The syn and anti isomers of 6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane-6,13-diamine (L1 and L2 respectively) have distinctly different shapes for binding metal ions. This is reflected in part in the stability constants for complexation (log K, 25°C, 1:1 complex), which have been determined for the d10 metal ions Zn2+(syn, 16.1; anti, 14.9), Cd2+(syn, 12.1; anti, 10.6), Hg2+(syn, 12.2; anti, 10.5), and Pb2+(syn, 11.8; anti, 10.8), none of which involves ligand-field stabilization contributions to the stability of complexes. The syn isomer, with its sterically undemanding ‘nest’ shape, displays higher K values than the anti isomer irrespective of metal ion size. Selection of the small zinc(II) ion compared to the larger cadmium(II) ion by both isomers is significant (Δ log K syn 4.0, anti 4.6). The syn isomer accommodates the large lead(II) ion, the [PbL1(H2O)][ClO4]2 complex crystallizing in the monoclinic P21/c space group, Z= 4, a= 8.774(3), b= 17.794(6), c= 15.738(5)Å, β= 109.67(3)°. The lead(II) ion is co–ordinated by all six amines [Pb–N range 2.505(5)–2.809(7)Å], the water molecule [Pb–O 3.098(6)Å] and weakly chelated by a perchlorate ion [Pb–O 3.24(1), 3.298(9)Å]. By contrast, the slightly smaller mercury(II) ion is not accommodated in the cavity of the anti isomer, the isolated complex [(H2L2)(HgCl3)2][L2(HgCl2)2]·2H2O crystallizing in the triclinic P space group, Z= 1, a= 14.321(4), b= 10.317(2), c= 8.246(2)Å, α= 95.19(2), β= 96.24(2), γ= 97.68(2)°. Two separate molecular units co-exist in equal amounts in the structure, the first with mercury(II) ions bound to each pendant amine in a unidentate manner, whereas the second has two mercury ions chelated at opposite sides of the macrocycle to, in each case, the pendant amine and one secondary amine. In each unit, chloride ions complete the tetrahedral environment for the metal ion, with variable Hg–N [range 2.279(7)–2.364(7)Å] and Hg–Cl [range 2.370(3)–2.535(2)Å] distances.