Missing Link in the Growth of Lead-Based Zintl Clusters: Isolation of the Dimeric Plumbaspherene [CuPb].

We report here the structure of an endohedral plumbaspherene, [CuPb], 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 [CuPb] subunits linked through a CuPb 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 d( + 1)s configuration, which generates both a low-lying ( + 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.