Generation and Characterization of the Charge-Transferred Diradical Complex CaCO 2 with an Open-Shell Singlet Ground State.

The CaCO 2 complex is generated via the reaction of excited-state calcium atom with carbon dioxide in a solid neon matrix. Infrared absorption spectroscopy and quantum chemical calculations reveal that the complex has a planar four-membered ring structure with a strongly bent CO 2 ligand side-on coordinated to the calcium center in an η 2- O, O manner. The complex has an open-shell singlet ground state, which can be described as the bonding interactions between a Ca + (4s 1 ) cation in the doublet ground state and a doublet ground state CO 2 - anion. The analysis of the bonding situation suggests that the Ca-O 2 C bonds have a large (75%) electrostatic character. The covalent (orbital) interactions come from the coupling of the unpaired electrons of Ca + and CO 2 - giving rise to electron-sharing bonding and a stronger contribution from dative bonding (Ca + )←(CO 2 - ). The atomic orbitals (AOs) of Ca + that are engaged in the covalent bonds are the 4s AO for the electron-sharing bonds and the 3d AOs for the dative bonds. This is further evidence for the assignment of the heavier alkaline-earth atoms as transition metals rather than main-group elements.