Several 1,3-dithia-2-silacyclopent-4-ene (dithiasilole) derivatives were prepared by the reactions of cis-NaS-HC=CH-SNa with the corresponding dichlorosilanes. The compounds formed charge-transfer (CT) complexes with tetracyanoethylene in CH2Cl2. The UV-vis spectra of the CT complexes indicated that the ionization potential of 2,2-bis(trimethylsilyl)dithiasilole was smaller than that of 2,2-dimethyldithiasilole. The results were explained by the sigma (Si-R)-pi interactions resulting in a higher HOMO level. Ab initio MO calculations supported the experimental results.