Clusters with seven or more transition-metal atoms

3 Clusters with seven or more transition-metal atoms 

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. 

The metal cluster of [RhioP(CO)22] forms a deltapolyhedron, which has g = 142, as shown in Fig. 19.4.1(d). The skeleton of RhyP(CO)2i I2- is obtained by removal of a vertex transition-metal fragment. The skeletal valence electron count of [RhgPtCOhi]2- gives g = 142 — 12 = 130. 

The metal cluster of [Rh 11 (CO)23]3 is composed of three face-sharing octa-hedra, as shown in Fig. 19.4.1(e). The metal cluster of [Rhi2Sb(CO)27l3 consists of an icosahedron with an encapsulated Sb atom at its center. 

Generally, capped or decapped deltapolyhedral clusters are characterized by the number of skeletal valence electrons g 

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