Deltahedral Zintl Ions

The categorisation of the elements as metals, semi-metals and non-metals is based fundamentally on the physical properties of the condensed elemental solids. While these labels offer a fair deal of information on the elements, they are not particularly useful in predicting their tendency to undergo oxidation and/or reduction or in assessing their ability to form small polyatomic cluster species. Thus, the relatively widespread belief that metals can exclusively be oxidized to monoatomic cationic species is a simplification which, while appropriate in the majority of cases, has several noteworthy exceptions (e.g. Au-, Bi22-, Sb73- and E9x- (E = Si, Ge, Sn, Pb; x = 2-4).

Figure 1.





a) [(Pd-Pd)@Ge18]4-: the largest single cage deltahedron isolated to date. The two palladium atoms form a dimer encapsulated within an eighteen-atom germanium cage (red).

b) [E9Zn(C6H5)]3- (E = Si, Ge, Sn, Pb): the nine-atom cluster cage (red) acts as a six-electron donor to the Zn atom (pictured in yellow).

Deltahedral Zintl Ions. Recent years have witnessed a renaissance of Zintl cluster chemistry as the reactivity of the deltahedral anions of group 14 towards a series of transition metal reagents has been explored. Incorporation of transition metals has resulted in the isolation of a series of novel species in which the transition metal atoms play an essential role in the stabilization of large, otherwise unattainable geometries (as pictured in Figure 1a). Also of interest are recent findings which show that nine-atom Zintl ions may act a 6 electron donors to electrophilic fragments such as Ni(CO), M(CO)3 (M = Cr, Mo, W) or ZnPh (Figure 1b). From a coordination chemist’s viewpoint these species are of interest as they are comparatively similar to the numerous organometallic species containing cyclic polyene ligands such as cyclopentadiene that have been the subject of great interest throughout the history of organometallic chemistry.

The ability of Zintl ions to undergo nucleophilic and electrophilic substitution may also allow for their integration as backbones within ligand and spacer group moieties, with the aim of employing them for the construction of molecular devices and larger frameworks. The unique physical properties of these Zintl clusters allows for the development of interesting new species with potential applications in sensors, molecular wires and photochemical systems. The clusters isolated to date represent the first steps in a nascent area of chemistry where many interesting new discoveries await.

(b) Metalloid Clusters. Metalloid cluster compounds are defined as metal cluster species where the number of metal-metal contacts outnumber metal-ligand contacts, and in which the metal atoms of the cluster core take on a close-packed arrangement reminiscent of the bulk metal. These species offer a unique insight into the rather ill-defined area of chemistry that spans between isolated molecular species and solid-state compounds with extended structures. Small, cage-like clusters have been relatively well studied and are found to generally follow established rules for electron counting and isolobal relationships. Their structures and bonding are fairly well understood and can be rationalized within the context of said rules. However, this is not the case for large clusters with high nuclearities (n>12) and dimensions beyond that of the nanometer. These species lack such a cohesive theoretical foundation and, furthermore, many of them seem to disobey traditional electron counting rules, which ultimately makes discussion of structure and bonding very difficult.



Figure 2. [Zn9Bi11]5- : Zn (yellow) and Bi (green) atoms arrange to form an intermetalloid cluster comprised of a Zn-centered, Zn8Bi4 icosahedron (blue), in which seven of the icosahedral faces are capped by bismuth atoms. The cluster represents a fragment of an otherwise unknown binary Bi/Zn alloy.


To date the chemistry of such clusters has been predominantly limited to the group 13 elements and to a few precious metals such as palladium and gold. However, recent findings show that controlled oxidation of negatively charged metal species such as the dimeric Bi22- molecule may be used as a viable synthetic route towards a series of novel metalloid species such as [Zn9Bi11]5- (Figure 2). These species prove that such nanometric species are available in other areas of the periodic table. Isolation of such compounds is paramount in establishing a library of cluster species which may be used for the development of a unified theory for bonding in solids.

(c) Nanoparticle synthesis. The aforementioned Zintl clusters and metalloid species represent intermediates in the oxidative transformation of negatively charged metal and semi-metal species to the bulk element. As such they are molecular ‘snap-shots’ of a synthetic procedure which may ultimately be used to obtain homo- and hetero-metallic colloids and nanoparticles. A similar approach involving the reduction of cationic metal complexes in the presence of strongly coordinating ligands has already been employed as a synthetic technique towards precious metal nanoparticles. However, this approach often suffers from poor polydispersities due to the kinetic nature of the reaction products. An inverse approach involving the mild oxidation of negatively charged species may be used as a synthetic alternative offering greater molecular control over the species synthesized as well as a wider gamut of available nanoparticle compositions.

Dept. of Chemistry, University of Oxford

http://www.chem.ox.ac.uk/researchguide/jmgoicoechea.html