Reduction chemistry of neptunium cyclopentadienide complexes: from structure to understanding.

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)], its bis-acetonitrile adduct [Np(Cp)(NCMe)], and its KCp adduct K[Np(Cp)] and [Np(Cp')] (Cp' = CHSiMe) have been made and characterised providing the first single crystal X-ray analyses of Np Cp complexes. In all NpCp derivatives there are three Cp rings in η-coordination around the Np centre; additionally in [Np(Cp)] and K[Np(Cp)] one Cp ring establishes a μ-η-interaction to one C atom of a neighbouring Np(Cp) unit. The solid state structure of K[Np(Cp)] is unique in containing two different types of metal-Cp coordination geometries in the same crystal. Np(Cp) units are found exhibiting four units of η-coordinated Cp rings like in the known complex [Np(Cp)], the structure of which is now reported. A detailed comparison of the structures gives evidence for the change of ionic radii of -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(Cp')] by KC 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(Cp')]. 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.