Neutral metal clusters

An alternative to the formation of neutral metal chelates for solvent extraction is that in which the species of analytical interest associates with oppositely charged ions to form a neutral extractable species.6 Such complexes may form clusters with increasing concentration which are larger than just simple ion pairs, particularly in organic solvents of low dielectric constant. The following types of ion association complexes may be recognised. 

Neutral metal carbonyl clusters exemplified by Ir4(CO)i2, IrelCOlie, and Rh6(CO)i6 are adsorbed intact from solution (e.g., n-pentane) onto more-or-less neutral supports such as y-Al203 or Ti02. The clusters on these supports can often be extracted intact into solutions such as tetrahydrofuran. 

In contrast, when neutral metal carbonyl clusters are adsorbed on basic supports such as MgO or La203, surface anions typically form (e.g., [HIr4(CO)ii] and [IrslCOlis] from Ir4(CO)i2 and Ir6(CO)i6, respectively). 

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

This is one of two articles in this volume concerned with the borane-carborane structural pattern. In the other (see Williams, this volume, p. 67) Williams has shown how the pattern reflects the coordination number preferences of the various atoms involved. The purpose of the present article is to note some bonding implications of the pattern, and to show its relevance to a wide range of other compounds, including metal clusters, metal-hydrocarbon n complexes, and various neutral or charged hydrocarbons. 

Structure of neutral and charged noble metal clusters... 

Figure 1. Equilibrium geometries of the two lowest energy isomers, Xy and Xy, of anionic, neutral, and cationic noble metal clusters. The roman numerals identify each clusters in Table 1.

Gold is one of the least reactive metals in bulk form. However, in recent years a considerable amount of theoretical and experimental works have studied the reactivity of small neutral and charged Au clusters towards different molecules, like H2, O2, CO, and organic radicals " . The reactivity depends on the size and charge state of the cluster. In the previous section we have studied the reactivity towards oxygen adsorption of anionic silver and gold clusters. In this section we study the reactivity of neutral gold clusters towards molecular O2 (subsection 6.1) and CO (subsection 6.2). 

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