The reaction of 1,1-dilithio-2,3,4,5-tetraphenylsilole (1) with 1,1-dichloro-2,3-diphenylcyclopropene (2) leads to the novel 1,4-disila-1,4-dihydropentalene (4), as well as an exceptionally stable diradical for which the structure 3 is suggested. The diradical is unreactive toward water, methanol, and chloroform; upon heating it transforms into 4. Structure 3 for the paramagnetic species is proposed on the basis of EPR data and theoretical calculations. The trans-trans isomer of diradical 3 was calculated to be more stable than its cis-cis isomer. The strong and stable EPR signal in the reaction mixture is probably due to the trans-trans isomer of diradical 3 in the triplet state. A reaction scheme describing the formation of 3 and 4 is presented.