Cluster Species of Alkali Metals

Alkali metal cluster formation presupposes that the element is in intermediate oxidation states between zero-valent and normal valence ones which are characteristic of metallic and salt-like species respectively. The chemistry of this class of compounds is therefore strongly associated with the stabilization of such states. 

The low number of valence orbitals available for cluster-ligand bonding commented on above in addition to the high reduction potentials of alkali metals cause alkali metal clusters to be rather unstable. Disproportionation into the metal and other products with the element in higher oxidation states is often a thermodynamic favorable process. 

The presence of any electron donor able to stabilize the cation will therefore favor cluster disruption. Such limiting conditions make it very difficult to find an adequate medium for the stabilization of such kinds of clusters. Solvent as well as counterions must be highly inert indeed, both as oxidation agent and as a Lewis base. Although the severity of such limiting conditions increases with increasing atomic weight of the metals, it has been possible to achieve the conditions under which clusters of the elements rubidium and cesium can exist, namely as the suboxide to be described in this Section. 

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Cluster Species of Alkali Metals Fig. 4.6. Structure of Rbg02... 

The study of bare metal clusters is central to the understanding of the links between solid state chemistry and that of discrete molecular species. Alkali metal clusters have been studied in molecular beams [12, 13], and the theoretical models proposed have attempted to interpret the abundances observed in the mass spectra of these clusters. These spectra show large abundances for specific numbers of metal atoms (N), the so-called magic numbers . Neutral alkali metal mass spectra show peaks at N = 2, 8, 20,40, 58, whereas cationic species show large abundances at N = 19, 21, 35, 41. The theoretical study of alkali metal clusters is simplified by the presence of only 1 valence electron per atom. 

In this section the calculated moments of inertia of alkali metal clusters, whose structures have been derived from ab initio calculations have been used successfully to interpret distortions from sphericality for 3-dimensional structures. The analysis has proved successful in relating these distortions to simple models of cluster bonding, and provides a useful tool for the prediction of stable cluster structures for as yet uncharacterised species. Distortions from circular to elliptical geometries for planar structures can be evaluated using a similar methodology. 

In the gas-aggregation source a metal is vaporized and introduced in a flow of cold inert gas in which the vapor becomes highly supersaturated. Clusters are mainly produced by successive single-atom addition in the build-up of larger species. This type of source has been used to produce continuous cluster beams of alkali elements. By using two separate ovens in the source, each containing separate materials, clusters with two elements can be produced as Ceo covered with alkali metals [83]. The limitation of this type of source is that only metals with a low melting point can be studied. 

Following a more simple hydrothermal procedure, Kornatowski et al. were able to incorporate up to ca. 1 wt.-% of V into the MFI framework starting from vanadates of alkali metals [101]. EPR analysis indicated that most of the vanadium is highly dispersed in the framework as distorted fourfold coordinated the remainder forming clustered extra-framework species [101]. After calcination in air, the signal attributed to isolated species vanished due to... 

A series of alkali metal and alkaline earth metal-arsenic clusters formulated as M[c-Ass] (M = Li, Na, K, Rb, Cs) andM [c-Ass]+(M = Be, Mg, Ca, Sr, Ba) were investigated by Xu and Jin [173] using DFT methods. All M[c-Assj andM [c-Ass]" clusters adopt a pentagonal pyramidal structure with Csv symmetry, its basal plane involving the planar pentagonal [c-Ass] anion. From molecular orbital and NICS analysis, it was established that each of these species had three delocalized n MOs that satisfied the 4h - - 2 electron counting rule and therefore exhibit n-aromatic character. 

Thus, the electronic structures of fullerene anionic species can be obtained by filling the 3-fold degenerate LUMOs of the cluster. Accordingly, the products of the intercalation of alkali metals in fullerenes are expected to show, depending on the intercalation degree, an enhanced conductivity. In some cases superconductivity has been also observed (vide infra). 

The polyhedral boranes and carboranes discussed above may be regarded as boron clusters in which the single external orbital of each vertex atom helps to bind an external hydrogen or other monovalent atom or group. Post-transition main group elements are known to form clusters without external ligands bound to the vertex atoms. Such species are called bare metal clusters for convenience. Anionic bare metal clusters were first observed by Zintl and co-workers in the 1930s [2-5], The first evidence for anionic clusters of post-transition metals such as tin, lead, antimony, and bismuth was obtained by potentiometric titrations with alkali metals in liquid ammonia. Consequently, such anionic post-transition metal clusters are often called Zintl phases. 

Up till now anionic mercury clusters have only existed as clearly separable structural units in alloys obtained by highly exothermic reactions between electropositive metals (preferably alkali and alkaline earth metals) and mercury. There is, however, weak evidence that some of the clusters might exist as intermediate species in liquid ammonia [13]. Cationic mercury clusters on the other hand are exclusively synthesized and crystallized by solvent reactions. Figure 2.4-2 gives an overview of the shapes of small monomeric and oligomeric anionic mercury clusters found in alkali and alkaline earth amalgams in comparison with a selection of cationic clusters. For isolated single mercury anions and extended network structures of mercury see Section 2.4.2.4. 

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