The structure of organoaluminum(halide) complexes, used as cocatalysts in Ziegler−Natta catalysis, is investigated by experimental far-infrared spectroscopy and by quantum simulations. Experimental and calculated far-infrared spectra agree very well, holding promise for the employed far-infrared/quantum simulation approach to reveal more detail on the catalytically active site in Ziegler−Natta catalysis. Whereas the species preferably form dinuclear aluminum complexes when the bridge can be formed by chlorine, on the basis of the calculated total energies and the observed infrared spectra, alkyl-bridged dimers are found to be much less stable.