Dedicated to Professor Wolfgang Weigand on the occasion of his 65 th birthdayThe synthesis and characterization of six new organometallic half-sandwich iridium(III) compounds containing modified 4,4'substituted 2,2'-bipyridines as the bidentate co-ligands were described. Thus the compounds [Ir(η 5 -C 5 Me 5 )(N^N)Cl]PF 6 [N^N: 4,4'-bpy-Ph, 1; 4,4'-bpy-Me, 2; 4,4'-bpy-nonyl, 3; 4,4'-bpy-CH 2 OH, 4; 4,4'-bpy-Cl, 5; 4,4'-bpy-NH 2 , 6 were obtained by bridgesplitting reactions from the precursor [{Ir(η 5 -C 5 Me 5 )(μ-Cl)Cl} 2 ] with the corresponding bidentate bipyridines. The X-ray singlecrystal structures of compounds 1, 2, 4 and 6 in the solid state were determined. To evaluate the cytotoxic properties of all six compounds, colorimetric assays (MTT assay) against two cancer cell lines, MCF-7 and HT-29, were performed. Most promising results were achieved for compounds 1 and 3 with nonpolar phenyl or nonyl group attached to the bipyridine ligand, while the substitution with less lipophilic groups led to the inactivation of the compound. The most remarkable biological activity showed compound 3 with an IC 50 value in the low macromolecular range and > 40-fold enhanced toxicity compared to cisplatin against both cell lines.

The synthesis and characterization of four compounds [M(η5‐C5Me5)(N^N)Cl]PF6 [N^N=4,7‐dichloro‐1,10‐phenanthroline with M=Rh, 1, and M=Ir, 2, and N^N=4'‐(4‐chlorophenyl)‐2,2':6',2''‐terpyridine in the κ2N,N'‐coordination mode with M=Rh, 3, and M=Ir, 4] are described. All compounds were characterized by spectroscopic means and their molecular structures in the crystal were confirmed by single‐crystal X‐ray diffraction studies. The cytotoxicity of all compounds was evaluated by MTT assay against the three cancer cell lines HeLa (cervical carcinoma), HT‐29 (colon adenocarcinoma) and MCF‐7 (human breast adenocarcinoma). The complexes 3 and 4 display promising activity with IC50 values of 1 μM. The rhodium(III) complex 1 also shows highly improved cytotoxicity compared to cisplatin against the cancer cell lines HT‐29 and MCF‐7. In contrast to this, the iridium(III) complex 2 is even less active against the HeLa cell line than cisplatin.