Density-functional theory methods were used to investigate the structure of dimeric aluminum (III)water complexes as a function of bridging group. The possibilities of oxygen, water, and hydroxylbridge ligands and a variety of structural arrangements, such as cis/trans, with respect to the relativeposition of hydroxyl ligands, were considered.Within the limit of our computational level, we foundthat electrostatic repulsion between hydroxyls is important in deciding the polyaluminum structure.Although the structures of aluminum-hexaaquo predominate, species with small number of chargesor a large number of hydroxyl ligands have a tendency toward a five-coordinate trigonal bipyramidalconfiguration. Because water is electronically neutral, it cannot provide enough negative charges asa bridge ligand to stabilize two Al (III) molecules. The energy differences among manyconfigurational isomers of hydroxyl Al are so small that they may coexist and convert into eachother easily at room temperature.