Two isostructural malonato-bridged complexes of formula {[M (H2O) 2][M (mal) 2 (H2O) 2]} n [M~ Co (II)(1), Ni (II)(2); H2mal~ malonic acid] have been synthesised and characterized by X-ray diffraction. Their structure consists of corrugated layers of trans-diaquabismalonatemetalate (II) and trans-diaquametal (II) units bridged by carboxylate–malonate groups in the anti–syn conformation. Two crystallographycally independent metal atoms occur in 1 and 2. The malonate anion acts simultaneously as a bidentate and bis-monodentate ligand. Variabletemperature (1.9–295 K) magnetic susceptibility measurements indicate the occurrence of weak antiferro-(1) and ferromagnetic (2) interactions between the cobalt (II)(1) and nickel (II) ions (2) through the anti–syn caboxylate–malonate bridge. A brief discussion on the structural diversity and crystal engineering possibilities of the malonate complexes with divalent first-row transition metal ions other than copper (II) is carried out.