Polymerization of Polyhedra

The geomeffy, bond valences, and polymerization ofhexavalent uranium polyhedra from 105 well-refined structures are analyzed. The Utu cation is almost always present in crystal stnrctures as part of a nearly linear (UOr)z* uranyl ion that is coordinated by four, five or six equatorial anions in an approximately planar arangement perpendicular to the uranyl ion, giving square, pentagonal and hexagonal bipyramids, respectively. The Utu-O7" bond length (Oy,: uranyl-ion O atom) is independent of the equatorial anions of the polyhedra;-averages of all polyhedra tlat contain uranyl ions ffs; I6lIJ6f-Or. = 1.79(3), mg0.-.9 a,= 1.79(4), and t8lu6+-Our = 1.78(3) A. Not a[ r6lu6+ polyhedra contain uranyl ions; there is a continuous series of coordiaation polyhedr4 from square bipyramidal polyhedra with uranyl ions to holosymmehic octahedral geometry. The mUo* and t8lu6+ polyhedra invariably contaitl a uranyl ion. The equatorial U6.-0 (0: O,-, OH-) bond-lengths of uranyl polyhedra depend upon coordhation number; averages for all polyhedra are t6lu6+-dq = 2,28(5), rlUot-$* = 2.37(9), afi t8tlJ6+-$q2.47 (12) A. Cunently available bond-valence parameters for U& are unsatisfactory for determining bond-valence sums. Coordination-specific bond-valence paxameters have been derived for U6|, together with parameters applicable to all coordination geometries. The parameters give bond-valence sums for Ue of -6 vlr and reasonable bond-valences for Uc,-Ou, bonds. The bond-valence paraneters facilitate the recognition of Ua, U5+ and U6| catiotrs in refined crystal structures. The crystal-chemical consfraints ofpolyhedral polymerization in uranyl phases are discussed.