Two different structural motifs observed for dimeric dialkylaluminum and dialkylgallium alkynides [R2E-C C-C6H5](2)

The dialkylaluminum and dialkylgallium alkynides [R2E-' CdropC-R'](2) (R = Me, CMe3; E = Al, Ga; R' = Ph) containing CdropC triple bonds attached to their central aluminum or gallium atoms are easily obtained by the reactions of dialkylelement chlorides with lithium alkynides or by treatment of the corresponding alkyne R-CdropC-H with dialkylaluminum or dialkylgallium hydrides. The first reaction is favored by the precipitation of LiCl, the second one by the formation of elemental hydrogen. All products form dimers in which the carbanionic carbon atoms of the alkynido groups adopt bridging positions, but, interestingly, different types of molecular structures were observed depending on the steric demand of the substituents terminally attached to the aluminum of gallium atoms. The small methyl substituents gave structures in which the aluminum or gallium atoms seemed to be side-on coordinated by the CdropC triple bonds of almost linear E-CdropC groups. In contrast, the more bulky tert-butyl groups forced an arrangement in which the CdropC triple bonds were perpendicular to the E-E axis of the molecules. Different bonding modes result, which were analyzed by quantum-chemical calculations.