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How are carboranes related to 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.
Carboranes are closely related to boranes as they are essentially boron hydride clusters that incorporate carbon atoms into their structure. Their relationship is defined by several key chemical and structural parallels:1
- Structural Similarity: Carboranes possess polyhedral molecular structures that are isoelectronicâmeaning they share the same number of electronsâwith polyhedral boranes. Like boranes, they utilize both ordinary two-centre bonds and three-centre, two-electron bonds, which are characteristic of electron-deficient cluster compounds.234
- Nomenclature: Carboranes employ the same structural prefixes as boranesâsuch as closo-, nido-, and arachnoâto describe the openness or closure of their polyhedral cages.23
- Chemical Behavior: Both classes of compounds exhibit similar trends in stability and reactivity based on their cage structure; for instance, closo-carboranes are generally more thermally stable than nido- or arachno-carboranes, mirroring the behavior of their borane counterparts.23
- Synthesis: Carboranes are often synthesized through the reaction of acetylene or its derivatives with boron hydrides, effectively substituting carbon into the boron framework.2