The equilibrium constants for [NpO2 center dot M](4+) (M = Al3+, In3+, Sc3+, Fe3+) in mu = 10 M nitric acid and [NpO2 center dot M](4+) in mu = 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 pi-donor with transition-metal cations and a sigma-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.