Experimental evaluation of the stabilization of the COT orbitals by 4f orbitals in COT2Ce using a Hubbard model

Strengths of covalent bonds in LnO2 determined from O K-edge XANES spectra using a Hubbard model

In LnO2 (Ln = Ce, Pr, and Tb), the amount of Ln 4f mixing with O 2p orbitals was determined by O K-edge X-ray absorption near edge (XANES) spectroscopy and was similar to the amount of mixing between the Ln 5d and O 2p orbitals. This similarity was unexpected since the 4f orbitals are generally perceived to be "core-like" and can only weakly stabilize ligand orbitals through covalent interactions. While the degree of orbital mixing seems incompatible with this view, orbital mixing alone does not determine the degree of stabilization provided by a covalent interaction. We used a Hubbard model to determine this stabilization from the energies of the O 2p to 4f, 5d(eg), and 5d(t2g) excited charge-transfer states and the amount of excited state character mixed into the ground state, which was determined using Ln L3-edge and O K-edge XANES spectroscopy. The largest amount of stabilization due to mixing between the Ln 4f and O 2p orbitals was 1.6(1) eV in CeO2. While this energy is substantial, the stabilization provided by mixing between the Ln 5d and O 2p orbitals was an order of magnitude greater consistent with the perception that covalent bonding in the lanthanides is largely driven by the 5d orbitals rather than the 4f orbitals.

Overlooked Effects of La-4f Orbitals in Endohedral Metallofullerenes

For endohedral metallofullerenes (EMFs), a central issue is how to correctly describe the intracluster and metal-cage interactions, which are critical for understanding their structures, stabilities, and various properties. In this work, density functional theory calculations were carried out for 13 La-based EMFs covering all four reported types and a rather wide cage size range (C₃₂-C₁₀₄). The results reveal that the usually core-like lanthanide 4f subshell may play a critical role in the structural characteristics, energetic stabilities, frontier orbital energy levels, metal charges, and chemical reactivities of these endofullerenes. Regardless of the encapsulated forms, the La-4f contributions to the chemical bonding and structural stability increase with the reduced cage sizes because of the gradually enhanced cage confinement. The combination of metal-to-nonmetal charge transfer and compression of the cage cavity exposes and effectively activates the otherwise chemically inert 4f orbitals. By disclosing the important role of long-neglected metal orbitals inside fullerenes, the current work not only deepens our understanding of EMFs, but also provides new insights into the chemical bondings in general confined spaces at the subnanometer scale.

Electronic structure studies reveal 4f/5d mixing and its effect on bonding characteristics in Ce-imido and -oxo complexes

This study presents the role of 5d orbitals in the bonding, and electronic and magnetic structure of Ce imido and oxo complexes synthesized with a tris(hydroxylaminato) [((2-^tBuNO)C₆H₄CH₂)₃N]³⁻ (TriNO_x³⁻) ligand framework, including the reported synthesis and characterization of two new alkali metal-capped Ce oxo species. X-ray spectroscopy measurements reveal that the imido and oxo materials exhibit an intermediate valent ground state of the Ce, displaying hallmark features in the Ce L_III absorption of partial f-orbital occupancy that are relatively constant for all measured compounds. These spectra feature a double peak consistent with other formal Ce(iv) compounds. Magnetic susceptibility measurements reveal enhanced levels of temperature-independent paramagnetism (TIP). In contrast to systems with direct bonding to an aromatic ligand, no clear correlation between the level of TIP and f-orbital occupancy is observed. CASSCF calculations defy a conventional van Vleck explanation of the TIP, indicating a single-reference ground state with no low-lying triplet excited state, despite accurately predicting the measured values of f-orbital occupancy. The calculations do, however, predict strong 4f/5d hybridization. In fact, within these complexes, despite having similar f-orbital occupancies and therefore levels of 4f/5d hybridization, the d-state distributions vary depending on the bonding motif (Ce Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O vs. CeN) of the complex, and can also be fine-tuned based on varying alkali metal cation capping species. This system therefore provides a platform for understanding the characteristic nature of Ce multiple bonds and potential impact that the associated d-state distribution may have on resulting reactivity.

Ce(iv) complexes with multiple bonds display similar f⁰ fractions, but different f/d hybridization, 5d-orbital energies, and TIP levels.