The reaction of the cluster complexes [Pt-3(mu -CO)(3)(PR3)(3)] (PR3 = PPh3 1, PPh(2)Bz 2, PCy3 3 and P(i)pr(3) 4) with dialkyl acetylenedicarboxylate, R'O2CC drop CCO2R' (R= CH3 or Bu-t) have been examined under various conditions. At low temperature the alkyne reacts quantitatively giving the unstable adducts [Pt-3(CO)(3)(PR3)(3)(mu (3)-alkyne)]. The stereochemistry of the intermediate [Pt-3(CO)(3)(PPh3)(3)(mu (3)-(BuO2CC)-Bu-t drop (CCO2Bu)-Bu-t)] 5has been deduced from low temperature Pt-195-{H-1}, P-31-{H-1} and C-13-{H-1} NMR spectra. At higher temperature a fluxional process renders two of the three platinum atoms equivalent on the NMR timescale. At room temperature the alkynes convert the starting clusters 1-4 to the stable dinuclear complexes [Pt-2(CO)(2)(PR3)(2)(mu -R'O2CC drop CCO2R')] (R'= CH3, PR3, = PPh3 6; R = Bu-t, PR3 = PPh3 7; PPh(2)Bz 8; PCy3 9 and (PPr3)-Pr-i 10) in which the alkyne coordination has the C-C bond collinear with the Pt-Pt bond (6 and 7) or perpendicular to the Pt-Pt axis (9 and 10). The stereochemistry of these two types of dinuclear complexes has been established by NMR and IR studies and confirmed by X-ray diffraction analyses of 8 and 9.