By controlled Anderson type rearrangement reactions complexes of the general formula trans-[(OsCl4)-Cl-IV(Hazole)(2)], where Hazole = 1H-pyrazole, 2H-indazole, 1H-imidazole, and 1H-benzimidazole, have been synthesized. Note that 2H-indazole tautomer stabilization in trans-[(OsCl4)-Cl-IV(2H-indazole)(2)] is unprecedented in coordination chemistry of indazole. The metal ion in these compounds possesses the same coordination environment as ruthenium(III) in (H(2)ind)[(RuCl4)-Cl-III(Hind)(2)], where Hind = 1H-indazole, (KP1019), an investigational anticancer drug in phase I clinical trials. These osmium(IV) complexes are appropriate precursors for the synthesis of osmium(III) analogues of KP1019. In addition the formation of an adduct of trans-[(OsCl4)-Cl-IV(Hpz)(2)] with cucurbit[7]uril is described. The compounds have been comprehensively characterized by elemental analysis, El and ESI mass spectrometry, spectroscopy (IR, UV-vis, ID and 2D NMR), cyclic voltammetry, and X-ray crystallography. Their antiproliferative acitivity in the human cancer cell lines CH1 (ovarian carcinoma), A549 (nonsmall cell lung carcinoma), and SW480 (colon carcinoma) is reported.