Nido Carboranes skeletal shapes

Figure 3.1. Deltahedral and deltahedral fragment skeletal shapes of typical closo, nido, and arachno boranes and carboranes.

Entries in the second column of Figure 3.1 and in Figures 3.3 and 3.4 show the deltahedral fragment skeletal shapes most commonly encountered among boranes and carboranes of formulae B H +4, CB iH +3, C2B 2H +2, and so on, all of which can be regarded as derived from hypothetical anions [B H ]4 by protonation and/or isoelectronic replacement of B by C. Their molecular formulae are sometimes written as CxB H +4, where x + y = n, the total number of skeletal atoms. The skeletal structures of these nido species. 

To help identify isomers, the shape of a particular nido carborane can be identified in its name by a Roman numeral indicating the connectivity of the site left vacant, that is, the number of skeletal atoms surrounding the open face. For example, the carboranes with six skeletal atoms related to BeHio shown in Figure 3.3 are said to have mdo-6(y structures, while those related to B10H14 shown in Figure 3.4 have nz6Zo-10(VI) structures (see also Table 3.1). 

The structural patterns illustrated in Figure 3.1, and the shapes shown in column 3 for arachno species, are essentially those first suggested by Williams in 1971,2 and have stood the test of time remarkably well. However, the shapes shown should be regarded as illustrative rather than definitive. As with the nido species, alternative shapes to those shown in Figure 3.1 are possible for arachno species, with different sites left vacant on the parent deltahedron. We have already noted that many nido boranes and carboranes with eight skeletal atoms show deviations in their atom connectivities from those shown in Figure Nevertheless, apart from these particular exceptions,... 

---