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Elucidating molecular-level interactions between dihydrogen (H 2 ) and uranium oxides reveals fundamental insights into the intrinsic H 2 activation mechanisms underlying processes such as heterogeneous catalysis over uranium oxides and corrosion of uranium induced by H 2 . Herein, the reactions of H 2 with uranium monoxide (UO) molecules have been investigated via a combination of matrix-isolation infrared spectroscopy and quantum chemical calculations. A side-on bonded H 2 complex, OU(η 2 -H 2 ), is identified at 3733.7 and 800.3 cm -1 . This species is regarded as a crucial intermediate along H 2 activation pathways. Bonding analysis reveals cooperative U(π 5 f /6d ) → H 2 (σ*) π // backdonation and U ← H 2 (σ) σ donation in OU(η 2 -H 2 ) that facilitate the activation of the H 2 moiety. Upon λ > 550 nm photoirradiation, OU(η 2 -H 2 ) isomerizes into H 2 UO, indicating the homolytic H 2 cleavage on UO. Mechanistic details of H 2 adsorption and dissociation on UO molecules have been further elucidated.

Fundamental investigation of metal-CO interactions is of great importance for the development of high-performance catalysts to CO activation. Herein, a series of side-on bonded mononuclear lanthanide (Ln) oxocarbonyl complexes OLn(η 2 -CO) (Ln = La, Ce, Pr, and Nd) have been prepared and identified in solid argon matrices. The complexes exhibit uncommonly low C-O stretching bands near 1630 cm -1 , indicating remarkable C-O bond activation in these Ln analogues. The η 2 -CO ligand in OLn(η 2 -CO) can be claimed as an anion on the basis of the experimental observations and quantum chemistry investigations, although the CO anion is commonly considered to be unstable with electron auto-detachment. The CO activation in OLn(η 2 -CO) is attributed to the photoinduced intramolecular charge transfer from LnO to CO rather than the generally accepted metal → CO π back-donation, which conforms to the traditional Dewar-Chatt-Duncanson motif. Energy decomposition analysis combined with natural orbitals for chemical valence calculations demonstrates that the bonding between LnO and η 2 -CO arises from the combination of dominant ionic forces (>76%) and normal Lewis "acid-base" interactions. The fundamental findings provide guidelines for the catalyst design of CO activation.

Investigations of the interactions of uranium trioxide (UO 3 ) with other species are expected to provide a new perspective on its reaction and bonding behaviors. Herein, we present a combined matrix-isolation infrared spectroscopy and theoretical study of the geometries, vibrational frequencies, electronic structures, and bonding patterns for a series of dinitrogen (N 2 ) complexes with UO 3 moieties UO 3 (η 1 -NN) 1-4 . The complexes are prepared by reactions of laser-ablated uranium atoms with O 2 /N 2 mixtures or laser-ablated UO 3 molecules with N 2 in solid argon. UO 3 (η 1 -NN) 1-4 are classified as "nonclassical" metal-N 2 complexes with increased Δν(N 2 ) values according to the experimental observations and the computed blue-shifts of N-N stretching frequencies and N-N bond length contractions. Electronic structure analysis suggests that UO 3 (η 1 -NN) 1-4 are σ-only complexes with a total lack of π-back-donation. The energy decomposition analysis combined with natural orbitals for chemical valence calculations reveal that the bonding between the UO 3 moiety and N 2 ligands in UO 3 (η 1 -NN) 1-4 arises from the roughly equal electrostatic attractions and orbital mixings. The inspection of orbital interactions from pairwise contributions indicates that the strongest orbital stabilization comes from the σ-donations of the 4σ*- and 5σ-based ligand molecular orbitals (MOs) into the hybrid 7s/6d x 2 - y 2 MO of the U center. The electron polarization induced by electrostatic effects in the N inner ← N outer direction provides complementary contributions to the orbital stabilization in UO 3 (η 1 -NN) 1-4 . In addition, the reactions of UO 3 with N 2 ligands and the origination of the nonclassical behavior in UO 3 (η 1 -NN) 1-4 are discussed.

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 O 2 M(η 1 -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. O 2 Nd(η 1 -CO) bears little resemblance to the other complexes in bonding characters because of the weak interactions between the NdO 2 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.