Electron counting in cationic and anionic silver clusters doped with a 3d transition-metal atom: endo- vs. exohedral geometry

Electron counting is a concept that often governs properties of molecules, clusters, and complexes. Here we explore silver clusters doped with a transition-metal atom, where it has been an issue whether or not 3d electrons delocalize to participate in electron counting. The experiment is performed on Ag M (M = Sc-Ni) clusters to examine their stability through chemical reactivity, enabling systematic control of the number of valence electrons by the cluster size, the charge state, and the transition-metal element across the periodic table. It is revealed for 18-valence-electron clusters that 3d electrons participate in electron counting to show exceptional stability only when the transition-metal atom is endohedrally doped, except for Cr and Mn doping that forces 3d electrons to localize. We thus present new entries for superatomic metal clusters as well as a geometric factor that regulates the behavior of 3d electrons in the nanoscale regime.