Routes to mononuclear Os(II) and Os(0) cluster complexes containing P-donor ligands (PR3, PP) were studied using the osmium salts Y2[OsX6] (Y = NH4, K; X = Cl, Br, I) as precursors. Carbonylation of the precursors and then subsequent reaction with tertiary phosphines afford neutral complexes of the type cis,cis,trans-[OsX2(CO)2(PR3)2] (X = Cl, Br, I, PR3 = tertiary phosphine), which were characterized by combination of infrared and Raman spectroscopy, NMR (1H, 13C, 31P) spectroscopy and elemental analysis. The compounds cis,cis,trans-[OsBr2(CO)2{Ph2P(CH2C6H5)}2] (2b), cis,cis,trans-[OsBr2(CO)2{P(CH2C6H5)}3] (3b) and cis,cis,trans-[OsBr2(CO)2{P(C6H11)}2] (4b) have been further characterized by single crystal X-ray diffraction. Thermogravimetric analysis of these complexes shows multiple stages of decomposition that can be directly related to the loss of ligands.Microwave-promoted reaction of the osmium salt precursor with bidentate phosphine ligands affords the neutral complexes [OsX2(PP)2] (X = Cl, Br, I; PP = bidentate phosphine ligand). A stepwise substitution pathway has been suggested resulting in the formation of compounds cis/trans-[OsX2(PP)2] in which the ligands display different reactivity. The size of the bidentate phosphine ligand determines the stereochemistry of the product complex. Reaction with short ligands such as dppm selectively yield the cis-[OsX2(PP)2] isomers. Whilst reaction with dppe and dppp under the same conditions gives the trans-[OsX2(PP)2] isomers. Steric bulkiness of the ligands influences the stereochemistry. The cis-[OsX2(PP)2] isomers are thermally stable with onset of decomposition at temperatures above 300 °C. In contrast, the trans-[OsX2(PP)2] isomers were found to be thermally less stable and found to decompose in the range 195 – 251 °C. Microwave irradiation of [Os3(CO)12] in the presence of PPh3 in acetonitrile solution gave a mixture of disubstituted and trisubstituted phosphine cluster complexes which have been characterized by IR, Raman, 1H and 31P (where applicable) NMR spectroscopies. Variation of solvents influences substitution and give a mixture of monosubstituted and disubstituted phosphine cluster. Complexes of the type [OsX2(PP)2] exhibited both anticancer and antimicrobial activity. In some instances the activity was comparable to that of the reference drugs. The complexes cis,cis,trans-[OsX2(CO)2(PR3)2] and phosphine substituted derivatives of [Os3(CO)12] exhibited moderate antibacterial activity against Gram-negative bacterial strains and some fungal strains under study. / Chemistry / M. Sc. (Chemistry)