Six novel ruthenium(II)- and osmium(II)-arene complexes with indoloquinoline modified ligands containing methyl and halo substituents in position 8 of the molecule backbone have been synthesised and comprehensively characterised by spectroscopic methods (1H, 13C NMR, UV–Vis), ESI mass spectrometry and X-ray crystallography. Binding of indoloquinolines to a metal-arene scaffold makes the products soluble enough in biological media to allow for assaying their antiproliferative activity. The complexes were tested in three human cancer cell lines, namely A549 (non-small cell lung cancer), SW480 (colon carcinoma) and CH1 (ovarian carcinoma), yielding IC50 values in the 10−6–10−7 M concentration range after continuous exposure for 96 h. Compounds with halo substituents in position 8 are more effective cytotoxic agents in vitro than the previously reported species halogenated in position 2 of the indoloquinoline backbone. High antiproliferative activity of both series of substances may be due at least in part to their potential to act as DNA intercalators.