The reactions of the cluster complexes [Pt-3(mu -Co)(3)L-3], where L = PPh3 1a, PPh(2)Bz 1b and PCy3 1c, with activated mono-olefins have been studied under preparative and equilibrium conditions. At low temperature the olefins react quantitatively giving the adducts [Pt-3(mu -CO)(3) L-3(olefin)] (olefin = trans-di-cyanoethene, DCE 2a-2c, maleic anhydride, MA 3a -3c). The stereo-chemistry of these unstable clusters has been deduced from low temperature P-31. C-13. Pt-195-NMR and I.R. spectra. At higher temperature there adducts in presence of excess of olefin convert quantitatively to stable mononuclear Pt(O) complexes [Pt(CO)L(olefin)] (olefin = DCE 4a-4c, MA 5a-5c. maleimide. MI 6a-6c and 1-4-nayhthoquinone, NQ 7a. 7c).