New (arene)ruthenium(II) derivatives containing neutral HL or anionic L(-) ligands (arene = p-cymene or benzene, HL in general, in particular HL(1) = bis(pyrazol-1-yl)acetic acid and HL(2) = bis(3,5-dimethylpyrazol-1-yl)acetic acid) have been synthesized and analytically and spectrally characterized. The ligands in neutral form coordinate ruthenium in a chelating kappa(2)-N,N'-bidentate fashion affording 1:1 derivatives of formula [Ru(arene)(HL)Cl]Cl, where the inner Cl can be replaced by a phosphine. These derivatives show very high conductance values in water, due to the contribution of H(3)O(+) produced by deprotonation of the -COOH fragment in HL ligands and consequent formation of 1:2 electrolytes such as [Ru(arene)(kappa(3)-N,N',O-L)]Cl(2) species. However, the remaining derivatives contain monoanion L(-) ligands coordinating in the tripodal kappa(3)-N,N',O-tridentate fashion. The solid-state X-ray structure of the complex [Ru(eta(6)-p-cymene)(kappa(3)-N,N',O-L(1))]PF(6) confirmed such behavior. The redox properties of those compounds have been investigated by cyclic voltammetry and controlled potential electrolysis, which, on the basis of their measured Ru(II/III) oxidation potentials, have allowed for the ordering of the HL and L(-) ligands according to their electron-donor character. This is accounted for by DFT calculations, which show a relevant contribution of L ligand orbitals to the highest occupied molecular orbitals (HOMOs) when they are coordinated in the monoanionic tridentate form, while for derivatives containing neutral HL ligands, the main contribution to the HOMOs comes from orbitals of the metal and chlorine atoms, the overall contribution from the bidentate HL ligand orbitals being small. Values of the Lever electrochemical E(L) ligand parameter (a measure of the net electron donor character of a ligand) have been estimated for the above and related acylpyrazolonate ligands, as well as for the eta(6)-coordinated benzene and cymene.