Ru(II)-polypyridyl complexes exhibit antitumor properties that can be systematically tailored by means of adjusting the ligand environment. In this work, the effect of incorporating pi-extended moieties into anionic N<^>O- based chelating ligands on the cytotoxic properties of Ru compounds is explored. Four new Ru(II) complexes, [Ru(bpy)(2)(dphol) [PF6] (1; bpy = 2,2'-bipyridine, dphol = dibenzo [a,c]phenazin-10-olate), [Ru(phen)(2)(dphol)] [PF6] (2; phen = 1,10-phenanthroline), [Ru(bpy)(2)(hbtz)][PF6] (3; hbtz = 2-(benzo[d]thiazol-2-yl)phenolate), and [Ru(phen)(2)(hbtz)][PF6] (4) were synthesized and thoroughly characterized. In vitro cytotoxicity was investigated in human lung adenocarcinoma (A549) cells, which revealed that 4 is the most cytotoxic compound (IC50 = 0.8 mu M) in the series including a control compound [Ru(bpy)2(quo)][PF6] (5; quo = 8-hydroxyquinolinate) and is nearly 8-fold more cytotoxic than cisplatin. An investigation of the mechanism of cell death led to the finding that compounds 1-4 disrupt the mitochondrial transmembrane potential (Delta Psi(m)) in a concentration-dependent fashion, which is an event associated with the intrinsic pathway of apoptosis. Moreover, compound 4 triggers the activity of caspase-3/7, which eventually induces the apoptotic cellular death of A549 cells. Thus, increasing the overall lipophilicity of the Ru compounds by introducing g-extended moieties in the anionic N<^>O- ligand is a successful strategy for realizing a new family of pro-apoptotic compounds with a [(RuN5O)-N-II](+) coordination environment.