Abstract
The equilibrium constants for [NpO2·M]4+ (M = Al3+, In3+, Sc3+, Fe3+) in μ
= 10 M nitric acid and [NpO2·Ga]4+ in μ = 10 M hydrochloric acid media have been
determined. The trend in the interaction strength follows: Fe3+ > Sc3+ ≥ In3+ > Ga3+ ≫
Al3+. These equilibrium constants are compared to those of previously reported values for
NpO2
+ complexes with Cr3+ and Rh3+ within the literature. Thermodynamic parameters and
bonding modes are discussed, with density functional theory and natural bond orbital
analysis indicating that the NpO2
+ dioxocation acts as a π-donor with transition-metal
cations and a σ-donor with group 13 cations. The small changes in electron-donating ability
is modulated by the overlap with the coordinating metal ion’s valence atomic orbitals.
