covalent contributions to the ground state can have different impacts on the overall stability of f-element metallocenes. 8 8 2 8 8 2 5,4 8 8 5,4 8 8 2 3 2 6 2u 2u 8 8 2 8 8 2 2- 1 3+ 4+ 2-

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