Infrared Spectroscopy of [M(CO2)n]+ (M = Ca, Sr, and Ba; n = 1-4) in the Gas Phase: Solvation-Induced Electron Transfer and Activation of CO2

Abstract

Cationic complexes of heavy alkaline earth metal and carbon dioxide [M(CO₂)_n]⁺ (M = Ca, Sr, and Ba) are produced by a laser vaporization-supersonic expansion ion source in the gas phase and are studied by infrared photodissociation spectroscopy in conjunction with quantum chemistry calculations. For the n = 1 complexes, the metal-ligand binding arises primarily from the electrostatic interaction with the CO₂ ligand bound to the metal (+I) center in an end-on η¹-O fashion. The more highly coordinated complexes [M(CO₂)_n]⁺ with n ≥ 2 are characterized to involve a [M²⁺(CO₂^-)] core ion with the CO₂^- ligand bound to the metal (+II) center in a bidentate η²-O, O manner. The activation of CO₂ in forming a bent CO₂^- moiety occurs via solvation-induced metal cation-ligand electron transfer reactions. Bonding analyses reveal that the attractive forces between M²⁺ and CO₂^- in the core cation come mainly from electrostatic attraction, but the contribution of covalent orbital interactions should not be underestimated. The atomic orbitals of metal dications that are engaged in the orbital interactions are ns and (n - 1)d orbitals.