Origin of the different reactivity of the high-valent coinage-metal complexes [RCuIIIMe3]- and [RAgIIIMe3]- (R=allyl)

High-valent tetraalkylcuprates(III) and -argentates(III) are key intermediates of copper- and silver-mediated C-C coupling reactions. Here, we investigate the previously reported contrasting reactivity of [(RMMe3)-Me-III](-) complexes (M=Cu, Ag and R=allyl) with energy-dependent collision-induced dissociation experiments, advanced quantum-chemical calculations and kinetic computations. The gas-phase fragmentation experiments confirmed the preferred formation of the [RCuMe](-) anion upon collisional activation of the cuprate(III) species, consistent with a homo-coupling reaction, whereas the silver analogue primarily yielded [AgMe2](-), consistent with a cross-coupling reaction. For both complexes, density functional theory calculations identified one mechanism for homo coupling and four different ones for cross coupling. Of these pathways, an unprecedented concerted outer-sphere cross coupling is of particular interest, because it can explain the formation of [AgMe2](-) from the argentate(III) species. Remarkably, the different C-C coupling propensities of the two [(RMMe3)-Me-III](-) complexes become only apparent when properly accounting for the multi-configurational character of the wave function for the key transition state of [RAgMe3](-). Backed by the obtained detailed mechanistic insight for the gas-phase reactions, we propose that the previously observed cross-coupling reaction of the silver complex in solution proceeds via the outer-sphere mechanism.