Anionic Copper Clusters Reacting with NO: An Open-Shell Superatom Cu18-

Gas-phase metal clusters have been a subject of research interest for allowing reliable strategies to explore the stability and reactivity of materials at reduced sizes with atomic precision. Here we have prepared well-resolved copper cluster anions Cu-n(-) (n = 7-37) and systematically studied their reactivity with O-2, NO, and CO. We found remarkable stability of an open-shell cluster Cu-18(-), which is comparable with the closed-shell clusters Cu-17(-) and Cu-19(-) within the picture of an electronic shell model. Even without having a magic number of valence electrons, intriguingly, the unpaired electron on the singly occupied molecular orbital of Cu-18(-) is mainly contributed by the central copper atom, while the other 18 delocalized valence electrons occupy the lower-energy superatomic orbitals of the cluster. The finding of such an open-shell superatom Cu-18(-), with an electron configuration of 1S(2)1P(6)1D(10)2S(1)parallel to 1F(0), is interesting in the sense that an elementary cluster of coinage metal atoms could still behave as a superatom mimicking coinage metals like silver or gold atoms with an empty f orbital. The superatomic stability of this Cu-18(-) cluster is reinforced by the unique electrostatic interaction between the Cu- core and Cu-17 shell, which provides new insights into the chemistry of metal clusters.