The preparation of two new [18]aneS₄N₂ macrocycles with conjugated links to redox-active Co centres has been achieved [(CpCo)₂S₄N₂ and (CpCo)S₄N₂ via reaction of an [18aneS₄N₂ bisthione macrocycle with CpCoLL, LL = COD or (CO)₂. Optimisation of this reaction had lead to a high yielding and selective route to the ligand containing one Co centre. The characterisation of these complexes is described in detail and includes a study of the spectro-electrochemical, electrochemical, EPR and UV/Vis spectroscopic data. The second section of this thesis concentrates on complexation studies of these redox-active cobalt macrocycles with a variety of transition metal ions. The metal ions are a mixture of late transition metal and heavy metal ions and include possible pollutant species present within aqueous environmental systems. They include, Ag^I, Fe^II, Co^II, Ni^II, Cu^I, Cu^II, Pb^II, Cd^II, Hg^II, and Sn^II. The studies was carried out in variety of solvent systems and complexations were followed by modifications in electrochemical behaviour and UV/Vis spectroscopy. Strong interactions were observed between the Co centre and the guest metal ion in the macrocycle in several cases and encouraging results are reported indicating the use of these systems in Ion Sensing. In particular the complexation study carried out with Ag^I and Ni^II proved to be the most significant. A route to the method of complexation of the ions, Ag^I, Fe^II, Co^II, Ni^II, Cu^I, and Cu^II is proposed with a suggestion for an alternative pathway for the complexation of the heavier metals, Sn^II, Pb^II, Hg^II and Cd^II is also given.