The synthesis of the cyclodisilazane Ib in a ring-expansion reaction is the first example of a reductive insertion into the nitrogen-nitrogen single bond accompanied by migration of a phenyl group from the silicon to a nitrogen atom. An analogous ring expansion is observed in the synthesis of the cyclodisilazane 2b, which is the first reaction that involves migration of a hydrogen atom from a silicon to a nitrogen atom. A crystal structure is presented for compound Ib. Quantum chemical calculations support the experimental findings. The reaction enthalpy Delta H-R degrees (298 K) of the isomerization reaction converting the three-membered-ring (Me3SiN)(2)SiF2 (3a) into the cyclodisilazane Me2Si(NSiMe3NMe)SiF2 (3b) is calculated to be -72.1 kcal mol(-1) The transition state and the unimolecular reaction mechanism are discussed in detail.