The major aim of this work has been to extend the range of known molybdenum and tungsten complexes which have some relevance to nitrogenase. Many of these compounds may also be of import to the hydrodesulphurization process. Chapter 1 contains a brief introduction to the chemistry of sulphur which is pertinent to the remainder of this work. Chapters 2 and 3 are concerned with the reaction of the complex {M02&gS2) with various nitrogen-, oxygen- and sulphur-containing ligands, and the characterization of these products by spectroscopic and analytical methods. Chapter 4 details the reaction of some of these products with H+ and compares them to known Mo-H and Mo-S-H species. Chapter 5 explains the unexpected formation of the lattice adduct [WSC14].S8 from W(CO)6 and S2Cl2, and compares its crystal structure and reactions with the known compound [WSCl4J. Chapters 6 and 7 describe the preparation, characterization and reactions of the complexes [MoH(SAr)3(PMe2Ph)2l and [MoH(SAr)3(PMePh2) ] (SAr - tipt or tmt). The crystal structures of the complexes [MoH(tipt)3 (PMe2Ph)2 ] and [Mo(tipt)2(00)2(PMe2Ph)2] are described, and many of the reactions of these compounds are compared with those of the analogous tungsten compounds lWH(SAr)3(PMe2Ph)2] (SAr - tipt or tmt). In Chapter 8 a general discussion and summary of the preceding Chapters is given, together with some suggestions for further work.iii.