Abstract
closo-Carboranes that can be formally divided into rings and caps follow a six-electron rule. The relative stability of various isomers within a given carborane depends on the size of the ring on which the polyhedral structure is based. With three-and four-membered rings the CH group fits in as the best cap; the overlap of the orbitals of CH with the orbitals of three-and four-membered borocycles is favourable. Thus 1,5-C2B3H5 is more stable than 1,2-C2B3H5. The BH group with more diffuse orbitals overlaps better with the orbitals of a five-membered ring;1,7-C2B5H7 is less stable than 1,2-C2B5H7, which, in turn, is less stable than 2,4-C2B5H7. The orbitals of BH are not diffuse enough to overlap favourably with the orbitals of a six-membered ring; C2B6H8 prefers a dodecahedron to a hexagonal bipyramid. Caps with more diffuse orbitals such as BeH, Li, and transitional-metal fragments should stabilize polyhedral based on six-membered rings.
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