Infrared Spectroscopy of Gas-Phase M⁺(CO₂)_n (M = Co, Rh, Ir) Ion-Molecule Complexes

The structures of gas-phase M⁺(CO₂)_n (M = Co, Rh, Ir; n = 2-15) ion-molecule complexes have been investigated using a combination of infrared resonance-enhanced photodissociation (IR-REPD) spectroscopy and density functional theory. The results provide insight into fundamental metal ion-CO₂ interactions, highlighting the trends with increasing ligand number and with different group 9 ions. Spectra have been recorded in the region of the CO₂ asymmetric stretch around 2350 cm^-1 using the inert messenger technique and their interpretation has been aided by comparison with simulated infrared spectra of calculated low-energy isomeric structures. All vibrational bands in the smaller complexes are blue-shifted relative to the asymmetric stretch in free CO₂, consistent with direct binding to the metal center dominated by charge-quadrupole interactions. For all three metal ions, a core [M⁺(CO₂)₂] structure is identified to which subsequent ligands are less strongly bound. No evidence is observed in this size regime for complete activation or insertion reactions.