Carbon Dioxide Activation at Metal Centers: Evolution of Charge Transfer from Mg .+ to CO 2 in [MgCO 2 (H 2 O) n ] .+ , n= 0–8

We investigate activation of carbon dioxide by singly charged hydrated magnesium cations Mg  .+ (H 2 O) n , through infrared multiple photon dissociation (IRMPD) spectroscopy combined with quantum chemical calculations. The spectra of [MgCO 2 (H 2 O) n ] .+ in the 1250–4000 cm −1 region show a sharp transition from n= 2 to n= 3 for the position of the CO 2 antisymmetric stretching mode. This is evidence for the activation of CO 2 via charge transfer from Mg  .+ to CO 2 for n ≥3, while smaller clusters feature linear CO 2 coordinated end‐on to the metal center. Starting with n= 5, we see a further conformational change, with CO 2 .− coordination to Mg 2+ gradually shifting from bidentate to monodentate, consistent with preferential hexa‐coordination of Mg 2+ . Our results reveal in detail how hydration promotes CO 2 activation by charge transfer at metal centers.