Triangular-metal faces, capping

Addition of either nucleophilic or electrophilic metallic species can result in the capping of triangular- or square-metal faces in carbonyl clusters. These redox reactions provide high yield syntheses of higher nuclearity clusters and somewhat resemble surface reconstruction on metals. With a few examples,... 

A cluster framework such as that seen in U has also been observed for its heavier homologues (14). Apparently, the latter topology is the result of a close packing of anionic pnictide moieties with alkali metal ions that maximizes ionic interactions. The topology of the rhombodo-decahedral Li6PeSi2 framework in U has been simply described as a Pe octahedron representing the anionic partial structure, of which the triangular faces are capped by two p.3-RSi moieties and six /t3-Li ions as counterion partial structure (Fig. 3). 

The structures of 204-207 have been established in the solid state by single crystal X-ray diffraction. Clusters 205 and 206 are based on the Ru6C octahedral skeleton. The [2.2]paracyclophane ligand in 205 coordinates over a triangular metal face, and in 206 one [2.2]paracyclophane adopts a similar face-capping bonding mode and the other coordinates in a terminal mode to an apical Ru atom. The metal polyhedron in 204 is relatively open in comparison to the octahedron having only nine Ru-Ru bonds. A carbide atom occupies the central cavity and interacts with five of the six Ru atoms. 

The trimeric structure of [Th(TPP)(0H)2]3-2H20 has been confirmed by Kadish et al. (1988). The six hydroxyl ligands in the molecular unit form a trigonal prism of which the triangular faces are capped by two water molecules and the rectangular faces are capped with three thorium atoms (fig. 2a). The three domed TPP units lie parallel to the threefold axis (fig. 2b) and each of the thorium atom assumes a distorted square antiprismatic geometry. The Th-Th separation is 3.960(3) A and does not suggest any metal-metal interaction. 

In addition to the close- and nido-metal-carbonyl clusters already mentioned, with structures based on the octahedron, another interesting category of structure that is found among metal-carbonyl clusters is one in which n skeletal atoms are held together formally by n skeletal bond pairs. These adopt structures based on polyhedra with ( — 1) vertices, as might be expected. The extra metal atom caps one of the triangular faces of the closo residue, where the three vacant orbitals that it can formally furnish for cluster bonding enable it to bond to the 3 metal... 

This derivative of the [OMo6Ojg]z structure (Section 4.6.8.1), by addition of an ML3 cap on a triangular (ji-0)3 face of the (fi-0)i2 cuboctahedron, is proposed269 for [(C0)3ReNb2W4019]3 (prepared by [Nb2W4Oi9]4 + [(CO)3Re(MeCN)3]+ in MeCN), which belongs to a class of mixed mode clusters amalgamating polyoxometallates and metal carbonyls. 

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