The work reported in this thesis is divided into two distinct areas. The first involves the synthesis of monoaza- and diaza-[12]-ring macrocycles, with differing side-arm N-substituents. The twelve-membered macrocycles possess a convenient ring-size for exploring the stability and selectivity of complexation of small cations, in particular those from groups IA and IIA. Amide substituents on nitrogen were expected to function as effective σ-donors to cations with high charge density (e.g. Li (^+), Ca (^2)(^+)), because of their high ground state dipole moments. The effect of the length of the side-arms attached to nitrogen on the complexation has also been studied. Complexation behaviour has been probed using (^13)C NMR spectroscopy, titration calorimetry, and fast-atom bombardment mass spectroscopy. Copper (II) complexes of three of the [12]-ring cycles have also been characterised by X-ray crystallographic analysis. The second area involves the study of a series of macrocyclic ligands capable of forming homo- and hetero-dinuclear complexes. In particular, ligands containing the pyridyl-dithio (PyS(_2)) binding unit and a polyether chain linking the two sulphur atoms have been examined. The three binding atoms of each PyS (_2) group define three corners of a fairly rigid square planar environment which favours the formation of square planar d(^8) complexes. Accordingly, complexation with rhodium (I) [and (III)], palladium (II), and platinum (II) has been investigated: the structural properties of these complexes have been determined by the use of FT NMR and X-ray crystallography.