5f, 5g, 6f, 8f, 9g, log, l l g ) of which only 9 g (R = Ph) corresponds to the spirosilanes and -germanes. This is the result of effective competition between intramolecular and intermolecular organoboration in the case of the tetra-1-alkynyltin compounds. The protodeborylation of l a , b and 2d, e with MeCOzH leads to 12a, b and 13d, e, respectively. 12a isomerises by UV irradiation to ally1 isomers 12a'. From 12a with 2 equiv. of maleic anhydride the 1:2 addition compound 14a is obtained, the autaddition of 12a, d leads to 15a, d. Spiro compound 12a reacts with (OC)5Fe or CpCo(C,H,), to give the cyclodiastereomeric q4-complexes [(OC)3Fe]z-12a (16al-a,; X-ray structure analysis of meso16a1), (OC)3Fe-12a (17a) and (CpCo),-12a (18al-a4). -All products were characterized by multinuclear NMR, including measurements of the coupling constants 1J(13C13C), 2J(zgSizgSi), nJ(29Si13C) and nJ("gSn'3C).

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Nicht nur die topochemisch erlaubten, sondern auch die zum Teil gebildeten topochemisch verbotenen Produkte der Photodimerisierung von kristallinen Butenoliden 1 lassen sich mit dem hier vorgestellten modifizierten Cohen‐Modell qualitativ richtig vorhersagen. Damit ist der Weg frei für die Anwendung standardisierter Molecular‐Modeling‐Verfahren auf Probleme der Organischen Festkörperchemie.