Bis(hexamethyldisilazyl)tin(II) in excess reacts with the molecular alumosiloxane (Ph2SiO)8(AlO(OH))4to yield the well known tin(II) derivative (Ph2SiO)8(AlO2)4Sn2and the novel bicyclic [(Me3Si)2N]Sn[N(SiMe3)SiMe2CH2]2Sn[N(SiMe3)2] containing a tin-tin bond, which is formally related to the starting molecule by a missing hydrogen atom. From X-ray diffraction studies on single crystals it follows that the new compound has almost C2point symmetry (the twofold axis intersecting the tin-tin bond) and that this tin-tin bond is the fusion edge of anellated fivefold Sn-Sn-N-Si-C rings. The bond connecting the almost tetrahedrally coordinated tin atoms (2.737(2) Å) is astonishingly very short. This feature is also reflected in the spectroscopic data (1H,13C,21Si,119Sn-NMR) of the compound with a coupling constant1J (117Sn/119Sn) of 9529 Hz.