The formation of the diazadisilacyclopentane 2 in a ring expansion reaction is the first example of an SiR2-CH2 insertion into the nitrogen-silicon single-bond accompanied by migration of a hydrogen atom from a carbon to a nitrogen atom [J. Organonzet. Chem. 1994, 482, 285]. Density functional calculations support the experimental findings and suggest a reaction mechanism which includes two four-center transition states and an intermediate silahydrazone (3b). The reaction enthalpy Delta H-R degrees (298 K) of the isomerization reaction converting the three-membered ring (Me3SiN)(2)SiH2 (1b) into the diazadisilacyclopentane Me2Si(NHNSiMe3CH2)SiH2 (2b) is calculated to be -23 kcal mol(-1). Another intermediate of the system shows an unexpected electronic structure with a lone electron pair at the SiH2 silicon atom. The transition states and the unimolecular reaction mechanism are discussed in detail, The calculated geometry for the product 2b is in very good agreement with X-ray structure data.