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Chelating diphosphine ligands have been prepared for two main applications: cytotoxic Ru complexes and organopalladium alkylation catalysts. By applying rational design ideas to the ligand targets, the desired properties of the resulting metal complexes have been achieved. Ruthenium diphosphine complexes derived from [RuCl(DMSO-S)2(cis-tach)]Cl (2.1) have been synthesised and assessed as cytotoxic agents by physical inorganic measurements, in vitro assays, and imaging techniques. The structures of the diphosphine ligand backbone and metallacycle size both have a significant influence on cytotoxicity. Therefore, diphosphines have been specifically designed to be capable of interacting with DNA by intercalation. Complexes with flexible aliphatic backbones (2.7-2.8) or planar aromatic backbones (2.17-2.19) are the most active, and we have demonstrated cytotoxicity may be the result of multiple mechanisms. The diphosphine bis((diphenylphosphino)methyl)glycine L3.1 is a versatile amide coupling partner with a variety of aryl and alkyl amines, producing fluorescent ligand conjugates. In addition, L3.1 forms a Ru chelate complex [RuCl(cis-tach)(L3.1)]Cl (3.1) which serves as a Ru precursor to fluorescent complex conjugates by coupling of the carboxylic acid functional group. The photophysical properties of the resulting diphosphine ligands and Ru complex conjugates have been assessed, and the methodology has been applied to other fluorescent, therapeutic agents. The cytotoxicity of a pyrene complex conjugate [RuCl(cis-tach)(L3.2)]Cl (3.3) has been investigated in collaboration with the University of York and compared to (cis-tach)Ru complexes 2.17-2.19. Changing from an all-carbon backbone to a PCNCP backbone does not have a detrimental effect on cytotoxicity and provides a useful point of derivatisation for metal diphosphine drug conjugates. A series of primary, secondary, and tertiary 1-pyrenylphosphines L4.1-L4.3 has been synthesised. The luminescence and air stability properties of these ligands and their associated (arene)Ru complexes have been investigated and it was found that phosphine oxidation increases fluorescence quantum yield, whereas Ru coordination dramatically quenches emission. The resistance to air-oxidation of 1-pyrenylphosphines has been explored experimentally and by computational studies on the phosphine radical cations. A series of aminophosphine pincer ligands L5.1-L5.9 have been synthesised and their derived organopalladium complexes with both symmetric (5,5- and 6,6-) and nonsymmetric (5,6-) metallacycles were prepared. The ligands that form non-symmetric metallacycles have been shown to have a dramatic impact on the Pd coordination chemistry. When applied as catalysts in allylic alkylation reactions, the larger 5,6- and 6,6- metallacycles show superior catalytic activity to the widely studied 5,5-metallacyclic systems. By optimising the reaction, we have found organopalladium catalysts that perform allylation of dimethylmalonate with high activity and excellent linear selectivity under mild conditions.