The novel sandwich complex Cp(2)*Al(2)I(2), which was recently synthesized by Minasian and Arnold, has been characterized using ab initio and density functional methods. A large family of related compounds was also investigated. Although a few Al(II)-Al(II) bonds are known, this is the first such bond to be supported by Cp-type ligands. In addition, in the remarkable Cp(4)*Al(4) synthesis by Roesky, Cp(2)*Al(2)I(2) is the Al(II) intermediate; Cp(4)*Al(4) is important as a precursor to novel organoaluminum species. Halogen and ligand effects on the AlAl bond in Cp(2)*Al(2)I(2) were systematically explored by studying a series of 20 Cp(2)*Al(2)I(2) derivatives using density functional theory with relativistic basis sets for the halogens. Comparison was made with the focal point treatment, which uses extrapolation to estimate the full configuration interaction and complete basis set limit energy. Torsional potential energy curves, natural population analyses, and enthalpies of hydrogenation were computed. Using the focal point approach, torsional barriers were computed with 0.05 kcal mol(1) uncertainty. The interplay of steric and electronic effects on the torsional potential energy curves, enthalpies of dehydrogenation reactions, and geometries is discussed. In species with small ligands (R = H, Me), hyperconjugative effects determine the torsional landscape, whereas steric repulsions dominate in species with Cp* alkyl ligands. Species with Cp ligands represent an intermediate case, thus providing insight into how ligands modulate the structures and properties of small metal clusters.