Metal Cluster Geometries

Table 3.1. Cluster valence electron counts for representative metal-cluster geometries...

Olusters of the heavier elements undergo coupling reactions with salts of Ag(I), Au(I), Hg(I), Hg(II), and Tl(III), in which the basic metal cluster geometry is preserved. Even in these cases it is difficult to predict the nuclearity of the mixed species formed. These reactions may result in the simple coupling of two cluster units, as in the formation of [ Ru6C(CO)i6 2T1] (Fig. la) shown in Eq. (7) (320). 

In all the calculations, no surface reconstruction nor surface relaxation have been considered, that is, the metal cluster geometry was kept frozen. The nearest-neighbour distance was taken from the bulk and is 2.55612 A. This cluster forms a compact section of the corresponding ideal Cu(lOO) surface. 

Iq this study the B3LYP hybrid method proposed by Becke [31] and included in the Gaussian 98 [30] package was used. This method includes a mixture of Hartree-Fock and DFT exchange terms associated with the gradient corrected correlation functional of Lee et al [32], In all the calculations the metal cluster geometry was kept frozen. 

Figure 8. A representation of typical metal cluster geometries in transition metal carbonyl clusters. For clarity, the carbonyl ligands have been omitted. These compounds can profitably be viewed as metallic cluster fragments which are effectively isolated from adjacent cluster units by inert sheaths of carbonyl ligands. Taken from Johnson et...

Characterization of these clusters indicate an unusual 2n framework electron count having geometries reminiscent of stricdy metallic clusters (11,164). 

Metal-promoted alkyne-insertion reactions afford another good method (see structure 12 for cluster geometry and numbering)... 

Synthesis and characterization of an Al69 cluster which can be regarded as a slightly modified Al77 cluster show the extreme sensitivity of the cluster geometry— even with the same ligand—to reaction conditions and number of ligands. Many more clusters will have to be synthesized to get a consistent picture of their formation and, consequently, the mechanism of metal formation which represents, as already mentioned, one of the oldest chemical processes in history. 

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