A Molecular Octafluoridoneptunate(IV) Anion in (NH4)4[NpF8] and Theoretical Investigations of the [MF8]4– System (M = Th – Bk)

Abstract

Olive‐green single crystals of ammonium octafluoridoneptunate(IV), (NH4)4[NpF8], were obtained by converting NpO2 to a green neptunium tetrafluoride hydrate with hydrofluoric acid and subsequent treatment of the fluoride with an aqueous NH4F solution. The crystal structure of the compound was determined by single‐crystal X‐ray diffraction and observed to be isotypic to the uranium analogue. In (NH4)4[NpF8], molecular [NpF8]4– anions, which can either be described as a distorted square‐antiprism or a bicapped trigonal prism, are present which are bound to the NH4+ ions via N–H···F hydrogen bonds. Quantum‐chemical calculations of [MF8]4– anions show that the M–F bonds are highly ionic and the energy differences between different ligand arrangements likely can be overcome by lattice energies of different crystal structures in the solid state.