The importance of the central N atom of biguanide (L) in the stability of transition metal complexes [ML2]​2+ (M = Cu, Ni) is strikingly demonstrated by replacing the NH by CH2. The novel ligand thus obtained is malonic diamidine (L')​. [ML'2]​Cl2 (M = Cu, Ni) were prepd. and their structures detd. by x-​ray diffraction. Unlike [ML2]​2+, the [ML'2]​2+ complexes are not planar, one of the C atoms connecting the amidine groups being above and the other below the plane of the coordination square. Bond distances and angles in the 2 types of complex cations are discussed. The fact that the 2 amidine groups share no electrons and do not act in unison is no doubt the reason for the decreased stability of [ML'2]​2+ in comparison with [ML2]​2+. The latter therefore are stabilized by resonance energy to an appreciable extent.