The structures of some dimers of carbon monoxide—an infrared matrix isolation spectroscopic and ab initio molecular orbital study

Insights into the Metal-CO Bond in O2M(η1-CO) (M = Cr, Mo, W, Nd, and U) Complexes

Investigations on the structures and bonding properties of metal carbonyl compounds provide fundamental understandings on the origin of small-molecule activations. Herein, the geometry and bonding trends of a series of isovalent metal oxocarbonyl complexes O₂M(η¹-CO) (M = Cr, Mo, W, Nd, and U) were studied by combined matrix-isolation infrared spectroscopy and advanced quantum chemical calculations. The title complexes present red shift of C-O stretching bands in the range from 122 to 244 cm^-1, indicating the difference of CO activation ability for the series of isovalent metal dioxides. Density functional theory calculations predict T-shaped structures with a C_2v symmetry for all the title molecules. O₂Nd(η¹-CO) bears little resemblance to the other complexes in bonding characters because of the weak interactions between the NdO₂ and CO moiety. For the other complexes, natural localized molecular orbital analysis reveals a gradual increase of covalent character in M-CO bonds along the metal series Cr → Mo → W→ U. Energy decomposition analysis with natural orbitals for chemical valence calculations demonstrates that the M-CO bonding patterns conform to the conventional Dewar-Chatt-Duncanson motif. The contributions from orbital interactions in total attractions increase from Cr (41.7%) to U (52.7%). The breakdown of the orbital term into pairwise interactions shows that contributions of the M ← CO σ donation decrease from Cr (59.2%) to U (28.4%), while the M → CO π* backdonation increases significantly from Cr (23.8%) to U (67.3%). The more effective overlap and the better energy matching of U 5f and U 6d valence orbitals with CO π* orbitals result in much stronger U → CO π backdonation than the other metal elements.

Intermediates of Carbon Monoxide Oxidation on Praseodymium Monoxide Molecules: Insights from Matrix-Isolation IR Spectroscopy and Quantum-Chemical Calculations

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(η¹-CO) and OPr(η²-CO), have been trapped and characterized in argon matrixes. The intermediate OPr(η²-CO) shows an extremely low C-O stretching band at 1624.5 cm^-1. Quantum-chemistry studies indicate that the bonding in OPr(η¹-CO) is described as "donor-acceptor" interactions conforming to the Dewar-Chatt-Duncanson motif. However, the bonding in OPr(η²-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(η²-CO). Electronic structure analysis suggests that the two captured species exhibit Pr(III) OSs. Besides, the pathways of CO oxidation have been discussed.