Complexes of ruthenium and osmium with bidentate phosphine and arsine ligands have been prepared and their properties investigated. Ruthenium(III) complexes of the form trans-[Ru(L-L)₂X₂]⁺ ((L-L) = diphosphine or diarsine; X = Cl, Br) and the analogous osmium(III) and (IV) compounds, trans-[Os(L-L)₂X₂]⁺/2+ were synthesised and studied using u.v.-visible and infra-red spectroscopies. The redox properties of the complexes were studied using cyclic voltammetry and their properties compared. The stereochemistry was confirmed by x-ray crystal structures of trans-[Ru(o-C₆F₄(AsMe₂)₂Br₂]BF₄ and trans-[Os(o-C₆H₄(AsMe₂)₂)₂Cl₂]ClO₄. The variation of Os-As and Os-X (X = Cl, Br) bond lengths with osmium oxidation state was investigated by the use of Os L_m-edge EXAFS 9Extended X-ray Absorption Fine Structure). Ruthenium compounds with bidentate Group 16 ligands, trans-[Ru(L-L)₂X₂]ⁿ+ (N = 0, (L-L) = diselenoether or ditelluroether; n = 1, (L-L) = dithioether, diselenoether; X = Cl, Br) have also been prepared and studied spectroscopically and electrochemically. ⁷⁷Se and ¹²⁵Te n.m.r. spectra were recorded for the ruthenium(II) compounds. The trans stereochemistry was confirmed by the x-ray crystal structure of trans-[Ru(PhSeCH₂CH₂SePh)₂Cl₂]. Ruthenium-stibine compounds of the form trans-[Ru(SbPh₃)₄X_2⁰+ (X = Cl, Br) have been studied and their properties investigated. The six-coordination was confirmed by an x-ray crystal structure of trans-[Ru(SbPh₃)₄Cl₂] and this contrasts with the well-known five-coordinate phosphine compound [Ru(PPh₃)₃Cl₂]. A functionalised o-phenylene diphosphine has been prepared and bound to a polystyrene polymer-support. Nickel, platinum, rhodium and ruthenium complexes of this supported diphosphine and model precursors have been synthesised and their properties investigated by the use of diffuse reflectance u.v.-visible and metal-edge EXAFS spectroscopies.