Prismanes structure

Figure 10.12 (a) Prismane structure of [Ge6 CH(SiMej)2)6l (for clarity only the ipso C atoms of the R groups are shown), (b) The tetracycio structure of [GegBugBrj] with only the ipso C atoms of the Bu groups shown, (c) The cubane structure of [Geg(CMeEt2>g] again with only the ipso C atoms of the t-hexyl groups shown. 

Figure 7 shows the ORTEP drawing of the hexasilaprismane 1237. Its structural parameters are listed in Table 4. The crystal has a two-fold axis of symmetry. The skeleton has a slightly distorted prismane structure with two triangular units [Si-Si = 2.374-2.387 A (av. 2.380 A) and ZSi-Si-Si = 59.8-60.3° (av. 60.0°)] and three... 

The hexamer Ge6[CH(SiMe3)2]6 has a prismane structure as shown in Fig. 14.7.3(e). The Ge-Ge distances within the two triangular faces are 258 pm, being longer than those in the quadrilateral edges, which are 252 pm. On the other hand, the hexamer (SnPh2)6 exists in the chair conformation, (as shown in Fig. 14.7.3(f)) with Sn-Sn distances 277 to 278 pm. 

FIGURE 12. Calculated EnEg nHg isomers (E, E = C, Si, Ge) with planar, chair, boat and prismane structure. Relative energies are given in Table 22. The boat form is not an energy minimum on the PES. Reprinted with permission from Reference 101. Copyright 1994 American Chemical Society... 

Such properties are also reflected in the relative stability of the MgHg valence isomers (Table 2). It is well known that benzene (CgHg) is very stable due to cyclic delocalization of its six re electrons (aromatic stabilization), and it is much more stable than other strained valence isomers — Dewar benzene, benzvalene and prismane13,14. However, the tendency is completely reversed in the case of heavier atoms the isomers with a smaller number of double bonds are more favorable. As a result, the prismane structure becomes much more stable than the benzene structure on going from carbon to tin atoms10,15. 

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