Clusters with more than four transition-metal atoms

Clusters with more than four transition-metal atoms... 

It is possible that a heteronuclear molecular cluster can offer a suitable explanation of these latter results a greater variety of heteronuclear molecular clusters with different transition metals are known. However most of them possess three or four metal atoms and do not represent the best models for very small particles of alloys. Examples of heteronuclear clusters with more than four metal atoms are rare. An interesting example is given by the hexanuclear iridium-copper complex syn-thetized by Bruce et al [135] of formula Cu4lr2(PPh3)2 ( C = C-Ph)g. In this cluster the six metal atoms form a distorted octahedron in which two PhaP-Ir moieties are mutually trans four — C = C-Ph units are only a bonded to each Ir atom and four — C = C-Ph units (one from each Ir) form tt linkages to each of the four equatorial copper atoms (Figure 25). 

Boranes and carboranes are comprised of a-aromatic cage-like structures with boron and carbon vertices ranging from small tetrahedral to supra-icosahedral clusters. Despite more than four decades of work with this class of inorganic compound, there have been few reports of their photophysical properties, with a distinct paucity of information regarding luminescence. Carboranes have nevertheless been incorporated into photophysically active metal complexes, usually as ancillary components. But what of boranes, carboranes, and metallacarboranes (where a transition metal atom can either be an integral vertex in the polyhedral skeletal framewoik or an exterior component thereof) as chromophores themselves and their resultant optoelectronic behavior In this arena, there are very few contributors. 

A simpler way to compare electron counts in boranes and transition-metal clusters is to consider the different numbers of valence orbitals available to the framework atoms. Transition metals, with nine valence orbitals (one s, three p, and five d orbitals), have five more orbitals available for bonding than boron, which has only four valence orbitals these five extra orbitals, when filled as a consequence of bonding within the framework and with surrounding ligands, give an increased electron count of 10 electrons per framework atom. Consequently, a useful rule of thumb is to increase the electron requirement of the cluster by 10 per framework atom when replacing a boron with a transition-metal atom. In the example cited previously, replacing the six borons in closo- d with six cobalts should, therefore, increase the electron count from 26 to 86 for a comparable closo cobalt cluster. Co6(CO)i6, an 86-electron cluster, meets this requirement. 

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