Hydration States of Transplutonium (Am, Cm, Bk, and Cf) Ions Alter Electronic Structure

The most fundamental and widely studied form of metal-ion complexation is hydration. Nevertheless, the hydration of the heaviest elements with experimentally accessible bulk properties, namely americium, curium, berkelium, and californium, remains a subject of ongoing debate. Herein, the solid-state series of nona-aqua complexes from uranium to californium is completed through the single-crystal structure characterization of [An(H2O)9](CF3SO3)3 (An3+ = Bk3+ and Cf3+) at 100 K. In addition, the largely unexplored solid-state octa-aqua series, [An(H2O)8](Hdtp)(dtp)·H2O (An3+ = Am3+, Cm3+, Bk3+ and Cf3+, H2dtp = 2,3-di(tetrazol-5-yl)pyrazine) is synthesized and comprehensively characterized to elucidate how variations in hydration manifest in electronic spectra and structural metrics. By combining crystallographic and spectroscopic data with ab initio multiconfigurational wave function calculations, we demonstrate that differences between the two hydration motifs give rise to distinct trends in bond lengths, spectral features, and crystal-field strengths. Although the average An-OH2 bond lengths decrease with similar slopes in both [An(H2O)8]3+ and [An(H2O)9]3+ from Am3+ to Cf3+, the trends in individual An-OH2 bond length contractions differ between the two series. Remarkably, the crystal-field splitting in the octa-aqua complexes can be up to twice that observed in the nona-aqua complexes.