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Why are closo-boranes generally more stable than nido-boranes and arachno-boranes?
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cluster
Figure 1: The four stable geometric structures of the seven-atom cluster of argon, in order of increasing energy: (A) A pentagonal bipyramid. (B) A regular octahedron with one face capped by the seventh atom. (C) A regular tetrahedron with three of its faces capped by other atoms. (D) A trigonal bipyramid with two of its faces capped by other atoms; although this has the highest energy of the four structures, it is very close in energy to the tricapped tetrahedron.
The relative stability of borane clusters is primarily determined by their structural geometry and degree of closure. In the context of borane chemistry, these clusters are classified based on their polyhedral frameworks:
- Closo-boranes: These consist of complete, closed deltahedrons (polyhedrons with triangular faces) and are the most thermally stable and least reactive of the series.1
- Nido-boranes: These are nonclosed structures where the boron cluster occupies corners of an -cornered polyhedron, effectively representing a closo-structure with one missing vertex. They are generally more reactive and less thermally stable than closo-boranes.1
- Arachno-boranes: These are even more open, with boron atoms occupying corners of an -cornered polyhedron (two missing vertices). They are typically the least stable and most reactive of the three types.1
The stability of these clusters correlates with the number of valence electrons present, a relationship rationalized by Wade’s rules, which account for the distribution of three-centre, two-electron bonds within the cluster.21