Hexasilabenzenes

As has been shown by a calculation of the temperature evolution of the Gibbs energy differences in the isomeric Si6H6 (g), at a temperature above 340 K hexasilabenzene (Did structure) becomes relatively more stable than (288) [89CPL(161)175 90JST143]. 

The HOMO of hexasilabenzene is by ca 2 eV higher than in benzene, while the HOMO-LUMO gap is much smaller in 31 than in benzene60. This suggest that hexasilabenzene should be more reactive than benzene towards both electrophiles and nucleophiles. 

Another index of delocalization, devised and widely applied by Schleyer et al.,163 is the nuclear-independent chemical shielding (NICS) value. When calculated above the ring, this value corresponds to the -contribution which is a significantly negative quantity for situations with aromatic delocalization . Scheme 21 shows the A and NICS values computed by Schleyer et al.141 for benzene, hexasilabenzene, and hexazine. The three species are seen to possess equally aromatic electron sextets, as indeed anticipated from the appreciable vertical resonance ener-... 

We want to use this knowledge to tell us something about R2M=MR2 and RM=MR, about which we know considerably less. Maybe this information will help chemists make digermenes and distan-nenes that will retain their structures in solution or the gas phase. Maybe it will aid in the synthesis of stable triply bonded compounds of silicon and germanium, or hexasilabenzene ... 

Because H-Si-SiHs is nearly isoenergetic with H2Si = SiH2, it is not obvious which structure would be favored for hexasilabenzene, SieHe. Whereas proponents of aromaticity may take heart from the theoretical calculations that show the hexasilabenzene structure to be 34 kcal/mol more stable than the tris(silylene) form, they will be reminded of how little aromaticity has to do with the magic of six tt electrons by the revelation that the hexasilaprismane is calculated to be 14 kcal/mol more stable yet. 

Silicon-based polymers form a dimensional hierarchy from disilanes, to crystal silicon, and through polysilanes, ladder polymers, siloxenes, polysilane alloys, clusters, and amorphous silicons and include unsaturated systems, such as polysilenes, hexasilabenzenes, and so on. Their properties depend basically on the network dimensions and can vary from conducting (metallic) and semiconducting to insulating. 

Hexasilabenzene 97, being the complete silicon analogue of benzene, has attracted considerable theoretical interest and, as a result, our knowledge about this fascinating molecule is extensive although it had not yet been observed experimentally. It is interesting to follow the procession of the calculations for this molecule. 

FIGURE 41. The b2g vibrational mode (a) for the deformation of D6h hexasilabenzene to the D3d nonplanar structure (b, 6-31G ). Silicon and hydrogen atoms move alternately up (O) and down ( ). Reproduced by permission of the American Institute of Physics from Ref. 255. 

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