Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding† †Electronic supplementary information (ESI) available: General procedures, synthetic details, spectroscopic data, X-ray crystallographic data. CCDC 1524162–1524166. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7sc00034k Click here for additional data file. Click here for additional data file.

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(Cp0)3] (Cp0 1⁄4 C5H4SiMe3) have been made and characterised providing the first single crystal X-ray analyses of Np Cp complexes. In all NpCp3 derivatives there are three Cp rings in h-coordination around the Np centre; additionally in [Np(Cp)3] and K[Np(Cp)4] one Cp ring establishes a m-h-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(Cp)4 units are found exhibiting four units of h -coordinated Cp rings like in the known complex [Np(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 and Np. The rich redox chemistry associated with the syntheses is augmented by the reduction of [Np(Cp0)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 and Th complexes K(2.2.2-cryptand)[Th/U(Cp0)3]. Together, these spontaneous and controlled redox reactions of organoneptunium complexes, along with information from structural characterisation, show the relevance of organometallic Np chemistry to understanding fundamental structure and bonding in the minor actinides.