Transition metal clusters, boron atoms structure

There are still only a few examples of mixed metal-transition metal clusters containing boron atoms. The first to be reported was Cp2Co2Fe(CO)4B3H3 (92). This cluster results from the photolysis of 2-CpCoB4Hg [a basally substituted isolobal analog of pentaborane(9)] with Fe(CO)5. NMR spectroscopic data are consistent with the proposed structure shown in Fig. 17. 

Even though qualitative bonding descriptions of metal atom clusters up to six or seven atoms can be derived and in some cases correlated with structural detail, it is clear that most structures observed for higher clusters cannot be treated thus. Nor do the structures observed correlate with those observed for borane derivatives with the same number of vertices. Much of borane chemistry is dominated by the tendency to form structures derived from the icosahedron found in elemental boron. However, elemental transition metals possess either a close-packed or body-centered cubic arrangement. In this connection, one can find the vast majority of metal polyhedra in carbonyl cluster compounds within close-packed geometries, particularly hexagonal close-packing. 

Metallacarboranes are cluster compounds in which the cluster vertices comprise one or more metal atoms (main group metal, transition metal or rare earth [lanthanide or actinide] metal), one or more (most often two) carbon atoms, and boron atoms, the last usually in the majority. The total number of cluster vertices may vary but the vast majority of metallacarboranes have between 6 and 14 vertices moreover, the cluster may be either open or closed. However, the single largest group of metallacarboranes has 12 vertices and is structurally based on the closed icosahedron, 1. 

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. 

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