The synthesis and structure of cis-1,4-di(1-pyrenyl)decamethylcyclohexasilane (2), one of the first cis-1,4-substituted cyclohexasilanes, are reported. This isomer is unexpectedly formed as the major product in the reaction of a mixture of trans-and cis-1,4-dichlorodecamethylcyclohexasilane (4a and 4b, respectively) with 1-pyrenyllithium. Reaction of 4a,b with phenyllithium showed that preferential formation of the cis-product is a general feature of this kind of reaction. According to the X-ray crystal structures of two different solvates, the cyclohexasilane ring in 2 adopts a unique boatlike geometry, with the pyrenyl substituents occupying an equatorial or bisectional position. 1 H NMR data indicate that a boatlike geometry also occurs in solution. Furthermore, the X-ray crystal structure of trans-1,4dihydrodecamethylcyclohexasilane, which is an intermediate in the synthetic pathway to 2, is reported.