Multinuclear Pt(II) complexes represent a novel class of antitumor agents. In this work, a dinuclear monofunctional Pt(II) complex {[cis-Pt(NH₃)₂Cl]₂(4,4′-methylenedianiline)}(NO₃)₂ (1) was synthesized and characterized by ¹H NMR, electrospray mass spectrometry, and elemental analysis. The 2D [¹H,¹⁵N] heteronuclear single quantum coherence NMR spectra of ¹⁵N-labeled 1 revealed that the cationic core of this water-soluble complex hardly hydrolyzes in aqueous solution and reacts very slowly with glutathione. Hydrolysis appears not to be an essential step for the formation of Pt–guanosine-5′-monophosphate (5′-GMP) or Pt–DNA adducts because the complex can react readily with 5′-GMP and partially transform B-DNA into its Z form. Such properties are desired to achieve the goal of enhancing cytotoxicity and lowering side effects of Pt(II) complexes. In fact, complex 1 is highly cytotoxic against the murine leukemia (P-388) and the human non-small-cell lung cancer (A-549) cell lines, and it is more cytotoxic than cisplatin at most concentrations tested.