The dihydrazone of 2,6-diacetylpyridine is used as a N-donor ligand L to synthesise [ZnL 2 ](ClO 4 ) 2 (1), [CdL 2 ](ClO 4 ) 2 (2) and [HgL 2 ](ClO 4 ) 2 (3). The X-ray crystal structures of the three complexes have been determined. The asymmetric unit of 1 contains two independent cations of [ZnL 2 ] 2+ and four perchlorates. The metal atoms have distorted octahedral N 6 coordination sphere. The structure of 2 is a dimer [Cd 2 L 4 ] 4+ containing a crystallographic centre of symmetry and associated with four perchlorate anions. The cadmium atoms are bonded to two tridentate L and in addition form a bond to a free NH 2 end of an adjacent CdL 2 2+ unit. Thus Cd(II) in 2 is seven coordinate. The cation in 3 is a monomer with an HgN 6 2+ core. The Lewis acidity in the three metal atoms follow the order Zn(II)>Cd(II)>Hg(II). The order of the tendency of acquiring a coordination number higher than six is Zn(II)<Cd(II)<Hg(II). A balance between these two counteracting factors is attained in Cd(II) making 2 a 7-coordinate dimer.