Abstract
Reactions of laser‐ablated lanthanum atoms with CO molecules in solid argon have been studied. The neutral lanthanum monocarbonyl (LaCO), produced upon sample deposition at 7 K, exhibits a CO stretching frequency of 1772.7 cm^−1^; to the best of our knowledge this is the lowest yet observed for a terminal CO in a neutral metal–carbonyl molecule (MCO, M=metal atom), implying anomalously enhanced metal‐to‐CO back‐bonding. The infrared (IR) absorption band at 1145.9 cm^−1^ is assigned to the CO stretching mode of the side‐on‐bonding CO in the La~2~[η^2^(μ~2~‐C,O)] molecule. This CO‐activated molecule undergoes an UV/Vis‐photoinduced rearrangement to the CO‐dissociated molecule, c‐La~2~(μ‐C)(μ‐O). Density functional theory (DFT) calculations have been performed on these molecules, the results of which lend strong support to the experimental assignments of the IR spectra. LaCO is predicted to have a quartet ground state, corresponding to a linear geometry. Its formation involves La 6s→4f promotion, which increases the strength of LaCO bonding by decreasing the σ repulsion and, remarkably, by increasing the La 5d and 4f→CO 2π back‐bonding. The observations schematically depict the whole process, starting with the interaction of CO with metal and ending with CO dissociation by the lanthanum dimer.