A family of diruthenium complexes with ligand-unsupported Ru-Ru bonds has been systematically synthesized, and their crystal structures and physical properties have been examined. A simple, useful reaction between Ru-2(OAc)(4)Cl (OAc- = acetate) and catechol derivatives in the presence of bases afforded a variety of diruthenium complexes, generally formulated as [Na-n{Ru-2(R(4)Cat)(4)}] (n = 2 or 3; R-4 = -F-4, -Cl-4, -Br-4, -H-4, -3,5-di-t-Bu, and -3,6-di-t-Bu; Cat(2-) = catecholate). The most characteristic feature of the complexes is the formation of short ligand-unsupported Ru-Ru bonds (2.140-2.273 angstrom), These comprehensive studies were carried out to evaluate the effects of the oxidation states and the substituents governing the molecular structures and physicochemical properties. The Ru-Ru bond distances, rotational conformations, and bending structures of the complexes were successfully varied. The results presented in this manuscript clearly demonstrate that the complexes with ligand-unsupported Ru-Ru bonds can sensitively respond to redox reactions and ligand substituents on the basis of the greater degree of freedom in their molecular structures.