The geometric structure and conformational properties of the saturated five-membered-ring compound 2,2-di-tert-butyl-1,3-diaza-2-silacyclopentane, (t-Bu)(2)Si(NH)(2)(CH2)(2), was investigated by gas electron diffraction and quantum chemical methods (B3LYP and MP2 with 6-31G* basis sets). The compound exists as a mixture of two conformers, both possessing a twist conformation and C-2 symmetry. In the prevailing form (76(6) % at 305 K) the N-H bonds stagger the adjacent CH2 groups, and in the minor form the N-H bonds eclipse the CH2 groups. This conformational mixture corresponds to a free energy difference of DeltaGdegrees = 0.69(19) kcal/mol. The B3LYP method predicts a preference for the eclipsed conformer. The largest torsion occurs around the C-C bond with tau(NCCN) = 29.2(24)degrees. The degree of puckering in the title compound is considerably smaller than that in silacyclopentane with tau(CCCC) = 49.7(14)degrees. This has been rationalized by larger angle strain in the title compound.