Clusters, metal electron counting

When the number of metal atoms in a cluster increases, the geometries of the clusters become more complex, and some are often structurally better described in terms of capped or decapped polyhedra and condensed polyhedra. For example, the first and second clusters listed in Table 19.4.3 are a capped octahedron and a bicapped octahedron, respectively. Consequently, capping or decapping with a transition-metal fragment to a deltapolyhedral cluster leads to an increase or decrease in the cluster valence electron count of 12. When a transition-metal atom caps a triangular face of the cluster, it forms three M-M bonds with the vertex atoms, so according to the 18-electron rule, the cluster needs an additional 18 - 6 = 12 electrons. The parent octahedron of [Os6(CO)is]2- has g = 86, the monocapped octahedron Osy(CO)2i has g = 98, and the bicapped octahedron [Oss(CO)22]2- hasg = 110. 

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

Behavior characteristic of transition metals but not p-block elements shows up in another guise in M4E4 cubane clusters. Two common types are illustrated in Figure 5.24. These clusters are characterized by versatile redox activity with little geometric change and a number of electron counts for the same cluster shape and connectivity. For example, cubane clusters with electron counts (treated as four-metal clusters) from 52 to 72 eve are known. As we have seen, this implies a small HOMO-FUMO gap for the structure type. 

For the mononuclear and dinuclear compounds, except V(CO)6, the mnnber of valence electrons per metal atom is 18. The EAN (effective atomic number) of the metal is the number of electrons of the d" configuration added to twice the number of CO groups, one electron being further added in the case of metal-metal bonded dinuclear systems. For trinuclear, tetranuclear, and hexanuclear compoimds (metal-carbonyl clusters), the cluster valence electron counting is 48, 60, and 86, respectively. As the [K(cryptand 2.2.2)] derivative. As the tetraalkylammonium derivative. As the tetrachloroaluminate. 

S22.9 There are four relevant pieces of information provided. First of all, the fact that [Fe4(Cp)4(CO)4] is a highly coloured compound suggests that it contains metal-metal bonds. Second, the composition can be used to determine the cluster valence electron count, which can be used to predict which polyhedral structure is the likely one ... 

Although the above mentioned face- and edge-halide-bridged clusters are characterised by 84 and 76 valence electrons respectively, it was apparent by the early 1970 s that the great majority of octahedral transition metal clusters possess 86 electrons. Braterman rationalised the 86 cluster valence electron count for octahedral metal carbonyl clusters, such as [Co6(CO)i6]75), [Ru6C(CO)17]76) and [Co6(CO)i4]4- 77) in terms of localised M-CO... 

Table 2.11. Correlation between metal polyhedron geometry and cluster valence electron counting. Structural properties of selected rhenium clusters...

Table 3.3. Cluster valence electron counting in main group-transition metal mixed clusters...

We will define the rules that allow us to understand and construct bimetallic complexes, then we will provide the most salient features of metallic clusters including electron counting (limits of the 18-electron rule and Wade s rules) and the isolobal analogy. 

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

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