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

Abstract

The equilibrium constants for [NpO2·M](4+) (M = Al(3+), In(3+), Sc(3+), Fe(3+)) 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: Fe(3+) > Sc(3+) ≥ In(3+) > Ga(3+) ≫ Al(3+). These equilibrium constants are compared to those of previously reported values for NpO2(+) 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 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.