Metal Chalcogenide Clusters with Closed Electronic Shells and the Electronic Properties of Alkalis and Halogens

Clusters with filled electronic shells and a large gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are generally energetically and chemically stable. Enabling clusters to become electron donors with low ionization energies or electron acceptors with high electron affinities usually requires changing the valence electron count. Here we demonstrate that a metal cluster may be transformed from an electron donor to an acceptor by exchanging ligands while the neutral form of the clusters has closed electronic shells. Our studies on Co₆Te₈(PEt₃)_m(CO)_n (m + n = 6) clusters show that Co₆Te₈(PEt₃)₆ has a closed electronic shell and a low ionization energy of 4.74 eV, and the successive replacement of PEt₃ by CO ligands ends with Co₆Te₈(CO)₆ exhibiting halogen-like behavior. Both the low ionization energy Co₆Te₈(PEt₃)₆ and high electron affinity Co₆Te₈(CO)₆ have closed electronic shells marked by high HOMO-LUMO gaps of 1.24 and 1.39 eV, respectively. Further, the clusters with an even number of ligands favor a symmetrical placement of ligands around the metal core.