A one-electron reduction of osmium(IV) complexes trans-[(OsCl4)-Cl-IV(Hazole)(2)], where Hazole = 1H-pyrazole ([1](0)), 2H-indazole ([2](0)), 1H-imidazole ([3](0)), and 1H-benzimidazole ([4](0)), afforded a series of eight new complexes as osmium analogues of KP1019, a lead anticancer drug in clinical trials, with the general formula (cation)[trans-(OsCl4)-Cl-III(Hazole)(2)], where cation = H(2)pz(+) (H(2)pz[1]), H(2)ind(+) (H(2)ind[2]), H(2)im(+) (H(2)im[3]), Ph4P+ (Ph4P[3]), nBu(4)N(+) (nBu(4)N[3]), H(2)bzim(+) (H(2)bzim[4]), Ph4P+ (Ph4P[4]), and nBu(4)N(+) (nBu(4)N[4]). All complexes were characterized by elemental analysis, H-1 NMR spectroscopy, electrospray ionization mass spectrometry, UV-vis spectroscopy, cyclic voltammetry, while H(2)pz[1], H(2)ind[2], and nBu(4)[3], in addition, by X-ray diffraction. The reduced species [1](-) and [4](-) are stable in aqueous media in the absence of air oxygen and do not react with small biomolecules such as amino acids and the nucleotide 5'-dGMP. Cell culture experiments in five different human cancer cell lines (HeLa, A549, FemX, MDA-MB-453, and LS-174) and one noncancerous cell line (MRC-5) were performed, and the results were discussed and compared to those for KP1019 and cisplatin. Benzannulation in complexes with similar structure enhances antitumor activity by several orders of magnitude, implicating different mechanisms of action of the tested compounds. In particular, complexes H(2)ind[2] and H(2)bzim[4] exhibited significant antiproliferative activity in vitro when compared to H(2)pz[1] and H(2)im[3].