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© 2021 American Chemical Society.Identifying reaction intermediates in gas–phase investigations will provide understanding for the related catalysts in fundamental aspects including bonding interactions of the reaction species, oxidation states (OSs) of the anchored atoms, and reaction mechanisms. Herein, carbon monoxide (CO) oxidation by praseodymium monoxide (PrO) molecules has been investigated as a model reaction in solid argon using matrix–isolation IR spectroscopy and quantum–chemical calculations. Two reaction intermediates, OPr(1–CO) and OPr(2–CO), have been trapped and characterized in argon matrixes. The intermediate OPr(2–CO) shows an extremely low C–O stretching band at 1624.5 cm–1. Quantum–chemistry studies indicate that the bonding in OPr(1–CO) is described as "donor–acceptor"interactions conforming to the Dewar–Chatt–Duncanson motif. However, the bonding in OPr(2–CO) results evidently from a combination of dominant ionic forces and normal Lewis "acid–base"interactions. The electron density of the singly occupied bonding orbital is strongly polarized to the CO fragment in OPr(2–CO). Electronic structure analysis suggests that the two captured species exhibit Pr(III) OSs. Besides, the pathways of CO oxidation have been discussed.
