Matrix-Isolation Infrared Spectra and Electronic Structure Calculations for Dinitrogen Complexes with Uranium Trioxide Molecules UO 3 (η 1 -NN) 1-4 .

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.