Carbon Dioxide Activation by Alkaline-Earth Metals: Formation and Spectroscopic Characterization of OCMCO3 and MC2O4 (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 MC₂O₄ and the carbonate-carbonyl complexes OCMCO₃ (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 M²⁺ and (CO₃)^2- or (C₂O₄)₂^- in the OCMCO₃ and MC₂O₄ 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.

Infrared spectra and structures of the neutral and charged CrCO2 and Cr(CO2)2 isomers in solid neon

The reactions from codeposition of laser-ablated chromium atoms with carbon dioxide in excess neon are studied by infrared absorption spectroscopy. The species formed are identified by the effects of isotopic substitution on their infrared spectra. Density functional calculations are performed to support the spectral assignments and to interpret the geometric and electronic structures of the experimentally observed species. Besides the previously reported insertion products OCrCO and O2Cr(CO)2, the one-to-one Cr(CO2) complex and the one-to-two Cr(CO2)2 complex as well as the CrOCrCO and OCCrCO3 complexes are also formed. The Cr(CO2) complex is characterized to be side-on η(2)-C,O-coordinated. The Cr(CO2)2 complex is identified to involve a side-on η(2)-C,O-coordinated CO2 and an end-on η(1)-O-coordinated CO2. OCCrCO3 is a carbonate carbonyl complex predicted to have a planar structure with a η(2)-O,O-coordinated carbonate ligand. The CrOCrCO complex is predicted to be linear with a high-spin ground state. Besides the neutral molecules, charged species are also produced. The Cr(CO2)(+) and Cr(CO2)2(+) cation complexes are characterized to have linear end-on η(1)-O-coordinated structures with blue-shifted antisymmetric CO2 stretching vibrational frequencies. The OCrCO(-) anion is bent with the Cr-O and CO stretching frequencies red-shifted from those of OCrCO neutral molecule.