Interstitial boron atoms

Most metallaboride clusters are synthesized from metal-rich metallaboranes (Lewis base adducts of BH3 are often used as the source of the B atom) several different methods are used to transform B-H bonds into B-M interactions and hence isolate a naked boron atom. Two recent review articles have been devoted to this subject.l l 

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Fig. 10-1.—Structure of tetragonal boron as viewed in the direction of the c axis. One unit cell is shown. Two of the icosahedral groups (light lines) are centered at z , and the other two (heavy lines) at. The interstitial boron atoms (open circles) are at (0, 0, 0) and (j, ). Numbers identify...

The basic structural feature is boron icosahedra. In the unit cell, there are 50 boron atoms, with 150 valence electrons and 148 boron-boron nearest neighbors (bond positions). These 50 boron atoms form four nearly regular icosahedra plus two interstitial boron atoms in the unit cell. Within each B12 icosahedron, each B atom has five adjacent atoms at the distance 1.797 0.015 A. Each icosahedral B atom has an interstitial B atom lying 1.62 0.02 A from it. Furthermore, each of the interstitial B atoms has four nearest neighbors (icosahedral B atoms). This implies that the icosahedral B atoms have ligancy 6, and the interstitial B atoms have ligancy 4. 

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