The area of ruthenium(H) and osmium(H) polypyridyl chemistry has been the subject of intense investigation over the last half century. In chapter 1, topics relevant to the studies presented in this thesis are introduced. These areas include the basic principles behind the ground and excited state properties of Ru(II) and Os(II) polypyridyl complexes, complexes incorporating the 1,2,4-triazole moiety and the application of deuteriation to inorganic photophysics. Chapter 2 details experimental and basic synthetic procedures employed in the studies presented in later chapters. A limited discussion of practical aspects of both synthetic procedures and physical measurements is included, in particular where major difficulties were encountered and where improvements to standard procedures were made. A central theme to this thesis is the application of deuteriation as a spectroscopic probe. In order to fully exploit its potential fully, a general and systematic approach to the deuteriation of polypyridyl type ligands is required. In chapter 3 a range of isotopomers of heteroaromatic compounds containing pyrazyl-, pyridyl-, 1,2,4-triazole-, thienyl-, methyl-, and phenyl- moieties, are reported. The application of deuteriation in inorganic chemistry as a spectroscopic probe both in simplification of NMR and Raman spectra and as a probe into the excited state structure of heteroleptic complexes is the focus of chapter 4. Deuteriation is employed extensively to probe the excited state structure of several series of Ru(II) and Os(II) polypyridyl complexes. In particular the effect of deuteriation on emission lifetime and ground and excited state resonance Raman spectra is investigated. In chapter 5, the phenomena of temperature dependent dual luminescence observed for the mononuclear complex [Ru(bpy)2 (pztr)]+ forms the basis of a wider investigation of related complexes in an effort to gain more insight into the nature of the phenomenon. In addition some fundamental studies into the picosecond excited state processes of [Ru(bpy)3]2+ are presented. In these studies deuteriation shows itself as a powerful tool in effecting small but important perturbations. In Chapter 6 the separation, characterisation and photophysical properties of the stereoisomers of mono- and bi-nuclear Ru(II) polypyridyl complexes is examined. In particular the importance of chirality both in terms of solvent and in complex in determining the circular dichroism, ]H NMR spectroscopy and photophysical properties is investigated. In chapters 7 and 8, attention is turned to binuclear systems incorporating 1,2,4-triazole moieties. The effects of variations in the bridging ligand in these systems (e.g., distance and spacer groups, pyrazine vs. triazole etc.) are examined. Deuteriation is employed in some of these systems as a tool in assessing the localisation of the lowest emissive excited state on particular moieties of the complexes.