© 2018 American Chemical Society. Complexation of Np(V) with three structurally related amine-functionalized diacetamide ligands, including 2,2′-azanediylbis(N,N′-dimethylacetamide) (ABDMA), 2,2′-(methylazanediyl)bis(N,N′-dimethylacetamide) (MABDMA), and 2,2′-(benzylazanediyl)bis(N,N′-dimethylacetamide) (BnABDMA), in aqueous solutions was investigated. The stability constants of two successive complexes, namely, NpO2L+and NpO2L2+, where L stands for the ligands, were determined by absorption spectrophotometry. The results suggest that the stability constants of corresponding Np(V) complexes follow the trend: MABDMA > ABDMA ≈ BnABDMA. The data are discussed in terms of the basicity of the ligands and compared with those for the complexation of Np(V) with an ether oxygen-linked diacetamide ligand. Extended X-ray absorption fine structure data indicate that, similar to the complexation with Nd3+and UO22+, the ligands coordinate to NpO2+in a tridentate mode through the amine nitrogen and two oxygen atoms of the amide groups. Computational results, in conjunction with spectrophotometric data, verify that the 1:2 complexes (NpO2(L)2+) in aqueous solutions are highly symmetric with Np at the inversion center, so that the f-f transition of Np(V) is forbidden and NpO2(L)2+does not display significant absorption in the near-IR region.