Some Aspects of Structure and Bonding in Binary and Ternary Uranium(VI) Oxides

Abstract

Aspects of the structure and bonding in binary and ternary uranium oxides are considered using group theoretical and geometrical ideas. A 13-orbital sd5f 7 bonding manifold for the uranium atoms is sufficient for regular octahedra having six double bonds, elongated “reverse uranyl” octahedra having four equatorial bonds of average order 21/2, and flattened uranyl octahedra or pentagonal bipyramids having two axial triple bonds. This is consistent with the uranium−oxygen bond lengths in ternary uranium oxides (uranates) exhibiting these structures. A key factor in determining structural features of uranates UXOYZ- is the formal negative charge per uranium atom (Z/X). A Z/X ratio of 2 corresponds to the tetraoxouranates M2IUO4 and MIIUO4, which have structures consisting of chains or layers of flattened UO2aO4/2e uranyl octahedra or layers of UO2aO6/3e uranyl hexagonal bipyramids with the interstitial counterions bonded to 3−8 oxygen atoms. Most cation-poor uranates with Z/X < 2 have layered structures related to those of UO3 or U3O8 consisting of equatorial planes containing quadrilaterals and/or pentagons, short strong axial uranium−oxygen bonds, and interstitial cations between the layers. Cation-rich uranates with Z/X > 2 contain either chains of UO4eO2/2a “reverse uranyl” elongated octahedra (e.g., Ca2UO5 or Na4UO5) or regular UO6 octahedra (e.g., Ca3UO6 or Li6UO6).