This paper focuses on the electron transfer reaction, in water, between aquaethylenediaminetetraacetatoruthenate(III) and aromatic diamines, particularly 9,10-diaminophenanthrene (pha), as well as on the chemical characterization of the corresponding a-diimine ruthenium(II) complexes. The redox chemistry of these complexes at all their accessible metal- and ligand-centered oxidation states in slightly acidic (pH 4.7) aqueous solutions was investigated by means of electrochemistry and spectro-electrochemistry. All these redox species were found to be rather stable in solid state or aqueous solutions and totally interconvertible in reversible chemical/electrochemical processes. Comparison of the metal-centered redox potentials of the [Ru-III/II(edta)(L)](-/2-) (L = 9,10-phenanthrenequinone diimine, o-benzoquinone diiimine, 1,10-phenanthroline and 2,2'-bipyridine) complexes revealed that the quinone diimines possess stronger pi-acceptor abilities than 1,10-phenanthroline and 2,2'-bipyridine, supporting a more efficient stabilization of the Ru(II) species, by means of pi-backbonding interactions, in the cases of the quinonoid ligands.