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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.

The reported actinide-based endohedral clusterfullerenes (ECFs) are rather scarce thus far. Though several members have been detected in mass spectra, their exact structures and properties mostly remain unclear. Herein, density functional theory calculations revealed that the U2O@C72observed in recent experiments should be U2O@D2(10611)-C72, U2O@C1(10610)-C72, or U2O@Cs(10616)-C72. Featuring two pairs of fused pentagons, their outer cages all break the well-known isolated pentagon rule. U2O@D2(10611)-C72is the first clusterfullerene based on the D2(10611)-C72cage, which only encapsulated dimetals (Sc2, La2, Ce2, Pr2) before. It is also the first time to reveal that C1(10610)-C72and Cs(10616)-C72can serve as the parent cage of an endohedral fullerene. Interestingly, the three isomers could interconvert with each other via Stone–Wales transformation with one internal U atom dynamically changing its orientation according to the position of pentagon adjacencies. A common electronic structure of (U4+)2(O)2–@C726–can be formally assigned to the three ECFs but with obvious covalent character for both U–O and U–C bonds. Their spatially extended U-5f orbitals substantially enhance the metal-cage interactions. Their various spectra were also simulated to assist future experiments. Moreover, our work shows that the careful choice of exchange-correlation functionals is rather critical for the structural characterization of ECFs.

Heavy alkaline earth metals (Aes) are usually considered to engage in chemical bonding by donating the two electrons on ns atomic orbitals (AOs). In this work, a series of typical endohedrally doped cage clusters Ae@cage (Ae = Ca, Sr, Ba; cage = C32, C74, C94, B40, Si20, Sn12, Au16) were thoroughly investigated by means of density functional theory calculations. We found that their occupied molecular orbitals have ∼1 to 14% contributions from Ae-(n– 1)d AOs due to electron back-donation from the cage. Though the amount is small, it is hard to ignore: with the d orbitals, all these endohedral clusters exhibit obviously shortened Ae–cage distances, greatly enhanced encapsulation stabilities, changed highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gaps, and much lowered Ae valences far from ideal +2. Evidently, the valence orbitals of Ca/Sr/Ba in these systems should include both ns and (n– 1)d. By disclosing the critical role of unnoticed metal orbitals, our work provides completely new insights into the cluster field.