Intermolecular interactions between the oxo group of an actinyl cation and other metal cations (i.e., cation cation interactions) are dependent on the strength of the actinyl bond. These cation cation interactions are prominently observed for the neptunyl cation [Np(V)O-2](+) and are sufficiently stable enough to explore using a variety of chemical techniques. Herein, we investigate these intermolecular interactions in the neptunyl 18-crown-6 system, because this macrocyclic ligand provides both stable coordination and the proper sterics to engage the oxo group in bonding with both low-valent metal cations and neighboring neptunyl units. We report the structural and spectroscopic characterization of five neptunyl, [Np(V,VI)O-2](+,2+), compounds: Np1a ([NpO2(18-crown-6)] ClO4), Np1b ([NpO2(18-crown-6)]AuCl4), Na-Np ([Np(V)O-2(18-crown-6) (Na(H2O)(18-crown-6)][Np (VI)O2Cl4], Np-Np ([NpO2(18-crown-6)](NpO2Cl2NO3)], and Np-Cl (NpO2Cl(H2O)1.75). Each of these compounds were prepared from the ambient reactions of Np(V) in HX (where X = Cl, NO3) with the 18-crown-6 ether molecule. Structural information obtained from single-crystal X-ray diffraction data was paired with solid-state and solution Raman spectroscopy to provide information on the interaction of the neptunyl oxo atom with neighboring cations. Neptunyl (Np=O) bond lengths are not perturbed upon interaction with the Na+ cation (Na-Np), but elongation is observed upon formation of a neptunyl neptunyl interaction (Np-Np). This is also the first structurally characterized isolated, molecular complex that contains a simple T-shaped neptunyl-neptunyl interaction. Raman spectroscopy indicates little perturbation to the neptunyl bond until the formation of the neptunyl-neptunyl motif, which also results in activation of the v3 asymmetric stretch. Additional spectroscopic studies indicated that the neptunyl 18-crown-6 inclusion complexes form in solution and persist in the presence of other low-valence cations.