A DFT and Matrix-Isolation IR/UV-Visible Study of High-Coordinated Lanthanide-CO Complexes

Recent joint mass spectrometric and IR photodissociation studies have provided proof on the existence of octa-coordinated ionic lanthanide-carbonyl complexes under those extreme gaseous conditions. In contrast, in older literature concerning cryogenic studies of neutral Ln(CO)_x species, the highest coordination was assigned to hexa-coordinated Ln(CO)₆ molecules. The present study aims to clarify the above controversy using matrix isolation spectroscopy and DFT calculations. In order to ensure the maximum possible coordination, the Ln(CO)_x complexes were synthesized in neat CO cryogenic matrices at 10 K and were investigated by infrared and UV-visible spectroscopy. The formed complexes were identified on the basis of the characteristic CO stretching frequencies of the ground-state molecules predicted by DFT calculations. Our joint experimental-theoretical analysis confirmed the preference of octa-coordinated Ln(CO)₈ complexes in cryogenic neat CO matrices.