Two mononuclear mixed-ligand ruthenium(III) complexes with oxalate dianion (ox(2-)) and acetylacetonate ion (2,4-pentanedionate, acac(-)), K-2[Ru(ox)(2)(acac)] (1) and K[Ru(ox)(acac)(2)] (2), were prepared as a candidate for a building block. In fact, reaction of complex 2 with manganese(II) sulfate gave a heterometallic tetranuclear complex, TBA[Mn-[]{(mu-ox)Ru-III(acac)(2)}(3)] (5) in the presence of tetrabutylammonium (TBA) bromide. The H-1 NMR, UV-Vis, selected IR and FAB mass spectral data of these complexes are presented. Both mixed-ligand ruthenium(III) complexes gave a Nernstian one-electron reduction step in 0.1 mol dm(-3) Na2SO4 aqueous solution on a mercury electrode at 25 degreesC. Comparison of observed reversible half-wave potentials with calculated values for a series of [Ru(ox)(n)(acac)(3-n)](n-)(n = 0-3) complexes by using Lever's ligand electrochemical parameters is presented. (C) 2003 Elsevier B.V. All rights reserved.