Pentagonal faces

In the early 1960s it was recognized (6,187) that there were bonding similarities between the pentagonal face of the isomeric nido-Q fri ]f, ions and the well-known cyclopentadienide ion (Cp ) (Fig. 16). The isomeric nido-Q fri ]f, ions, which are commonly known as dicarboUide ions, and... 

Figure 6.21 Relation between C2B9Hn and C sHs". In this formalism the c/o.ro-carboranes C2B10H12 are considered as a coordination complex between the pentahapto 6-electron donor C2B9H] - and the acceptor BH (which has 3 vacant orbitals). The o/oso-structure can be regained by capping the open pentagonal face with an equivalent metal acceptor that has 3 vacant orbitals.

Figure Three represernarions of the structure of Cm- (a) normal ball-and-stick model (b) the polyhedron derived by truncating the 12 vertices of an icosahedron to form 12 symmetrically separated pentagonal faces (c) a conventional bonding model.

As a second illustrative aspect of the boron/carbon connection, we consider the dicarbollide ion, C2B9H112-, which can be pictured as derived from Bi2Hi22- by removing one BH(t) vertex and replacing two adjacent borons of the exposed pentagonal face by carbon atoms, as shown in Fig. 3.116. This ion exhibits remarkable electronic mimicry of the cyclopentadienyl ion (C5H5-, Section 3.3.2), forming similar coordination and sandwich complexes with many metallic species.162... 

Start with a 12- or 13-vertex gallium polyhedron containing some quadrilateral and/or pentagonal faces. This polyhedron is conveniently called the core polyhedron. 

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