The thesis addresses the potential application of ruthenium(II)-cobalt(III) heterodinuclear complexes as a new selective cancer treatment. The selectivity is to be achieved through the use of visible light to trigger activation of the drug. The majority of work conducted relates to the design and synthesis of the bridging ligand for the final ruthenium(II)-cobalt(III) heterodinuclear complex. In Chapter 2, a potential bridging ligand based on a functionalised terpyridine is described. The intention was to bind the ruthenium(II) metal centre to the terpyridine end of the bridging ligand and have a secondary binding domain available for coordination of the cobalt(III) metal centre. However, a reductive step in the synthetic pathway failed to produce the desired product and this potential bridging ligand had to be abandoned. In Chapter 3, two series of bridging ligands are described. The first of these series is based on Jurgen Sauer’s ‘LEGO’ system. In addition to describing the free synthesis of these ligands, their synthesis on a ruthenium(II) metal centre is described. The second series is based on disubstituted-1,2,4,5-tetrazines. These compounds are only able to be directly synthesised as the non-coordinated ligand. Coordination of these ligands to a single ruthenium(II) metal centre is then described. Ruthenium(II) complexes of both ligand series are then exposed to several transition metals and their ability to coordinate a second metal centre investigated. The formation of ruthenium(II)-cobalt(III) heterodinuclear complexes, using the ligand series detailed in Chapter 3, is described in Chapter 4. These complexes are formed by reacting the ruthenium(II) complex of the bridging ligand with either Co(en)₂(OTf)₂ or Co(tren)(OTf)₂. These heterodinuclear complexes exhibit photo-activated ligand release, which makes them candidates for development as a potential cancer treatment. The non-bridging ligands coordinated to the cobalt(III) metal centre in Chapter 4 were not cytotoxic. In order to make the system biologically active these ligands need to be changed. Chapter 5 describes how nitrogen mustards (a class of cytotoxic DNA alkylators) could be introduced as the non-bridging ligands. This involves the synthetic strategy of forming the cobalt(III) complex of the alcohol precursor of a nitrogen mustard. This precursor complex is then converted into the nitrogen mustard complex and coordinated to the ruthenium(II) bound bridging ligand. The synthetic strategies outlined in this thesis can be applied to a wide range of potential bridging ligands and could potentially lead to a large number of ruthenium(II)-cobalt(III) heterodinuclear complexes being synthesised. One journal article based on this research has been accepted for publication, in the Australian Journal of Chemistry. Three more articles are in preparation.