A Combined Density Functional Theory and Spectrophotometry Study of the Bonding Interactions of [NpO2·M]4+ Cation–Cation Complexes

The equilibrium constants for [NpO 2 ·M] 4+ (M = Al 3+ , In 3+ , Sc 3+ , Fe 3+ ) in μ = 10 M nitric acid and [NpO 2 ·Ga] 4+ in μ = 10 M hydrochloric acid media have been determined. The trend in the interaction strength follows: Fe 3+ > Sc 3+ ≥ In 3+ > Ga 3+ ≫ Al 3+ . These equilibrium constants are compared to those of previously reported values for NpO 2 + complexes with Cr 3+ and Rh 3+ within the literature. Thermodynamic parameters and bonding modes are discussed, with density functional theory and natural bond orbital analysis indicating that the NpO 2 + 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.