Origin of the different reactivity of the high‐valent coinage‐metal complexes [RCu( iii )Me(3)](−) and [RAg( iii )Me(3)](−) (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 [RM( iii )Me(3)](−) 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 [AgMe(2)](−), 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 [AgMe(2)](−) from the argentate(iii) species. Remarkably, the different C−C coupling propensities of the two [RM( iii )Me(3)](−) complexes become only apparent when properly accounting for the multi‐configurational character of the wave function for the key transition state of [RAgMe(3)](−). 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.