Na[trans-RuCl4(R2SO)(L)] and mer,cis-[RuCl3(R2SO)(R2SO)(L)] (L = nitrogen ligand) were prepared and their spectroscopic characterization in the solid state (IR) and in solution (NMR, UV-visible) is described.The cyclic voltammetry of the complexes performed in aqueous solution shows in every case a monoelectronic rapid Ru(III) /Ru(II) electron transfer.The observed formal potentials are much more pos. than those reported for other Ru(III) complexes.The net charge, together with the π acidic ability of DMSO-S, are the factors responsible for this behavior.The crystal structures of Na[trans-RuCl4(DMSO-S)(NH3)]·2DMSO (5a) (triclinic, space group P1̅, Z = 2, R = 0.039), Na[trans-RuCl4(DMSO-S)(Im)]·H2O·Me2CO (5b) (triclinic, space group P1̅, Z = 2, R = 0.031) and mer,cis-RuCl3(DMSO-S)(DMSO-O)(NH3) (6a) (monoclinic, space group P21/n, Z = 4, R = 0.022) were determinedIn 5a and 5b, the DMSO ligand is S-bonded to Ru, with Ru-S bond distances (trans to N) of 2.2797(7) and 2.2956(6) Å, resp., while in 6a, one DMSO, trans to N, is S-bonded (Ru-S, 2.2714(6) Å), and the other, trans to Cl, is O-bonded (Ru-O, 2.070(2) Å).The Ru-Cl bond distance trans to O is 2.3207(7) Å.The Ru-Cl bond distances trans to Cl are similar in all three compounds averaging 2.343(6) Å.The antitumor activity of some of the new compounds is currently under investigation.Their redox potentials suggest the possibility that they might undergo an easy biol. reduction in vivo.