Abstract The d 8 complexes, M(CO)(L)(PPh 3) 3 (M = Ru, Os; L = CO, CNR; R = p-tolyl) and OsCl(NO)(PPh 3) 3, all form simple 1 : 1 π-adducts with tetrafluoroethylene and maleic anhydride, with the overall coordination geometry being dependent upon the relative electron-withdrawing properties of the olefin and the relative electron richness of the metal fragment. In all the tetrafluoroethylene complexes of ruthenium examine, the geometry of the complex involves cis-triphenylphosphine ligands. The X-ray crystal structure of Ru(C 2F 4(CO) 2(PPh 3) 2 is presented as an example of this structural class. Ru(C 2F 4)(CO) 2(PPh 3) 2 crystallises in the monoclinic space group P2 1/ a with a cell having dimensions a = 35.940(2), b = 10.655(7), c = 18.559(6) Å and β = 93.21(3)°, with two crystallography independently molecules in the asymmetric unit ( Z = 8). The osmium complexes do not display this strong preference for cis-triphenylphosphine ligands and Os(C 2F 4)(CO) 2(PPh 3) 2 exists as two separate but interconvertible isomers in solution. One isomer has the same relative geometry as Ru(C 2F 4)(CO) 2(PPh 3) 2. The other has trans-triphenylphosphine ligands. This temperature-dependent equilibrium has been studied by variable-temperature NMR and has Δ H° = −15 kJ mol −1 and Δ S° = −60 J K −1 mol −1. The maleic anhydride complexes also show a variation in geometry from the usual trans triphenylphosphine arrangement. The X-ray crystal structure of Os(maleic anhydride)(CO) 2(PPh 3) 2 is reported along with the solution structures of other maleic anhydride complexes. Os(maleic anhydride)(CO) 2(PPh 3) 2 crystallises in the monoclinic space group P2 1/ c with a cell having dimensions a = 23.667(2), b = 20.306(1), c = 16.147(1) Å and β = 93.20(1)°, with two crytallographically independent molecules in the asymmetric unit ( Z = 8). The triphenylphosphine ligands are again cis but only one lies in the plane of the osmium and the coordinated olefin.