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How do closo-, nido-, and arachno-boranes differ in the openness of their structures?
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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.
In the study of borane clusters, the terms closo-, nido-, and arachno- describe the degree of openness in the boron framework, which is determined by the number of missing vertices from a complete deltahedron (a polyhedron with triangular faces).12
- Closo-boranes: These structures represent the most closed form, consisting of a complete deltahedron where n boron atoms occupy all n corners of the polyhedron.1
- Nido-boranes: Derived from the Latin word for "nest," these are nonclosed structures. The boron cluster occupies n corners of an (n + 1)-cornered polyhedron, meaning the structure is a closo-polyhedron with one missing vertex.12
- Arachno-boranes: Meaning "spider's web," these clusters are more open than nido-structures. The boron atoms occupy n contiguous corners of an (n + 2)-cornered polyhedron, which corresponds to a closo-polyhedron with two missing vertices.12
These structural classifications are essential for understanding the electron-deficient nature of these compounds, where multicentre bonding allows for complex polyhedral arrangements.13