Treatment of trans-[PtCl2(NCR)(2)] (1; R = Et (1a), Ph (1b)) with 1,3-diiminoisoindoline (2) gives access to the corresponding symmetrical (1,3,5,7,9-pentaazanona-1,3,6,8-tetraenato)Pt(II) complexes [PtCl{NH=C(R)N=C(C6H4)-NC=NC(R)=NH}] (3). The reactions of 1 with one equivalent of 1,1,3,3-tetramethylguanidine (4), 1,3-diphenylguanidine (6), or acetone oxime (8) leads to the formation of mixed asymmetrical Pt(II) complexes trans-[PtCl2{NH=C(R)N=C(NMe2)(2)} (NCR)] (5), [PtCl{NH=C(R)NC(NHPh)=NPh}(NCR)] (7), or trans-[PtCl2{NH=C(Ph)ON=CMe2)(NCPh)] (9), respectively, as a result of nucleophilic addition to one of the nitrile ligands in I. Treatment of 5, 7, and 9 with one equivalent of 2 leads to complexes 3. The complexes were characterized by IR. H-1, C-13{H-1}, and Pt-195 NMR (for 3) spectroscopies, ESI+-MS, elemental analyses, and X-ray diffraction (for 3). Complex 3a has an asymmetric unit with five independent Pt molecules of the same chemical composition and two molecules of water, resulting in a total of 40 molecules of the complex and sixteen guest water molecules per unit cell. Theoretical calculations revealed that the most plausible mechanism of formation of complexes 3 includes stepwise nucleophilic addition of 2 to one of the nitrile ligands in 1, a first cyclization upon formation of the Pt-N bond and elimination of HCl, and a second nucleophilic addition/cyclization.