Bis(hexamethyldisilazyl)tin(II) in excess reacts with the molecular alumosiloxane (Ph
2
SiO)
8
(AlO(OH))
4
to yield the well known tin(II) derivative (Ph
2
SiO)
8
(AlO
2
)
4
Sn
2
and the novel bicyclic [(Me
3
Si)
2
N]Sn[N(SiMe
3
)SiMe
2
CH
2
]
2
Sn[N(SiMe
3
)
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 C
2
point 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 (
1
H,
13
C,
21
Si,
119
Sn-NMR) of the compound with a coupling constant
1
J (
117
Sn/
119
Sn) of 9529 Hz.