Chemical clusters from solid state systems at high temperatures. Interstitials as a means to stability and versatility

The reactivity of many “naked” clusters and the means by which these may be isolated as stable “chemical” clusters are briefly considered for the orbitally-rich transition metals, where clusters must be sequestered by coordination with ligands, and for the heavier main group elements that utilize only three p orbitals in bonding. Numerous “naked” clusters with favorable close-shell configurations are known for the latter group both as solids at room temperature and in cluster beams. A few binary transition-metal clusters stable at elevated temperatures are obtained with chloride, sulfide, etc. as ligands. Greater versatility and variety have recently been found for electron-precise clusters that require an interstitial atom Z within M6X12-type clusters for stability, as with the A1X[Zr6(Z)X12]Xn family and rare-earth-element analogs where 22 different examples of Z and many structure types are known for X = Cl, Br, I. A newer and even broader area of cluster chemistry involves interstitials bonded within chains of confacial octahedra in a variety of polar intermetallic phases that occur in the Mn5Si3 structure ([Mn3Si3.Mn2]), where the large differences in valence state energies of the component atoms generally assure that valence bonding levels on the main-group element are filled first. The electron-rich Zr5Sb3 host is found to form Zr5Sb3Z phases with at least 16 examples of Z. Band calculational results for the binary host and for the sulfide are considered. Examples of similar investigations are described for the Zr5Sn3, Zr5Pb3, La5Ge3, La5Pb3, and M5B3, M = Ca, Sr or Ba and B = Sb or Bi. Valence-precise (Zintl) phases are achieved in some M5B3Z compounds from the last two groups.