A new mixed valent neptunium compound [BuMeIm](5)[Np(NPO2)(3)(H2O)(6)Cl-12] was synthesized by evaporation of an ethyl acetate-acetone mixture where the salt [BuMeIm](2)[NpCl6] was dissolved. The crystal structure was determined using single crystal X-ray diffraction; it consists of four crystallographically unique Np centers with two different coordination environments in two different oxidation states. In addition, five crystallographically independent dialkylimidazolium cations stabilize the crystal through C-H center dot center dot center dot Cl hydrogen bonds. [Np(NPO2)(3)(H2O)(6)Cl-12] unit presents a highly symmetric structure with the formation of three Np(IV)-NP(V)O-2(+) bonds. According to the crystal structure and density functional theory (DFT) calculations, the stability of the three Np4+-NpO2+ interactions is due to the presence of chloride ions around NPO2+ which brings electronic charge into the system and also to the presence of water molecules which create hydrogen bond with chloride ions,