Post transition elements

The metals which form silicate and aluminosilicate minerals are the more electropositive metals, i.e. those in Groups 1, 2 and the 3d transition series (except Co), together with Y, La and the lanthanoids, Zr, Hf, Th, U and to a much lesser extent the post-transition elements Sn", Pb", and Bi ". 

None of the three elements is particularly abundant in the earth s crust though several minerals contain them as major constituents. As can be seen from Table 13.1, arsenic occurs about halfway down the elements in order of abundance, grouped with several others near 2 ppm. Antimony has only one-tenth of this abundance and Bi, down by a further factor of 20 or more, is about as unabundant as several of the commoner platinum metals and gold. In common with all the post-transition-element metals. As, Sb and Bi are chalcophiles, i.e. they occur in association with the chalcogens S, Se and Te rather than as oxides and silicates. 

Homopolyatomic ions of the post-transition elements — synthesis, structure and bonding, J. D. Corbett, Prog. Inorg. Chem., 1976, 21,129-158 (61). 

The need for better performing and secure anodic materials in lithium-ion batteries compared with those based on carbon, has boosted research in various domains. Lithium/post-transition element (Al, Si, Sn, Sb...) binary systems have been widely investigated. Owing to the numerous intermetallic compounds that... 

More often, polyhedral clusters with strong metal-metal (or metalloid-metalloid) bonding are the major structural motifs of classic Zintl phases. These are nominally salts composed of reduced p- (i.e., post-transition) elements that are usually inter-bonded into closed shell polyanions plus active metal cations, originally the alkali... 

Some post-transition elements (or the corresponding radicals) containing 3 or more electrons in their valence shell are able to assist the formation of clusters by bonding to several metal atoms. Typical examples of this behaviour are the extraordinarily easy syntheses of large series of compounds such as Co3 (CO)9 (p3-E) (E = Al, CR, CX, GeR, P, As, PS, S, Se, PR, SR) 201 209) and Fe3 (CO)9 (p3 -E)2 (E = S, Se, Te, NR, PR). This type of stabilization is usually found in trinuclear clusters although a few examples in tetranuclear clusters are known, for instance ... 

Rules for counting the number of skeletal electrons provided by each vertex atom need to be established in order to determine the number of skeletal electrons in polygonal and polyhedral clusters of the post-transition elements. The rules discussed above for polyhedral boranes can be adapted to bare post-transition metal vertices as follows ... 

Hetero-atomic clusters, moreover, may be derived from the binary structures mainly through the introduction of late transition or earlier post-transition elements. Examples of ternary alloys containing such structures are the alkali metal salts of centred clusters In10Me10 (Me = Ni, Pd, Pt), Tl12 Me12- (Me = Mg, Zn, Cd, Hg), etc. The crystal structure of the phase Na T Cdi x)27 (0.24 < x < 0.33)... 

The first evidence that post-transition elements, the metals especially, could be reduced to highly colored anions was published over 90 years ago by Joannls (O who discovered that sodium and lead or their alloys dissolve In liquid ammonia to yield an Intensely green-colored solute. A stoichiometry of 2.25 lead atoms per sodium ( ) for what was evidently an anion led Kraus ( ) to formulate this as Pbg ". Until the past decade the principal Information regarding this and many other species were the stoichiometries obtained by Zintl and coworkers from... 

The homopolyatomlc or Zlntl anions Incorporating post-transitions elements of groups IV and V which have been so synthesized and structurally characterized are listed In Table I, together with their observed symmetries and other ions or molecules with which they are Isoelectronlc. As a matter of fact only three of those given, Sng ", Sb " and were observed... 

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. 

Modem work on these and related bare post-transition element clusters began in the 1960s after Corbett and coworkers found ways to obtain crystalline derivatives of these post-transition element clusters by the use of suitable counterions. Thus, crystalline derivatives of the cluster anions had cryptate or polyamine complexed alkali metals as countercations [8]. Similarly, crystalline derivatives of the cluster cations had counteractions, such as AlCLj, derived from metal halide strong Lewis acids [9]. With crystalhne derivatives of these clusters available, their structures could be determined definitively using X-ray diffraction methods. 

An essential feature of the Wade-Mingos rules [13-16] is the partitioning of the valence electrons of the cluster atoms into skeletal electrons and external electrons. For polyhedral boranes, the external electrons are used to form bonds to external groups, such as hydrogen atoms in the prototypical borane dianions However, for bare post-transition element clusters, these external electrons formally correspond to nonbonding electron pairs. Subsequent experimental and theoretical... 

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. 

To understand the unusual structures found in bare post-transition element clusters and underlying reasons for differences in the structures between analogous boranes and post-transition element clusters, we carried out extensive density functional theory (DFT) studies of bare germanium clusters during the past decade [18-20, 69-72]. In three cases (Fig. 5), these studies predict lowest energy structures for Ge clusters that are different from the well-established structures of the isovalent boranes and carboranes C2B 2H . Thus, the lowest energy... 

The next magic number for jellium clusters is 40. This is a particularly important magic number in cluster chemistry, since numerous 40 valence electron bare clusters with 9 to 11 vertices of the post-transition elements in Groups 13 to 15 are known as isolable species in intermetallics or salts with suitable counterions. Examples of such species include lun, Geg", and Big, all of which have been isolated in intermetallics (for Inn ) or as stable salts with suitable counterions (Geg" and Big ) and characterized by X-ray crystallography. 

Acknowledgment The author is indebted to the US National Science Foundation for support of this workunder Grant CHE-0716718. This article is dedicated to the memory of Prof. loan Silaghi-Dumitrescu (1950-2009) in recognition of the pivotal role that he played in density functional theory studies on bare post-transition element clusters. 

Corbett JD (1985) Polyatomic Zintl anions of the post-transition elements. Chem Rev 85 383-397... 

Hexavalent molybdenum and tungsten, pentavalent vanadium and, to a more limited extent, niobium and tantalum form a very large number of polyoxoanions ( heteropolyanions such as [PW12O40]3- and isopolyanions such as [M07O24]6-). Unlike the polyoxoanions of the post transition elements the heteropolyanions for the most part are discrete, compact species of high... 

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