Confinement of Americium in a Silver Nanocluster Reduces its Bond Covalency

Understanding the origin and tunability of covalency in actinide-ligand bonding remains one of the central challenges in f-element chemistry, particularly for late actinides where 5f orbital participation is subtle and difficult to control experimentally. We report here the synthesis and characterization of the first actinide endohedral silver nanocluster, {Am(W₅O₁₈)₂@Ag₄₂(C≡C^tBu)₂₈(NO₃)₄}·(NO₃)·11H₂O (AmW₁₀@Ag₄₂), which represents the heaviest atom that is confined in a silver nanocluster. Suppression of americium-oxygen bond covalency upon confinement is observed that originates from the electrostatic compression and polarization of the AmW₁₀ unit, whereas the lanthanide analogs do not exhibit this phenomenon. Complementary theoretical calculations demonstrate that both energy-degeneracy and spatial overlap mechanisms contribute to AmW₁₀, whereas confinement in silver clusters significantly suppress orbital overlap-driven covalency, thereby providing a new strategy for deliberate manipulation of the electronic structure of actinide materials.