Matrix-Isolation Infrared Spectra and Electronic Structure Calculations for Dinitrogen Complexes with Uranium Trioxide Molecules UO3(eta(1)-NN)(1-4)

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 (N-2) complexes with UO3 moieties UO3 (eta(1)-NN)(1-4). The complexes are prepared by reactions of laser-ablated uranium atoms with O-2/N-2 mixtures or laser-ablated UO3 molecules with N-2 in solid argon. UO3(eta(1)-NN)(1-4) are classified as "nonclassical" metal-N-2 complexes with increased Delta v(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 UO3(eta(1)-NN)(1-4) are sigma-only complexes with a total lack of pi-back-donation. The energy decomposition analysis combined with natural orbitals for chemical valence calculations reveal that the bonding between the UO3 moiety and N-2 ligands in UO3(eta(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 sigma-donations of the 4 sigma*- and 5 sigma-based ligand molecular orbitals (MOs) into the hybrid 7s/6d(x2-y2 ) 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(eta(1)-NN)(1-4). In addition, the reactions of UO3 with N-2 ligands and the origination of the nonclassical behavior in UO3(eta(1)-NN)(1-4) are discussed.