Arachno-type 13-vertex cluster

At first glance, electron count in these clusters gives a remarkable analogy to the carbaboranes. Each aluminum atom contributes two electrons to the cluster, three electrons are from each carbon atom, and one electron is added by the bridging hydrogen atom. A total count of 32 electrons or 16 electron pairs results, which in accordance with the Wade rules [6] gives an arachno-type 13-vertex cluster. Indeed,... 

There is a series of metallaboranes of the type [SBgHjoM], [S2B7H7M], etc., where M seems to contribute two cluster electrons giving a nido-electron count. Nevertheless, these metallaboranes must be designated as arachno-dusters. Though the principal structure of the 10-vertex nido- and arachno-skeleton is the same, they differ structurally by the position of the bridging H atoms (which are the gunwale positions 5-10 and 7-8 in the arachno-case) and, moreover, in their physical properties, particularly in the sequence of the 11B NMR shift values. We will discuss the ar-10 clusters with formal ni-10 electron count later. 

Determine the type of borane cluster from Table 10.20, where n is the number of B atoms. Goso structures have n vertices. Nido structures (from the Greek word for nest) are based on the n + I deltahedron with a single vertex missing. Similarly, the arachno (spider), hypho (net), and klado (branch) boranes... 

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