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The work presented in this thesis attempts to elucidate the structure, nature of bonding in, and basic chemistry of the osmium carbonyl halides. An adaptation of a spectrophotometric method of analysis for osmium, involving the characterisation of the visible absorption spectrum of the [Os(thiourea)[6]][3+] ion, has been developed which has advantages over the current procedures in the literature . Syntheses of osmium carbonyl halides of the general formulae Os(CO)[4]X[2], Os(CO)[3]X[2] and Os(CO)[2]X[2] (where X = halogen), have been carried out at both atmospheric and higher pressures. Several new derivatives of these compounds of the type OsL[2](CO)[2]X[2] have been prepared through the replacement of carbon monoxide by neutral ligands capable of entering into pi-bonding with the central metal atom, notably substituted phosphines. Compounds were characterised by analysis and infrared spectroscopy. Structures have been suggested which account for the observed patterns of absorption bands occurring in the carbonyl stretching frequency region, and for solubility trends. Assignments have been made for the fundamental CO vibrations, and force constants calculated on the basis of models proposed by Cotton and Kraihanzel. Further work has been carried out to examine the effect of solvents on the fundamental carbonyl vibration frequencies, the results discussed, and a mechanism proposed which accounts for the observed phenomena. Both the solvent shifts and those frequency shifts of the carbonyl vibrations caused by a change in ligand substitution have been ascribed to a reasonably simple bonding picture in the compounds considered.