Electronic Structures and Unusual Chemical Bonding in Actinyl Peroxide Dimers [AnO] and [(AnO)(12-crown-4 ether)] (An = U, Np, and Pu).

As known, actinyl peroxides play important roles in environmental transport of actinides, and they have strategic importance in the application of nuclear industry. Compared to the most studied uranyl peroxides, the studies of transuranic counterparts are still few, and more information about these species is needed. In this work, experimentally inspired actinyl peroxide dimers ([AnO], An = U, Np, and Pu) have been studied and analyzed by using density functional theory and multireference wave function methods. This study determines that the three [AnO] have unique electronic structures and oxidation states, as [(UO)(O)], [(NpO)(O)], and mixed-valent [(PuO)(O)]. This study demonstrates the significance of two bridging oxo ligands with at most four electron holes availability in ionically directing actinyl and resulting in the unusual multiradical bonding in [(PuO)(O)]. In addition, thermodynamically stable 12-crown-4 ether (12C4) chelated [(AnO)(12C4)] complexes have been predicted, that could maintain these unique electronic structures of [AnO], where the An ← O dative bonding shows a trend in binding capacity of 12C4 from κ (U) to κ (Np) and κ (Pu). This study reveals the interesting electronic character and bonding feature of a series of early actinide elements in peroxide complexes, which can provide insights into the intrinsic stability of An-containing species.