2-(2-Chlorophenyl)imidazo[4,5-f ]1,10-phenanthroline (CIP) or 2-(2-nitrophenyl)imidazo[4,5-f ]1,10-phenanthroline (NIP) and their complexes [Ru(bpy)2(CIP)]21 and [Ru(bpy)2(NIP)]21 (bpy=2,29-bipyridine) have been synthesized and characterized. The binding of the two complexes to calf thymus DNA has been investigated with spectrophotometric methods and viscosity measurements. The experimental results indicate that the two complexes bind to DNA through a partial intercalative mode that is different from the bonding mode for their parent compound, [Ru(bpy)2(PIP)]21 (PIP=2-phenylimidazo[4, 5-f ]1, 10-phenanthroline). The crystal structure of [Ru(bpy)2(CIP)]-[ClO4]2?2H2O was determined by X-ray diffraction analysis; the imidazo[4, 5-f ]1, 10-phenanthroline moiety is not coplanar with the 2-chlorophenyl ring, having a dihedral angle of 44.58 in the CIP.