Enhanced Stability of Rhombic Dodecahedron Nb₁₅^- with Well-Organized Superatomic States

Well-resolved Nb^- clusters are produced and reacted with ethene and propene via a downstream flow tube reactor. Interestingly, the Nb^- clusters readily react with ethene and propene to form dehydrogenation products; however, Nb₁₅^- shows up in the mass spectra with prominent mass abundance indicating its inertness to react with olefins. For this cluster, we conduct photoelectron velocity map imaging (VMI) experiments and verify the stability of Nb₁₅^- within a highly symmetrical rhombic dodecahedron structure. Theoretical studies show that the stability of the Nb₁₅^- cluster is correlated with its superatomic nature pertaining to both geometric and electronic shell closures. Notably, the superatomic 1s orbital is dominated by the 5s electron of the central Nb atom, while the other superatomic orbitals are contributed by s-d hybridization, especially a remarkable contribution of s-d hybridization. Apart from the closed shells, the highly symmetric geometry of Nb₁₅^- is associated with a regular polyhedral structure directed by all rhombus facets, embodying a magic number for body-centered dodecahedra, indicative of enhanced stability as a double magic cluster free of olefin adsorption.