Closo-borane [BnHn]2- has the structure of a closed polyhedron of n boron atoms
bonded to n hydrogen atoms, as seen in the examples of a regular octahedron [B6H6]2- and an icosahedron [B12H12]2-. The boranes of this series do not contain B-H-B bonds.
Boranes BnHn+4, such as B5H9, form structures with B-B, B-B-B, and B-H-B bonds and
lack the apex of the polyhedron of closo boranes, and are referred to as nido type boranes.
Borane BnHn+6, such as B4H10, have a structures that lacks two apexes from the closo type
and are more open. Skeletons are also built by B-B, B-B-B, and B-H-B bonds, and these
are called arachno type boranes. The structures of typical boranes are shown in Fig. 4.3.