Spectroscopic properties of uranyl chloride complexes in non-aqueous solvents

Abstract

The complex formation of uranyl with chloride ions in non-aqueous solvents (acetone, tributylphosphate and methyl-i-butylketone) was studied by absorption, luminescence and MCD spectroscopy. The maximal coordination of the uranyl ion by chloride ions in these organic solvents was found to be four, which is in agreement with single crystal data. The MCD spectra unambiguously exclude the existence of [UO₂Cl₃]⁻ with D_3h coordination symmetry. For the intermediately formed complexes we propose a structure of the type: UO₂Cl₂(solv)₄, with an orthorhombic symmetry. For these intermediate complexes, the 19 500–22 000 cm⁻¹ region of the absorption spectrum exhibits an intensity enhancement with respect to that of the solvated uranyl ion. This intensity enhancement as well as the increase in total luminescence intensity is explained in terms of vibronic coupling of the first electronic state (Π_g) with the asymmetric stretching (ν_a:a_2u) and the bending (ν_b:e_u) vibrations of the uranyl ion and the out-of-plane bending mode of the equatorial ligands (ν₁₀:b_1u). The spectra are dominated by transitions arising from the σ_u⁺δ_u configuration. The spectroscopic properties of the UO₂Cl₂(TBP)_x complexes are compared with those of the UO₂(NO₃)₂(TBP)₂ complex (TBP = tributylphosphate).