AbstractMetalloenzymes preorganize the reaction environment to steer substrate(s) along the required reaction coordinate. Here, we show that phosphine ligands selectively facilitate protonation of binuclear silver hydride cations, [LAg₂(H)]⁺ by optimizing the geometry of the active site. This is a key step in the selective, catalysed extrusion of carbon dioxide from formic acid, HO₂CH, with important applications (for example, hydrogen storage). Gas-phase ion-molecule reactions, collision-induced dissociation (CID), infrared and ultraviolet action spectroscopy and computational chemistry link structure to reactivity and mechanism. [Ag₂(H)]⁺ and [Ph₃PAg₂(H)]⁺ react with formic acid yielding Lewis adducts, while [(Ph₃P)₂Ag₂(H)]⁺ is unreactive. Using bis(diphenylphosphino)methane (dppm) reshapes the geometry of the binuclear Ag₂(H)⁺ scaffold, triggering reactivity towards formic acid, to produce [dppmAg₂(O₂CH)]⁺ and H₂. Decarboxylation of [dppmAg₂(O₂CH)]⁺ via CID regenerates [dppmAg₂(H)]⁺. These gas-phase insights inspired variable temperature NMR studies that show CO₂ and H₂ production at 70 °C from solutions containing dppm, AgBF₄, NaO₂CH and HO₂CH.