The single crystal structures of complexes [CuL¹Br]ClO4 1, [CuL²Br]PF6 2, and [NiL²][ClO4]2 3 were determined (L¹ is 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene and L² is 3,6,10,16-tetraazabicyclo[10.3.1]hexadeca-1(16),12,14-triene). The asymmetric unit of 1 contains two [CuL¹Br]⁺ cations having different five-co-ordinated environments. One (A) exhibits a distorted square pyramidal arrangement, with the basal plane defined by three nitrogen atoms of the macrocycle and the bromine, and the apical position occupied by the nitrogen opposite to the pyridine ring. In the other (B) the donor atoms are distorted from this geometry towards a trigonal bipyramid with the equatorial plane formed by two nitrogen atoms of the macrocycle and Br, and the axial positions occupied by the nitrogen atoms contiguous to the pyridine ring. The complex cation [CuL²Br]⁺ 2 exhibits a distorted square pyramidal environment with the basal plane defined by the four nitrogen atoms of the macrocycle and the apical co-ordination by the bromine atom. In [NiL²]²⁺ 3 the four nitrogen atoms of the macrocycle form a distorted square planar environment around the nickel centre. Molecular mechanics calculations are used to determine the best-fit sizes for metal ions accommodated into L¹ and L² by evaluation of all sterically allowed conformers for five-co-ordination geometry. The results obtained, together with those of L³ (3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene), published previously, clearly establish the effect of macrocyclic cavity size on metal ion selectivity. These macrocycles prefer a planar conformation to accommodate small metal ions but folded conformations are preferred for longer Mâ N distances. The increase of the macrocyclic cavity size leads to an increase of the Mâ N(sp³) distances at which the folded conformer(s) become the most stable form: 1.90, 2.14 and 2.18 Ã for 12-, 13- and 14-membered macrocycles, respectively.