Pentagonal pyramidal carboranes

Figure 3.10. Localized bond schemes for square pyramidal and pentagonal pyramidal carboranes, showing only one of the several possible canonical forms in each case. (2,4-C2B4H8 and 2,3,5-6363117 incorporate CHB bridge hydrogens. Relative energies, jei in kcal per mole.)...

A nido-carborane mixture, formed from B Hjq with acetylenes under mild conditions, includes alkyl derivatives of nido-CBjH, nido-C2B4Hg, nido-2,3,4-C3B3H7, nido-C4B2Hg as well as nido-l,2-C2B3H7 (Fig. 1) . The reaction of the square-pyramidal nido-l,2-C2B3H7 with HC=CH produces pentagonal-pyramidal... 

The carboranes are a well-recognised class of compounds based on boron cages in which some of the boron atoms have been replaced by carbon atoms (Figure 9.2). Phosphacarboranes are similar-type compounds which contain both carbon and phosphorus substituted within the boron framework, with the P atoms having a pentagonal pyramidal arrangement of bonds to B and C atoms [42], Other heteroboranes are known in which B atoms have been replaced by S, As or Sb atoms. Present best-known heteroboranes are based on 11 or 12 atom frameworks as in Figure 9.3. 

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