Borohydride ions and carboranes

Structures of boranes (a) BioHig, (b) BigH2o (c) B18H22. (d) 820 16-Borohydride ions and carboranes 

Metal derivatives of boranes and related compounds are of at least three types  

A1(BH4)3 is a volatile liquid boiling at 44-5°C and the borohydrides M(BH4)4 of U, Th, Hf, and Zr are the most volatile compounds known of these metals. 

In the more covalent metal borohydrides BH4 is bonded to the metal by hydrogen bridges, as shown in Fig. 24.23 for Be(BH4)2, A1(BH4)3, [(C6Hs)3P]2.CuBH4,( and A1(BH4)3. N(CH3)3. Note the quite different mode of bonding of BH4 in the Be and A1 compounds, and the trigonal prismatic arrangement of the 6 H atoms around the metal atom in the latter. It was not possible to locate the H atoms in the room temperature form of A1(BH4)3. 

N(CH3)3, in which the gross structure of the molecule is tetrahedral, (d), but in the low temperature form the three BH4 groups are not equivalent. One is rotated so that one H (H ) occupies an apical position of a pentagonal bipyramidal group around Al, (e). In the molecule Zr(BH4)4 the BH4 groups are arranged tetrahedrally around the metal atom with triple bridges  

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