Quantum chemical calculations were performed to investigate the cooperative effect of the nitrogen and silicon atoms on the singlet-triplet energy spacing and the reactivity of the singlet state in 1,2-diazacyclopentane-3,5-diyls and 1,2-diaza-4-silacyclopentane-3,5-diyls. The largest singlet-triplet energy gap (ΔE^c_ST = -36.1 kcal/mol) found so far in localized 1,3-diradicals was in the C_2v symmetry of 4,4-difluoro-1,2-diaza-4-silacyclopentane-3,5-diyl at the UB3LYP/6-31G(d) level of theory. The cooperative effect was also found in the energy differences of singlet diradicals with the corresponding ring-closing compounds, bicyclo[2.1.0]pentane derivatives. The singlet state of the 1,2-diaza-4-silacyclopentane-3,5-diyls was calculated to be energetically more stable than the ring-closing compound. The notable finding on the stability of the singlet diradicals may be attributed to the resonance structures that specifically stabilize the singlet state of diradicals. The computational studies predict that the singlet 1,2-diaza-4-silacyclopentane-3,5-diyl is a persistent molecule under conditions without intermolecular-trapping reagents.