Oxidative addition of Me1 to [PdMe2(bpy)] (bpy = 2,2'-bipyridine) occurs by the S N~ mechanism. Evidence includes the observation of second-order kinetics in acetone solvent, with a large negative value for the entropy of activation, and the observation of a cationic species, [PdMe3(bpy)(CD3CN)]+I-, in CD3CN solvent. The reaction occurs more slowly than the analogous reaction of [PtMe2(bpy)], but the same mechanism operah. Reductive elimination from [PdIMe,(bpy)] to give ethane and [PdIMeCbpy)] follows good firsborder kinetics, occurs more rapidly in polar solvents, and is strongly retarded by added iodide. These observations are interpreted in terms of a mechanism that involves preliminary ionization of iodide followed by reductive elimination from the cation [PdMe,(bpy)]+. Studies by differential scanning calorimetry allow an estimate of the Pd-C bond energy of-130 kJ mo1-l to be obtained, and this value is considerably higher than the activation energy for reductive elimination of ethane from [PdIMe,(bpy)]. The reductive elimination step is therefore concerted, and possible mechanisms, which may involve direct CC coupling or CC coupling after an agostic CHPd interaction, are discussed. This work is relevant to catalytic CC coupling reactions using palladium complex catalysts.

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Abstract:Results consistent with the participation of vinyls in the initiation and of alkenyl species in the propagation steps of the Fischer-Tropsch reaction are reported. Substantial incorporation of I3C2 into the alkene and alkane (C,-C,) hydrocarbon products occurred when doubly labelled vinyls (I3C,H3Br, (13C2H,),Si, or 13C,H,) were added as molecular probes to the hydrogenation of carbon monoxide over rhodium/ceria/silica catalysts (1 atm, 220 "C). There was, by contrast, no significant incorporation of 13C1 into any of the organic products; thus cleavage of the C, probe did not occur. The degree of I3C2 incorporation decreased with increasing molecular mass of the hydrocarbon; this indicates that the probe molecule initiated but did not propagate. A mathematical model based on polymerisation of surface methylenes initiated by a vinyl, propagated by alkenyls and terminated by reaction with a surface hydrogen or by coupling has been developed to explain the 13C, incorporation data. Under

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