Matrix-Isolation Infrared Spectra and Electronic Structure Calculations for Dinitrogen Complexes with Uranium...

Investigations of the interactions of uranium trioxide (UO₃) 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 (N₂) complexes with UO₃ moieties UO₃(η¹-NN)_1-4. The complexes are prepared by reactions of laser-ablated uranium atoms with O₂/N₂ mixtures or laser-ablated UO₃ molecules with N₂ in solid argon. UO₃(η¹-NN)_1-4 are classified as "nonclassical" metal-N₂ complexes with increased Δν(N₂) 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 UO₃(η¹-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 UO₃ moiety and N₂ ligands in UO₃(η¹-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 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 UO₃(η¹-NN)_1-4. In addition, the reactions of UO₃ with N₂ ligands and the origination of the nonclassical behavior in UO₃(η¹-NN)_1-4 are discussed.