Carbon Dioxide Activation by Alkaline-Earth Metals: Formation and Spectroscopic Characterization of OCMCO 3 and MC 2 O 4 (M = Ca, Sr, Ba) in Solid Neon.

The reactions of alkaline-earth metal atoms (Ca, Sr, and Ba) with carbon dioxide are investigated using matrix isolation infrared spectroscopy in solid neon. The ground-state metal atoms react with two carbon dioxide molecules to produce the oxalate complexes MC ₂ O ₄ and the carbonate-carbonyl complexes OCMCO ₃ (M = Ca, Sr, Ba) spontaneously on annealing. The species are identified by the effects of isotopic substitution on their infrared spectra as well as density functional calculations. Bonding analyses reveal that the attractive forces between M ²⁺ and (CO ₃ ) ^{2 -} or (C ₂ O ₄ ) ₂ ^- in the OCMCO ₃ and MC ₂ O ₄ complexes come mainly from electrostatic attraction, but covalent orbital interactions also play an important role, which are dominated by the ligand-to-metal donation bonding. The calcium, strontium, and barium metal centers in these complexes use their ns and predominately ( n - 1)d atomic orbitals for covalent bonding that mimic transition metals.