The mechanism of the transmetalation step in the metal-carbon bond-formation process catalyzed by palladium complexes has been studied by spectroscopic and kinetic methods. The reaction of properly designed model complexes [eta(5)-(1-Ph2P-2,4-Ph-2)C5H2](CO)(3)MoPd(PR3)] (3, R = Ph; 15, R = Bu; 16, R = Me), resulting from oxidative addition of a Mo-I moiety to a palladium center, with aryltributyltinacetylicles Bu3Sn-C=C-(p-XC6H4) (11a, X = H; 11b, X = Cl) yields the products of transmetalation [eta(5)-(1-Ph-2- P-2,4-Ph-2)C5H2](CO)(3)MoPd(PR3)-C=C-(p-XC6H4) (5a,b). The reaction, which shows a strong dependence on the nature of the phosphine ligand PR3 (Ph > Bu > Me) and less so on the nature of the p-substituent X group, proceeds through two competing pathways, depending on the initial concentration of substrate. At high [3] (congruent to10(-2) M), the transmetalation proceeds through an intermediate species (12) formed by the interaction of complex 3 with 11 a. This associative complex accumulates in the presence of added PPh3 and has been characterized spectroscopically. At low [3] (congruent to10(-4) M), the reaction rate shows an inverse dependence on the concentration of the complex. This is due to the formation of a solvent-coordinate species (13), in which PPh3 has been substituted by a dimethylformamide (DMF) molecule, as shown by UV-vis and P-31 NMR spectroscopy. Values of k(obs) depend on the concentration and nature of the aryltributyltinacetylides, in agreement with the existence of a kinetically detectable intermediate. A dimeric iodide bridged complex [{eta(5)-(1-Ph2P-2,4-Ph-2)C5H2}(CO)(3)MoPdl](2) (14) has been obtained during attempts at isolating 13, which changes quantitatively into 13 upon dissolution in DMF and reacts with 11a to give the transmetalation product.