Incipient class II mixed valency in a plutonium solid-state compound

Electron transfer in mixed-valent transition-metal complexes, clusters and materials is ubiquitous in both natural and synthetic systems. The degree to which intervalence charge transfer (IVCT) occurs, dependent on the degree of delocalization, places these within class II or III of the Robin-Day system. In contrast to the d-block, compounds of f-block elements typically exhibit class I behaviour (no IVCT) because of localization of the valence electrons and poor spatial overlap between metal and ligand orbitals. Here, we report experimental and computational evidence for delocalization of 5f electrons in the mixed-valent Pu^III/Pu^IV solid-state compound, Pu₃(DPA)₅(H₂O)₂ (DPA = 2,6-pyridinedicarboxylate). The properties of this compound are benchmarked by the pure Pu^III and Pu^IV dipicolinate complexes, [Pu^III(DPA)(H₂O)₄]Br and Pu^IV(DPA)₂(H₂O)₃·3H₂O, as well as by a second mixed-valent compound, Pu^III[Pu^IV(DPA)₃H_0.5]₂, that falls into class I instead. Metal-to-ligand charge transfer is involved in both the formation of Pu₃(DPA)₅(H₂O)₂ and in the IVCT.