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 gasphase reactions, we propose that the previously observed cross-coupling reaction of the silver complex in solution proceeds via the outer-sphere mechanism.