The electrochemical properties of the Ru(II)-mono(bipyridine) complexes (4,4'-L-2-2,2'-bpy)Ru(CO)(2)Cl-2 (L = NH2, t-Bu, Me, H, Cl, Br, COOH, H2PO3, NO2) (L = 1-9, respectively) were studied in CH3CN by means of cyclic voltammetry. The effect of the electronic nature of the ligand substituent on the experimental redox data, on the computational HOMO-LUMO energy differences, and on the ionization energies was investigated. The computational work was carried out using the density functional hybrid method. The positive shift of the Ru(II)/Ru(III) potential values, which was induced by increasing the electron-withdrawing nature of the substituent. was found to correlate with the decreasing HOMO-LUMO energy difference and with the increasing ionization energies. The ligand electrochemical parameter, E-L(L), was derived from the experimental redox data for the 4,4'-L-2-2,2'-bipyridine ligands (L = NH2, t-Bu, Cl, NO2). The ligand parameters followed the electron-donor or electron-acceptor nature of the ligands studied. (C) 2003 Elsevier B.V. All rights reserved.