Zintl ions structure, 277

RESULTS AND DISCUSSION Structure factors and stability of Zintl ions... 

In an effort to make the distannyne, Ar Sn=SnAr (Ar = 2,6-(2,4,6-Pr 3C6H2)2C6H3), Ar SnH was heated in toluene hydrogen was eliminated but SnC bonds also were cleaved, and gave the paramagnetic cluster Ar 3Sn9 59 (g= ca. 2.031), with a structure that is based on the 21-electron [Sng]3- cage that is known as a Zintl ion.478... 

A comprehensive book on chemistry, structure and bonding of Zintl compounds has been edited by Kauzlarich (1996) in this book several aspects of Zintl phases such as structural patterns, molecular transition complexes of Zintl ions, transition metal Zintl compounds are deeply discussed. 

Structure and Bonding in Zintl Ions and Related Main Group Element Clusters... 

Abstract This chapter reviews the methods that are useful for understanding the structure and bonding in Zintl ions and related bare post-transition element clusters in approximate historical order. After briefly discussing the Zintl-Klemm model the Wade-Mingos rules and related ideas are discussed. The chapter concludes with a discussion of the jellium model and special methods pertaining to bare metal clusters with interstitial atoms. 

Early Efforts to Rationalize the Structures of the Zintl Ions The Zintl-Klemm Concept... 

The efforts to rationalize the formulas and structures of Zintl ions and related species predated extensive definitive structural information on anionic post-transition metal clusters obtained by Corbett and his group in the 1970s [8, 9]. After enough such structural information on the bare post-transition metal clusters became available, the resemblance of their polyhedra to the known polyhedral boranes became apparent. For this reason, the simple Zintl-Klemm concept has been largely superseded by newer, more advanced descriptions of chemical bonding in such clusters, initially those applied to the polyhedral boranes. 

The next development in the understanding of structure and bonding in the Zintl ions recognized their relationship to the polyhedral boranes and the isoelectronic carboranes. Then the Wade-Mingos rules [13-16], which were developed to understand the structure and bonding in polyhedral boranes, could be extended to isovalent Zintl ions and related post-transition element clusters. 

The first transition metal derivatives of a Zintl ion was prepared by Teixidor et al. in 1983 in reactions between Pt(PPli4)4 and en solutions of the Eg (E = Sn, Pb) [25, 26]. Despite being the first examples in this important class of clusters, the complexes have yet to be isolated and their structures and compositions remain unknown. The authors propose that complexes have a (PPh3)2PtSng stoichiometry and a nido-ty structure. Based on comparisons with NMR parameters from the past 30 years and the stoichiometry of the reactions described by Teixidor et al., we believe that the Rudolph compounds are most likely 22-electron cZos )-Pf E9Pt (PPh3) complexes. Our rationale is given below. 

The dynamic properties of these two complexes are perhaps the most surprising and interesting of the transition metal Zintl ion complexes. The clusters have markedly different structures but are both prolate in nature with several different Sn environments. The tin atoms at opposite ends of both structures are separated by nearly 9 A, but all 17 atoms are in fast intramolecular exchange on the NMR time scale. 

It has been shown that a variety of substituents can be attached to the outside of the group 14 Zintl ion clusters in exo positions (i.e., not vertex or interstitial positions) [70,73-78]. A variety of alkyl, aryl, and main group moieties have been attached to Ge9 and Sn9 clusters. The structures of these clusters are similar to some organos-tannane clusters prepared via different synthetic routes. This burgeoning class of compounds is rapidly developing however, little is known about the effect of the exo-substituents on the dynamic properties of the clusters. Only the RSng ions, where R = i-Pr, t-Bu, and SnCys, Sn- -Bu3, have been studied in detail [70]. 

In this chapter, the chemistry and the structures of Zintl ions and cage molecules are summarized and put in the context of intermetalloid clusters. The emphasis is put on the observation that irrespective of the starting material (Zintl ions or small low-valent organometallic compounds) the same types of intermetalloid clusters are formed. 

Considerable evidence exits of the survival of Zintl ions in the liquid alloy. Neutron diffraction measurements [5], as well as molecular dynamics simulations [6, 7], give structure factors and radial distribution functions in agreement with the existence of a superstructure which has many features in common with a disordered network of tetrahedra. Resistivity plots against Pb concentration [8] show sharp maxima at 50% Pb in K-Pb, Rb-Pb and Cs-Pb. However, for Li-Pb and Na-Pb the maximum occurs at 20% Pb, and an additional shoulder appears at 50% Pb for Na-Pb. This means that Zintl ion formation is a well-established process in the K, Rb and Cs cases, whereas in the Li-Pb liquid alloy only Li4Pb units (octet complex) seem to be formed. The Na-Pb alloy is then a transition case, showing coexistence of Na4Pb clusters and (Pb4)4- ions and the predominance of each one of them near the appropiate stoichiometric composition. Measurements of other physical properties like density, specific heat, and thermodynamic stability show similar features (peaks) as a function of composition, and support also the change of stoichiometry from the octet complex to the Zintl clusters between Li-Pb and K-Pb [8]. 

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