Photochemical activation of carbon dioxide in Mg(CO)(HO).

We combine multi-reference ab initio calculations with UV-VIS action spectroscopy to study photochemical activation of CO on a singly charged magnesium ion, [MgCO(HO)], as a model system for the metal/ligand interactions relevant in CO photochemistry. For the non-hydrated species, two separated Mg 3-3 bands are observed within 5.0 eV. The low-energy band splits upon hydration with one water molecule. [Mg(CO)] decomposes highly state-selectively, predominantly via multiphoton processes. Within the low-energy band, CO is exclusively lost within the excited state manifold. For the high-energy band, an additional pathway becomes accessible: the CO ligand is activated via a charge transfer, with photochemistry taking place on the CO moiety eventually leading to a loss of CO after absorption of a second photon. Upon hydration, already excitation into the first and second excited state leads to CO activation in the excited state minimum; however, CO predominantly evaporates upon fluorescence or absorption of another photon.