Some aspects of structure and bonding in binary and ternary uranium(VI) oxides

Aspects of the structure and bonding in binary and ternary uranium oxides are considered using group theoretical and geometrical ideas. A 13-orbital sd(5)f(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 2(1)/(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 U(X)O(Y)(Z-) is the formal negative charge per uranium atom (Z/X). A Z/X ratio of 2 corresponds to the tetraoxouranates M(2)(I)UO(4) and M(II)UO(4), which have structures consisting of chains or layers of flattened UO(2)(a)O(4/2)(e) uranyl octahedra or layers Of UO(2)(a)O(6/3)(e) 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 UO(3) or U(3)O(8) 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 UO(4)(e)O(2/2)(a) "reverse uranyl" elongated octahedra (e.g., Ca(2)UO(5) or Na(4)UO(5)) or regular UO(6) octahedra (e.g., Ca(3)UO(6) or Li(6)UO(6)).