Arachno Carboranes fragment

In 1971, a note 164) was published favoring the hypothesis that the carboranes, boranes, their isoelectronic anions, Lewis base adducts, and heteroatom-substituted analogs should be viewed as constructed about the vertices of either the most spherical series of triangular-faceted polyhedra (deltahedra) found to be characteristic of the dicarba-cZoao-carboranes (Fig. 1) or, with one lone exception, fragments of the series of deltahedra produced by the successive removal of the highest coordinated vertices that sequentially define the nido and arachno classes. This position was in conflict with the then prevalent shibboleth that all nido and arachno compounds [except B5H9 (I-N5)] had or would prove to have icosahedral fragment structures. 

Other Heteroboranes. Other well-documented families of heteroboranes include the azaboranes, phosphaboranes, arsenaboranes, stibaboranes, selenaboranes, and telluraboranes. Table 5 lists representative examples of heteroboranes from Groups 15 (V) and 16 (VI). The thiaboranes are the most extensively developed class of heteroboranes after the carboranes. The thiaboranes [arachno-6-SB9H12] [45979-10-0] and nido-6-SB6Hn [59120-72-8] (145) can be converted (eq. 65) to 1-SB9H9 /41646-564/, which has a nonicosahedral fragment structure (22). 

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

Other carboranes have the more open, nido (nest-like) and arachno (cobweblike) structures that are illustrated in columns 2,3, and 4 in Figure 3.1. Their structures can be regarded as fragments of the closo deltahedra. One vertex... 

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