3,4,7,8-Tetrasilacycloocta-l,5-diyne

Before going into a detailed account of the chemistry of phanes, the author will touch on 3,4,7,8-tetrasilacycloocta-l,5-diyne briefly, since the compound illustrates the importance of a—it mixing. The ionization potential of the Si-Si bond is estimated by photoelectron spectroscopy to be 8.69 eV (9). Thus, the HOMO level of the Si-Si is comparable to most HOMOs of tt systems. Consequently, the Si-Si bond can conjugate efficiently with carbon-carbon double and triple bonds, benzene rings, and other tt systems. Most Si-Si bonds are stable enough to construct sophisticated structures by themselves and with organic molecules (10). 

A further example that the c-bonds can influence the 7c-splitting drastically can be seen by comparing the first four bands of the PE spectra of 1,5-cyclooctadiyne (42a) with those of3,3,4,4,7,7,8,8-octamethyl-3,4-7,8-tetrasilacycloocta-l,5-diyne(43). ThePE spectrum of 43 is shown in Figure 14a. 

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