Treatment of diphenyl-diethynylsilanes, (H 5 C 6 ) 2 Si(CϵC-RЈ) 2 (RЈ = -C 6 H 5 , -4-Me-C 6 H 4 ), with two equivalents of dialkylaluminium hydrides, R 2 Al-H (R = -CMe 3 , -CH 2 CMe 3 ), afforded the corresponding dialkenylsilanes (3-5) by hydroalumination. The mixed alkenyl-alkynyl compounds resulting from the reduction of only one CϵC triple bond occurred as intermediates. Two of these (1 and 2) were isolated and characterized by crystal structure determinations. They show close interactions of the α-carbon atoms of the ethynyl groups with the coordinatively unsaturated aluminium atoms and a cis arrangement of H and Al atoms across the C=C double bonds. cis/trans Rearrangement took place upon the [a]

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Hydroalumination or hydrogallation of tri(ethynyl)silanes, RSi(CϵC-Ar) 3 (1a, R = Ph, Ar = Ph; 1b, R = Me, Ar = Ph; 1c, R = Me, Ar = C 6 H 4 Me), with the element hydrides H-EtBu 2 (E = Al, Ga) in stoichiometric ratios of 1:1 to 1:3 at ambient temperature yielded the addition products (PhCϵC) 2 (R)Si[(tBu 2 E)C=C(H)Ph] (2, R = Ph, E = Ga; 3a, R = Me, E = Al; 3b, R = Me, E = Ga), (PhCϵC)(Me) Si[(tBu 2 E)C=C(H)Ph] 2 (4a, E = Al, 4b, E = Ga) and (Me)Si[(tBu 2 Al) C=C(H)Ar] 3 (5, Ar = Ph; 6, Ar = C 6 H 4 Me). Compounds 2-4 show

Bis(phenylethynyl)silanes reacted with equimolar quantities of various dialkylaluminium hydrides, R2Al–H [R = CH2tBu, tBu, iBu, CH(SiMe3)2], by hydroalumination and addition of a Al–H bond to a C≡C triple bond. Alkenyl‐alkynylsilanes were formed, which had the aluminium atoms in geminal positions with the silicon atoms and adopt a cis arrangement of aluminium and hydrogen at the resulting C=C double bonds in most cases. cis/trans‐Rearrangement with the formation the trans‐addition products was observed only for the two sterically least shielded compounds. Intramolecular interactions of the coordinatively unsaturated aluminium atoms with the α‐carbon atoms of the intact alkynyl groups were detected by crystal structure determinations for the tert‐butyl and neopentyl derivatives, whereas they did not occur with bulky bis(trimethylsilyl)methyl substituents.