Two mononuclear ruthenium(iii) mer- and fac-isomers of the formula [RuCl3(PPh3)(dmpbt)] (where PPh3 = triphenylphosphine, dmpbt = 2-(3,5-dimethylpyrazoll-yl)benzothiazole) have been synthesised from the reaction of [RuCl3(PPh3)3] with a bidentate ligand - dmpbt. Appropriate reaction conditions allowed obtaining the two isomers separately without separation techniques. X-ray crystallography has determined the crystal and molecular structures of the new complexes. mer-Ru(iii) (1) crystallised in the monoclinic P2(1)/n group, and fac-Ru(iii) (2, 2') in the triclinic P1[combining macron] space group. The composition of the ruthenium coordination sphere was confirmed and characterised using spectroscopic techniques (FT-IR, UV-vis and EPR), elemental analysis and mass spectrometry (MS-FAB). The structures of the complexes obtained were analysed using X-ray and other spectroscopic methods (IR and UV-vis). The electrochemical properties of the ligand and the complex compound were identified using cyclic voltammetry, determining the potential and charge of faradaic processes. Both isomers are redox active and display quasi-reversible metal centered redox processes for the Ru(iii)/Ru(ii) pair. Moreover, preliminary tests of their biological activity were performed. The cytotoxicity of these compounds has been tested for human lung carcinoma (A549), chronic myelogenous leukemia (K562), human cervix carcinoma (HeLa) cells, acute lymphoblastic leukemia (MOLT-4), human breast adenocarcinoma cell line (MCF-7) and normal human umbilical vein endothelial cells (HUVEC). The ability to induce apoptosis has been demonstrated in caspase 3/7 activity assay. In addition, the lipophilicity of both isomers was described by a partition coefficient, log P, values of which were estimated by the shake-flask method. The interesting and promising preliminary results of the biological and chemical activities of the new octahedral mer/fac Ru(iii) complexes motivate further in vitro and in vivo studies.