Reactions of laser-ablated La atoms with CO2 molecules in solid argon and neon have been investigated using matrix-isolation infrared spectroscopy. On the basis of isotopic shifts, mixed isotopic splitting patterns, and CCl4-doping experiments, absorptions at 1839.9 and 753.6 cm(-1) in argon and 1855.9 and 771.3 cm(-1) in neon are assigned to the C-O and La-O stretching vibrations of the OLaCO molecule, respectively. Ultraviolet-visible photoinduced isomerization of OLaCO to La-(eta(2)-OC)O and OLa-(eta(2)-CO) have been observed under different wavelength photolyses in the solid matrix. The neon matrix experiments give the C-O and La-O stretching vibrations of the OLaCO- anion at 1769.5 and 779.3 cm(-1), respectively. Density functional theory calculations have been performed on these products, which support the experimental assignments of the infrared spectra. The present study reveals that the C-O stretching vibrational frequencies of OMCO decrease from Sc to La, which indicates an increase in metal d orbital -> CO pi* back-donation in this series.