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

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