The role of the 5f valence orbitals of early actinides in chemical bonding

Abstract One of the long standing debates in actinide chemistry is the level of localization and participation of the actinide 5 f valence orbitals in covalent bonds across the actinide series. Here we illuminate the role of the 5 f valence orbitals of uranium, neptunium and plutonium in chemical bonding using advanced spectroscopies: actinide M 4,5 HR-XANES and 3 d 4 f RIXS. Results reveal that the 5 f orbitals are active in the chemical bonding for uranium and neptunium, shown by significant variations in the level of their localization evidenced in the spectra. In contrast, the 5 f orbitals of plutonium appear localized and surprisingly insensitive to different bonding environments. We envisage that this report of using relative energy differences between the 5 f δ/φ and 5 f π*/5 f σ* orbitals as a qualitative measure of overlap-driven actinyl bond covalency will spark activity, and extend to numerous applications of RIXS and HR-XANES to gain new insights into the electronic structures of the actinide elements.