The complexes L5FeIIClBPh4 and L5FeII(H2O)(BPh4)2 (L5 = N,N,N‘-tris(2-pyridylmethyl)-N‘-methyl-ethane-1,2-diamine) have been isolated. Bernal et al. (Bernal, J.; et al. J. Chem. Soc., Dalton Trans. 1995, 3667−3675) have prepared this ligand and the corresponding complex L5FeIIClPF6. We obtained the structural data of L5FeIIClBPh4 by X-ray diffraction. It crystallizes in the orthorhombic space group P212121 with a = 17.645(7) Å, b = 16.077(6) Å, c = 13.934(5) Å, V = 3953(3) Å3, and Z = 4. It presents Fe(II)−N bond lengths close to 2.2 Å, typical of high-spin Fe(II). In solution the L5FeII(H2O)(BPh4)2 complex showed a dependence of spin state upon the nature of the solvent. It was high spin in acetone and changed to low spin in acetonitrile. This was detected by UV−vis spectroscopy and by 1H NMR. Bernal et al. (ibidem) showed that these complexes in the presence of an excess of H2O2 give a purple species, very likely the L5FeIII(OOH)2+ derivative, with spectroscopic signatures analogous to those of “activated bleomycin”. The formation of L5FeIII(OOH)2+ is confirmed here by electrospray ionization mass spectrometry. We found that a L5/Fe system gave single-strand breaks on plasmid DNA in the presence of either a reducing agent (ascorbate) and air or oxidants (H2O2, KHSO5, MMPP) at 0.1 μM concentration. The methyl group in L5 was substituted by a (CH2)5N(CH3)3 + group in order to get higher affinity with DNA. The corresponding ligand L5 + was used to prepare the complexes L5 +FeIIClY2 (Y = BPh4 -, PF6 -, ClO4 -) and L5 +FeIIBr(PF6)2. The crystal structure of L5 +FeIICl(ClO4)2 was solved. It crystallizes in the monoclinic space group P21/a with a = 14.691(2) Å, b = 13.545(2) Å, c = 17.430(2) Å, β = 93.43(1)°, V = 3462(1) Å3, and Z = 4. The Fe(II)−ligand distances are similar to those of L5FeIIClBPh4. At the relatively low concentration of 0.01 μM, L5 +FeIIBr2+ promoted DNA breaks. The reaction was not inhibited by hydroxyl radical scavengers. The reaction might involve a nondiffusible oxygen reactive species, either a coordinated hydroperoxide or a high-valent metal−oxo entity.