Probing the Electronic Structure and Chemical Bonding in Tricoordinate Uranyl Complexes UO₂X₃^– (X = F, Cl, Br, I): Competition between Coulomb Repulsion and U–X Bonding

Abstract

While uranyl halide complexes [UO2(halogen)n](2-n) (n = 1, 2, 4) are ubiquitous, the tricoordinate species have been relatively unknown until very recently. Here photoelectron spectroscopy and relativistic quantum chemistry are used to investigate the bonding and stability of a series of gaseous tricoordinate uranyl complexes, UO2X3(-) (X = F, Cl, Br, I). Isolated UO2X3(-) ions are produced by electrospray ionization and observed to be highly stable with very large adiabatic electron detachment energies: 6.25, 6.64, 6.27, and 5.60 eV for X = F, Cl, Br, and I, respectively. Theoretical calculations reveal that the frontier molecular orbitals are mainly of uranyl U-O bonding character in UO2F3(-), but they are from the ligand valence np lone pairs in the heavier halogen complexes. Extensive bonding analyses are carried out for UO2X3(-) as well as for the doubly charged tetracoordinate complexes (UO2X4(2-)), showing that the U-X bonds are dominated by ionic interactions with weak covalency. The U-X bond strength decreases down the periodic table from F to I. Coulomb barriers and dissociation energies of UO2X4(2-) → UO2X3(-) + X(-) are calculated, revealing that all gaseous dianions are in fact metastable. The dielectric constant of the environment is shown to be the key in controlling the thermodynamic and kinetic stabilities of the tetracoordinate uranyl complexes via modulation of the ligand-ligand Coulomb repulsions.

Photoelectron spectroscopy and theoretical studies of gaseous uranium hexachlorides in different oxidation states: UCl6q− (q = 0–2)

“…The diagonal matrix elements were replaced by the individual CCSD(T) or CR-EOM-CCSD(T) or CASPT2 state energies at the SR level, which are denoted as CASSCF/CCSD(T)/SO or CASSCF/CR-EOM-CCSD(T)/SO or CASSCF/CASPT2/SO methods, respectively. [25][26][27][56][57][58][59][60] In the CASSCF/CR-EOM-CCSD(T)/SO calculation for UCl 6 − , the CCSD(T) groundstate energy and the CR-EOM-CCSD(T) 5f 1 excited state energies were used as diagonal matrix elements of the SO coupling matrix. That is, the vertical excitation energies of UCl 6 2− and UCl 6 − at the SO level were calculated with the socalled CASSCF/CAS(2,7)PT2/SO and CASSCF/CR-EOM-CCSD(T)/SO methods, respectively.…”

“…3). The softness of the outer noble-gas core shells is well known for the bottom rows of the periodic table, [25][26][27]31,88,89 where the heavy noble-gas shells have an influence on the valency of the heavy metal atoms from the sand early f-blocks. Without accounting for the SO splitting of the Cl-3p t 1u MOs of 0.4 eV, the spectral simulations would become visibly worse for both UCl 6 − and UCl 6 2− , indicating the valence participation of the Rn-6p noble-gas shell.…”

“…24 Relevant to the current uranium halides, we have successfully applied ESI-PES to probe the electronic structures of a series of uranyl and uranium halide anions in conjunction with relativistic quantum chemical calculations. [25][26][27][28][29][30][31] In particular, we have produced [UF 5 ] − and [UF 6 ] − using ESI and obtained the PES spectra of [UF 5 ] − both at room temperature and low temperature, which yielded an accurate electron affinity (EA) for UF 5 as 3.885 ± 0.015 eV. 28,29 The [U(f 2 )F 5 ] − anion has a C 4v symmetry with two unpaired 5f electrons, occupying f z 3 and f xyz type orbitals, respectively.…”

Gas Phase Reactions of Ions Derived from Anionic Uranyl Formate and Uranyl Acetate Complexes

“…Electrospray ionization (ESI) provides easy access to a wide range of gasphase complexes containing uranium in high oxidation states (+5 and +6) for studies of intrinsic structure and reactivity (i.e., outside of the influence of solvent or other condensed phase effects) in a species specific fashion. For example, the transfer of uranium as mono-positive, pentavalent U V O 2 + from solution to the gas phase using ESI was first reported in 1992 [5] and since then, advances in the fundamental understanding of uranyl coordination chemistry have been made using the ionization method [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Most importantly, ESI has been used to generate gas-phase, doubly charged complexes containing UO 2…”

Tuning electronic structures of uranyl fluorides via increasing equatorial pyridyl number and extending pyridyl conjugation

“…[6][7][8][9][10][11][12] As one of the most thermodynamically stable uranium derivatives, the dioxo uranium(VI) cation is the most prevalent in the processing of nuclear fuel and the disposal of nuclear waste. [6] It has been indicated that the U=O bonds of uranyl determine its high-lying occupied orbitals while the low-lying unoccupied orbitals are dominated by U(f) character [10][11][12][13][14][15][16]. Usually, the linear uranyl cation is equatorially coordinated by 4 to 6 ligands.…”