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 closed-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, and so on 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 A_x^I[Zr₆(Z)X₁₂]X_n family or 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, 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.