Electronic Spectra of Pure Uranyl(V) Complexes:  Characteristic Absorption Bands Due to a U<sup>V</sup>O<sub>2</sub><sup>+</sup> Core in Visible and Near-Infrared Regions<sup>1</sup>

To clarify the electronic spectral properties of uranyl(V) complexes systematically, we measured absorption spectra of three types of pure uranyl(V) complexes:  [U<sup>V</sup>O<sub>2</sub>(dbm)<sub>2</sub>DMSO]<sup>-</sup>, [U<sup>V</sup>O<sub>2</sub>(saloph)DMSO]<sup>-</sup>, and [U<sup>V</sup>O<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>5-</sup> (dbm = dibenzoylmethanate, saloph = <i>N,N</i><i>‘</i>-disalicylidene-<i>o</i>-phenylenediaminate, DMSO = dimethyl sulfoxide). As a result, it was found that these uranyl(V) complexes have characteristic absorption bands in the visible−near-infrared (NIR) region, i.e., at around 640, 740, 860, 1470, and 1890 nm (molar absorptivity, ε = 150−900 M<sup>-1</sup>·cm<sup>-1</sup>) for [U<sup>V</sup>O<sub>2</sub>(dbm)<sub>2</sub>DMSO]<sup>-</sup>, 650, 750, 900, 1400, and 1875 nm (ε = 100−300 M<sup>-1</sup>·cm<sup>-1</sup>) for [U<sup>V</sup>O<sub>2</sub>(saloph)DMSO]<sup>-</sup>, and 760, 990, 1140, 1600, and 1800 nm (ε = 0.2−3.6 M<sup>-1</sup>·cm<sup>-1</sup>) for [U<sup>V</sup>O<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>5-</sup>. These characteristic absorption bands of the uranyl(V) complexes are attributable to the electronic transitions in the U<sup>V</sup>O<sub>2</sub><sup>+</sup> core because the spectral features are similar to each other despite the differences in the ligands coordinated to the equatorial plane of the U<sup>V</sup>O<sub>2</sub><sup>+</sup> moiety. On the other hand, the ε values of [U<sup>V</sup>O<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>]<sup>5-</sup> are quite smaller than those of [U<sup>V</sup>O<sub>2</sub>(dbm)<sub>2</sub>DMSO]<sup>-</sup> and [U<sup>V</sup>O<sub>2</sub>(saloph)DMSO]<sup>-</sup>. Such differences can be explained by the different coordination geometries around the center uranium in these uranyl(V) complexes. Consequently, the absorption bands of the uranyl(V) complexes in visible−NIR region were assigned to f−f transitions in the 5f<sup>1</sup> configuration.