Missing Link in the Growth of Lead-Based Zintl Clusters: Isolation of the Dimeric Plumbaspherene [Cu 4 Pb 22 ] 4 .

We report here the structure of an endohedral plumbaspherene, [Cu 4 Pb 22 ] 4- , the gold analogue of which was previously postulated to be a "missing link" in the growth of larger clusters containing three and four icosahedral subunits. The cluster contains two [Cu 2 Pb 11 ] 2- subunits linked through a Cu 2 Pb 4 trigonal antiprism. Density functional theory reveals that the striking ability of mixed Pb/coinage metal Zintl clusters to oligomerize and, in the case of Au, to act as a site of nucleation for additional metal atoms, is a direct consequence of their n d 10 ( n + 1)s 0 configuration, which generates both a low-lying ( n + 1)s-based LUMO and also a high-lying Pb-centered HOMO. Cluster growth and nucleation is then driven by this amphoteric character, allowing the clusters to form donor-acceptor interactions between adjacent icosahedral units or to additional metal atoms.