This thesis encompasses work on the coordination chemistry and catalytic activity of half sandwich ruthenium(II)-arene phosphine complexes. The investigation of phosphine dissociation and association reactions of these complexes represents the unifying theme of this study and, in addition to providing fundamental insights into the coordination chemistry of these compounds, allowed structure-activity relationships to be made and assisted in the development of new synthetic procedures. The thesis is organised into three parts according to the type of phosphine ligand investigated. The dissociation characteristics and catalytic activity of bis-phosphine complexes are presented in Chapter 2. The former properties were established by kinetic studies of ligand exchange and gasphase fragmentation analysis by tandem mass spectrometry, and helped to rationalise trends in the observed catalytic activity for the hydrogenation of styrene to ethylbenzene. The more active complexes were also shown to be chemoselective catalysts for the hydrogenation of aldehydes in the presence of olefinic bonds. Mechanistic studies were carried out in order to rationalise the preferential hydrogenation of aldehydes, which is consequently suggested to proceed via an ionic outer-sphere mechanism. Chapter 3 describes investigations on the coordination chemistry of ruthenium(II)-arene complexes containing diphosphine ligands. Complexes containing κ1-coordinated diphosphine ligands, with or without triphenylphosphine as a co-ligand, were prepared or characterised as the corresponding borane adducts. Mechanistic studies, including determination of enthalpies, entropies and, in two cases, volumes of activation, for the chelation reactions of the isolated κ1-diphosphine complexes were undertaken. The coordination chemistry of a chiral diphosphine, (R)-BINAPO, was also investigated; the synthesis of complexes containing this ligand in a range of coordination modes was achieved. The final chapter concerns the reactivity and catalytic activity of ruthenium(II) complexes containing (PPh2CH2)3CMe (triphos). Investigations of the catalytic activity of ruthenium(II)-arene complexes containing the κ2-coordinated triphos ligand were carried out together with pertinent reactivity studies and comparisons to structural analogues, which helped rationalise the observed activities. The reactivity and catalytic chemistry of one of the structural analogues, a ruthenium(II)-acetate κ3-triphos complex, was also investigated in detail as it proved to be a highly active catalyst for the hydrogenation of 1-alkenes.