A Combined Density Functional Theory and Spectrophotometry Study of the Bonding Interactions of [NpO 2 ·M] 4+ Cation-Cation Complexes.

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.