A DFT study of R-R reductive elimination (R = Me, Ph, vinyl) in plausible intermediates of Pd-catalyzed processes is reported. These include the square-planar tetracoordinated systems cis-[PdR(2)(PMe(3))(2)] themselves, possible intermediates cis-[PdR(2)(PMe(3))L] formed in solution or upon addition of coupling promoters (L = acetonitrile, ethylene, maleic anhydride (ma)), and tricoordinated intermediates cis-[PdR(2)(PMe(3))] (represented as L = empty). The activation energy ranges from 0.6 to 28.6 kcal/mol in the gas phase, increasing in the order vinyl-vinyl < Ph-Ph < Me-Me, depending on R, and ma < "empty" < ethylene < PMe(3) approximately MeCN, depending on L. The effect of added olefins was studied for a series of olefins, providing the following order of activation energy: p-benzoquinone < ma < trans-1,2-dicyanoethylene < 3,5-dimethylcyclopent-1-ene < 2,5-dihydrofuran < ethylene < trans-2-butene. Comparison of the calculated energies with experimental data for the coupling of cis-[PdMe(2)(PPh(3))(2)] in the presence of additives (PPh(3), p-benzoquinone, ma, trans-1,2-dicyanoethylene, 2,5-dihydrofuran, and 1-hexene) reveals that: (1) There is no universal coupling mechanism. (2) The coupling mechanism calculated for cis-[PdMe(2)(PMe(3))(2)] is direct, but PPh(3) retards the coupling for cis-[PdMe(2)(PPh(3))(2)], and DFT calculations support a switch of the coupling mechanism to dissociative for PPh(3). (3) Additives that would provide intermediates with coupling activation energies higher than a dissociative mechanism (e.g., common olefins) produce no effect on coupling. (4) Olefins with electron-withdrawing substituents facilitate the coupling through cis-[PdMe(2)(PR(3))(olefin)] intermediates with much lower activation energies than the starting complex or a tricoordinated intermediate. Practical consequences are discussed.

Bimetallic catalysis refers to homogeneous processes in which either two transition metals (TM), or one TM and one Group 11 (G11) element (occasionally Hg also), cooperate in a synthetic process (often a C-C coupling) and their actions are connected by a transmetalation step. This is an emerging research area that differs from the isolated or tandem applications of the now classic processes (Stille, Negishi, Suzuki, Hiyama, Heck). Most of the reactions used so far combine Pd with a second metal, often Cu or Au, but syntheses involving very different TM couples (e.g., Cr/Ni in the catalyzed vinylation of aldehydes) have also been developed. Further development of the topic will soon demand a good knowledge of the mechanisms involved in bimetallic catalysis, but this knowledge is very limited for catalytic processes. However, there is much information available, dispersed in the literature, coming from basic research on exchange reactions occurring out of any catalytic cycle, in polynuclear complexes. These are essentially the same processes expected to operate in the heart of the catalytic process. This Review gathers together these two usually isolated topics in order to stimulate synergy between the bimetallic research coming from more basic organometallic studies and the more synthetic organic approaches to this chemistry.

The hydration of phenylacetylene in the presence of the complex mer,trans-(PNP)RuCl 2 (PPh 3 ) in THF at 60°C leads to the cleavage of the C-C triple bond with formation of the carbonyl complex fac,cis-(PNP)RuCl 2 (CO) and toluene [PNP ) CH 3 CH 2 CH 2 N(CH 2 CH 2 PPh 2 ) 2 ]. A study under different experimental conditions, the use of model and isotope labeling experiments, and the detection of several intermediates, taken altogether, show that the C-C bond cleavage reaction comprises a number of steps, among which the most relevant to the mechanism are 1-alkyne to vinylidene tautomerism, conversion of a vinylidene ligand to hydroxycarbene by intramolecular attack of water, deprotonation of hydroxycarbene to σ-acyl, deinsertion of CO from the acyl ligand, and hydrocarbon elimination by protonation of the metal-alkyl moiety. The following intermediate species have been isolated and characterized: the vinylidene fac,cis-(PNP)RuCl 2 {CdC(H)Ph}, the (aquo)(σ-alkynyl) complex fac-(PNP)RuCl-(CtCPh)(OH 2 ), and the (benzyl)carbonyl mer-(PNP)RuCl(η 1 -CH 2 Ph)(CO). Other intermediates such as the σ-acyl mer-(PNP)RuCl(η 1 -COCH 2 Ph)(CO) have been intercepted by addition of appropriate reagents, while the independent synthesis of the aminocarbene complex fac,cis-(PNP)RuCl 2 {C(NC 5 H 10 )(CH 2 Ph)} and its reaction with water have provided evidence for the intermediacy of a hydroxycarbene species in the C-C bond cleavage reaction.