With the aim of assessing whether ruthenium(II) compounds with meridional geometry might be utilized as potential antitumor agents, a series of new, water-soluble, monofunctional ruthenium(11) complexes of the general formula mer[Ru(L-3)(N-N)X][Y](n) (where L-3 = 2,2':6',2 ''-terpyridine (tpy) or 4'-chloro-2,2':6',2 ''-terpyridine (Cl-tpy), N-N = 1,2-diaminoethane (en), 1,2-diaminocyclohexane (dach), or 2,2'-bipyridine (bpy); X = Cl or dmso-S; Y = Cl, PF6, or CF3SO3; n = 1 or 2, depending on the nature of X) were synthesized. All complexes were fully characterized by elemental analysis and spectroscopic techniques (IR, UV/visible, and 1D and 2D NMR), and for three of them, i.e., [Ru(Cl-tpy)(bpy)Cl][Cl] (3(Cl)), [Ru(Cl-tpy)(en)(dmso-S)][Y](2) [Y = PF6 (6(PF6)), CF3SO3 (6(OTf))] and [Ru(Cl-tpy)(bpy)(dmso-S)][CF3SO3](2) (8(OTf)), the X-ray structure was also determined. The new terpyridine complexes, with the exception of 8, are well soluble in water (>25 mg/mL). H-1 and P-31 NMR spectroscopy studies performed on the three selected complexes [Ru(Cl-tpy)(N-N)Cl](+) [N-N = en (1), dach (2), and bpy (3)] demonstrated that, after hydrolysis of the Cl ligand, they are capable of interacting with guanine derivatives [i.e., 9-methylguanine (9MeG) or guanosine-5'-monophosphate (5'-GMP)] through N7, forming monofunctional adducts with rates and extents that depend strongly on the nature of N-N: 1 approximate to 2 >> 3. In addition, compound 1 shows high selectivity toward 5'-GMP compared to adenosine-5'-monophosphate (5'-GMP), in a competition experiment. Quantitative kinetic investigations on 1 and 2 were performed by means of UV/visible spectroscopy. Overall, the complexes with bidentate aliphatic diamines proved to be superior to those with bpy in terms of solubility and reactivity (i.e., release of Cl- and capability to bind guanine derivatives). Contrary to the chlorido compounds, the corresponding dmso derivatives proved to be inert (viz., they do not release the monodentate ligand) in aqueous media.