The synthesis, spectroscopic and electrochemical characterisation of ruthenium (II) polypyridyl mononuclear complexes containing 1,2,4-tnazole and tetrazole moiety are described. Chapter one is an introduction relating to the work described in the thesis. The methods of characterisation, which are described in chapter two, include High Performance Liquid Chromatography, 'H-NMR, UV/Visible spectroscopy, fluorimetry, electrochemistry, spectroelectrochemistry, mass spectrometry, and lifetime emission measurements. Chapter three describes the synthesis of the new set of ligands and their mononuclear Ruthenium (II) complexes. Chapter four contains an extensive characterisation of Ru(II)(bpy)2 moiety complexes containing a 5-(2-pyridyl)-l,2,4-triazole ligand, (Apy), or a 5-(2-pyridyl) tetrazole ligand, ([ ipy), and their comparison with the archetype Ru(bpy)s2+. The examination of the acid-base chemistry of the complexes by UV/Visible spectroscopy revealed important information about the location of the excited state. Chapter five explores the spectroscopic, photophysical and electrochemical properties of the new [Ru(tpy)fApyA)] complexes, where (ApyA)2' is the 2,6 di-(l,2,4- triaz-3-yl)-pyridine ligand. The new species was exhaustively studied especially because it revealed to be one of the few example, available in literature, of emitting Ru(H) terpiridine complexes, with a lifetime of the excited state in the order of 100 nanoseconds. Chapter six describes the synthesis of complexes containing (ApyA)2' and terpyridine derivatives or vice-versa terpyridine and (ApyA)2" derivative ligands. One of them will be used as a photsensitiser in a photovoltaic cell. Attachment of the [Ru(II)(tctpy)(ApyA)] complex to nanocrystalline TiC>2 films indicates incident photonto- current efficiency (IPCE) of greater than 60%. Chapter seven exptores the use of a new 2,6 di-(tetraz-5-yl)-pyndine ligand, (Upyl ]) ", and the spectroscopic, photophysical and electrochemical properties of its Ru(II) complexes. Chapter eight is an attempt to rationalise the collected data of the Ru(tpy) moiety. The change of the energy levels in relation to the different ligands is analysed. The correlations between spectroscopic, photophysical and electrochemical data of the new complexes and the existent ones created an extensive knowledge of the tridentate Ru(H) complexes, that increases their availability as a photosensitive building block for a supramolecular system. Some suggestions for future work are also considered in the final Chapter. Finally two appendices are included in this thesis. The first is a literature survey which synopsises the last ten years scientific papers, on the Ru-tpy moiety, includmg investigations of their properties, use in analytical research or their use as building blocks in supramolecolar systems. The second appendix refers to the publications, poster presentations and oral presentations made during the course of the research.