Pure zyxwvutsrqpo & zyxwvutsrq Appl. Chem., Vol. 62, No. 1, pp. 103-1 12, 1990. zyxwvuts Printed in Great Britain. @ 1990 zyxwvutsrqp IUPAC Chemical clusters from solid state systems at high temperatures. lnterstitials as a means to stability and versatility John zyxwvutsrq 0. Corbett, Eduardo Garcia, Young-Uk Kwon and Arnold Guloy Ames Laboratory-DOE and Department o f Chemistry Iowa State University, Ames, IA 50011 Abstract - 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. shell configurations are known for the latter group both as solids at room temperature and i n cluster beams. 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 Ai[Zr6(Z)XI2]X, family and rare-earth-element analogs where zy 22 different examples of Z and many structure types are known for X = C1, 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 i n 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 o f Z. 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 M&, M = Ca, Sr o r Ba and 6 = Sb or Bi. phases are achieved i n some M5B3Z compounds from the last two groups. Numerous "naked" clusters with favorable close- A few binary transition-metal clusters stable Band Valence-precise (Zintl) INTRODUCTION Clusters of metal atoms are o f great interest i n materials chemistry as well as i n impinging areas o f chemistry and physics. clusters studied in the gas phase and a t the other, condensed nanoscale t o micron-sized particles with considerable potential for catalysis, microdevices, etc. (ref. 1). However, metal clusters seen in beam studies usually exhibit little stability discrimination with respect to size i n the range of roughly 5 - 25 o r more atoms, many appearing to have more of the character of van der Waals or "jellium" molecules than of "chemical" clusters with well defined, closed shell electronic configurations and high stabilities such as are known in more molecular systems, P4 and Cb0 for example (ref. 2). The equilibrium electronic structures of small "chemical" clusters of metals should naturally be better described by molecular orbital rather than by band methods suitable to the bulk. However, it appears likely that few if any metal clusters with these closed shell configurations and relatively high stabilities are achieved in most single component beams because of natural limitations on the valence electron counts that can be achieved. course unknown in condensed systems because of their intrinsic reactivity. will briefly explore the bases for the above behaviors and some exceptions thereto and then will outline the newer aspects of the more stable "chemical" clusters that may be achieved in binary and ternary systems. A new feature of this chemistry is the added versatility and stability achieved for transition metal cluster halides through manipulation of the cluster electron configurations and thereby the phases formed via the ehcapsulation of interstitial atoms within the clusters. We will also relate the beginnings of a new cluster chemistry that can be achieved in infinite chain structures of some electron-rich polar intermetallic phases with the Mn5Si3 structure and the broad interstitial chemistry that i s possible therein. At one end o f the spectrum are the "naked" homoatomic Examples o f these same clusters are of This article NAKED CLUSTERS Transition metals owe much of their prolific chemistry to the profusion of good bonding orbitals, namely (n-l)d, ns and np, but it i s this same orbital abundance that precludes the isolation of small naked transition-metal clusters that are stable in bulk. As an example, 103