Borane - Cluster Types

Cluster Types

It was realised in the early 1970s that the geometry of boron clusters are related and that they approximate to deltahedra or to deltahedra with one or more vertices missing. The deltahedra that are found in borane chemistry are (using the names favoured by most chemists)

One feature of these deltahedra is that boron atoms at the vertices may have different numbers of boron atoms as near neighbours. For example, in the pentagonal bipyramid, 2 borons have 3 neighbors, 3 have 4 neighbours, whereas in the octahedral cluster all vertices are the same, each boron having 4 neighbours. These differences between the boron atoms in different positions are important in determining structure, as they have different chemical shifts in the 11B NMR spectra.

• Diborane, B₂H₆
• Tetraborane, B₄H₁₀
• Pentaborane-, B₅H₉
• Decaborane-, B₁₀H₁₄
• B₁₂H₁₂2-

B₆H₁₀ is a typical example. Its geometry is, in essence, a 7-boron framework (pentagonal bipyramid), missing a vertex that had the highest number of near neighbours, e.g., a vertex with 5 neighbours. The extra hydrogen atoms bridge around the open face. A notable exception to this general scheme is that of B₈H₁₂, which would be expected to have a nido- geometry (based on B₉H₉2− missing 1 vertex), but is similar in geometry to B₈H₁₄, which is based on B₁₀H₁₀2−.

The names for the series of boranes are derived from this general scheme for the cluster geometries:-

• hypercloso- (from the Greek for "over cage") a closed complete cluster, e.g., B₈Cl₈ is a slightly distorted dodecahedron
• closo- (from the Greek for "cage") a closed complete cluster, e.g., icosahedral B₁₂H₁₂2−
• nido- (from the Latin for "nest") B occupies n vertices of an n+1 deltahedron, e.g., B₅H₉ an octahedron missing 1 vertex
• arachno- (from the Greek for "spiders web") B occupies n vertices of an n+2 deltahedron e.g. B₄H₁₀ an octahedron missing 2 vertices
• hypho- (from the Greek for "net") B occupies n vertices of an n+3 deltahedron possibly B₈H₁₆ has this structure, an octahedron missing 3 vertices
• conjuncto- 2 or more of the above are fused together, e.g., the edge or two vertex fused B₁₉H₂₂1−, face or three vertex fused B₂₁H₁₈1−, and four vertex fused B₂₀H₁₆