Transition metal clusters, boron atoms geometry

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. 

Wade has taken into account the fact that transition metals are electron-deficient in average-size clusters in order to rationalize their geometries and electron counts. He proposed that the metals must share their available electrons to form the cluster skeleton. Wade also noticed that the situation is analogous for polyborane clusters, a very rich and varied family of general formula BnH/ , the starting point of this analysis.In these compounds, each boron atom, that has 3 valence electrons. 

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