The syntheses of the platinum(II) and palladium(II) complexes of 1,6‐diaminotetrahydropyrrolo[2,3‐b]pyrrole‐2,5(1H,4H)‐dione (3) are described. These compounds contain a six‐membered chelate ring with four nitrogen atoms. An X‐ray diffraction study of the palladium(II) complex [Pd(3)Cl2] shows a distorted “sofa” conformation for this chelate ring. Both complexes exhibited in the 1H‐NMR spectrum an ABgem system for the amino protons. In vitro ICL50 and ICT50 and in vivo antitumour activities have been determined for these products. The corresponding dicarboxylato complexes have been obtained by the reaction of the platinum(II) complex [Pt(3)Cl2] with silver sulfate followed by the addition of barium hydroxide and oxalic, malonic, hydroxymalonic, and 1,1‐cyclobutanedicarboxylic acid.