Neptunium complexes in the formal oxidation states II, III, and IV supported by cyclopentadienyl ligands are explored, and significant differences between Np and U highlighted as a result. A series of neptunium(iii) cyclopentadienyl (Cp) complexes [Np(Cp)₃], its bis-acetonitrile adduct [Np(Cp)₃(NCMe)₂], and its KCp adduct K[Np(Cp)₄] and [Np(Cp')₃] (Cp' = C₅H₄SiMe₃) have been made and characterised providing the first single crystal X-ray analyses of Np^III Cp complexes. In all NpCp₃ derivatives there are three Cp rings in η⁵-coordination around the Np^III centre; additionally in [Np(Cp)₃] and K[Np(Cp)₄] one Cp ring establishes a μ-η¹-interaction to one C atom of a neighbouring Np(Cp)₃ unit. The solid state structure of K[Np(Cp)₄] is unique in containing two different types of metal-Cp coordination geometries in the same crystal. Np^III(Cp)₄ units are found exhibiting four units of η⁵-coordinated Cp rings like in the known complex [Np^IV(Cp)₄], the structure of which is now reported. A detailed comparison of the structures gives evidence for the change of ionic radii of ca. -8 pm associated with change in oxidation state between Np^III and Np^IV. The rich redox chemistry associated with the syntheses is augmented by the reduction of [Np(Cp')₃] by KC₈ in the presence of 2.2.2-cryptand to afford a neptunium(ii) complex that is thermally unstable above -10 °C like the U^II and Th^II complexes K(2.2.2-cryptand)[Th/U(Cp')₃]. Together, these spontaneous and controlled redox reactions of organo-neptunium complexes, along with information from structural characterisation, show the relevance of organometallic Np chemistry to understanding fundamental structure and bonding in the minor actinides.