Treatment of (silylalkynyl)benzenes with (Me3C)2GaH afforded stable cis‐addition products, for example, (Me3C)2GaC(SiMe3)C(H)C6H5 (1), while spontaneous cis/trans rearrangement was observed for sterically less shielded gallium hydrides. The corresponding trans‐di(tert‐butyl)gallium compounds (13, 14) were obtained by the reaction of C6H6−n[C(H)C(SiMe3)GaCl2]n (11, 12) with LiCMe3. In contrast, spontaneous isomerization took place upon reaction of (Me3C)2AlH with phenyltrimethylsilylethyne. In this case the cis isomer (17) was detected only at low temperature, while the trans product (18) formed quantitatively above 0 °C. Quantum‐chemical calculations showed that the trans forms are thermodynamically favored, essentially caused by a better mesomeric interaction of the CC double bonds with the phenyl groups, a smaller steric stress in the molecules, and a short bonding contact of the coordinatively unsaturated Al or Ga atoms to CH bonds of the aromatic rings. The rotation about the CC double bonds follows a zwitterionic mechanism, and the relatively small rotational barrier is further lowered by an interaction to a Lewis acidic lithium cation.

Hydrogallation of 1,4-bis(trimethylsilylethynyl)benzene and 1,3,5-tris(trimethylsilylethynyl)benzene with dialkylgallium hydrides R2GaH (R = Et, nPr, iPr, neopentyl, tBu) afforded the corresponding addition products with intact GaR2 groups and two or three alkenyl substituents. In all products the gallium atoms attacked those carbon atoms that are attached to the trimethylsilyl groups. The expected cis arrangement of gallium and hydrogen atoms at the CC double bonds was detected only with di(tert-butyl)gallium residues. Smaller alkyl groups gave the spontaneous formation of the trans-addition products. Cis/trans isomerization is an inevitable step for the formation of effective chelating Lewis acids, and in particular the trisalkene derivatives form interesting chalice-like hollows containing three Lewis-acidic centers at their inner surfaces.

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