N-Aminoguanidine (Amgu) was examined for its behaviour as a ligand for platinum(II) and palladium(II). Protonated AmguH+ can coordinate platinum as a monodentate ligand bound via the N4 amino group. Deprotonated, electrically neutral Amgu forms chelate complexes with both PtII and PdII; in these complexes Amgu is coordinated to the metal by the N4 amino group and the deprotonated N1 imino group, forming a five-membered ring. K[PtCl3(dmso)] reacted with Amgu·HCl to yield a chelate complex with N1 bound trans to S, suggesting that an exchange between the N1 and N4 binding sites followed the primary nucleophilic attack of N4 on platinum. Whereas reactions between K2PdCl4 and Amgu·HCl were rapid and straightforward, K2PtCl4 was easily reduced to metallic platinum by Amgu·HCl, especially in stoichiometric conditions. Solution NMR and X-ray crystallographic characterization is presented for five new complexes: trans-[PtCl2(AmguH-N4)2]Cl2·21/2H2O (3), trans-[Pt(Amgu-N1,N4)2](NO3)2 (4), [PtCl(Amgu-N1,N4)(dmso)]Cl·2H2O (5), [PdCl2(AmguH-N1,N4)] (6), and trans-[Pd(Amgu-N1,N4)2]Cl2 (7)