Colourless crystalline compounds of centrosymmetric [Np(NO ₃ ) ₆ ] ²⁻  were yielded from 3 M HNO ₃  aq in the presence of double-headed 2-pyrrolidone derivatives (L). In the obtained crystal structures, H ⁺  was also involved as a countercation to compensate for the negative charge of [Np(NO ₃ ) ₆ ] ²⁻ , where the initial hydration around H ⁺  was fully removed during crystallization despite it having the strongest hydration enthalpy. Instead, this anhydrous H ⁺  was captured by L to form a [H ⁺ ⋯L] _n  hydrogen bonded polymer. In [Np(NO ₃ ) ₆ ] ²⁻ , the Np ⁴⁺  centre is twelve-coordinated with 6 bidentate NO ₃ ⁻ , and therefore, present in an icosahedral geometry bearing inversion centre. In such a centrosymmetric system, any f–f transitions stemming from the 5f ³  electronic configuration of Np ⁴⁺  are electric-dipole forbidden. This is the reason why the compounds currently obtained were colourless unlike ordinary Np(IV) species, which are olive-green.