Crystal Chemistry of Rubidium Uranyl Molybdates: Crystal Structures of Rb6[(UO2) (MoO4)4], Rb6[(UO2)2O(MoO4)4], Rb2[(UO2)(MoO4)2], Rb2[(UO2)2(MoO4)3] and Rb2[(UO2)6(MoO4)7(H2O)2]

Five Rb uranyl molybdates have been prepared by solid-state reactions and hydrothermal syntheses. The structure of each compound has been investigated using single-crystal X-ray diffraction data collected using MoKalpha radiation and a Charge-Coupled-device-based area detector. The structure of Rb6[(UO2)(MoO4)4] (monoclinic, C2/c, Z=4,a=17.312(1) ?, b= 11.5285(8) ?, c=13.916(1) ?, beta=127.634(1)°, V=2199.4(3) ?3, R1=0.029) is based on finite clusters of composition [(UO2)(MoO4)4]6- composed of a UrO4 (Ur: UO22+ uranyl ion) tetragonal bipyramid that shares each equatorial vertex with an adjacent MoO4 tetrahedron. The structure of Rb6[(UO2)2O(MoO4)4] (triclinic, P-1, Z=2, a=10.1567(5) ?, b=10.1816(5) ?, c=13.1129(6) ?, alpha=76.921(1)°, beta=76.553(1)°, gamma=65.243(1)°, V=1184.8(1) ?3, R1=0.047) is based upon chains of composition [(UO2)2O(MoO4)4]6containing UrO5 pentagonal bipyramids and MoO4 tetrahedra. The structure of Rb2[(UO2) (MoO4)2] (monoclinic, P21/c, Z=8, a=12.302(1) ?, b=13.638(1) ?, c=13.508(1) ?, beta= 94.975(2)°, V=2257.8(4) ?3, R1=0.059) contains sheets of composition [(UO2)(MoO4)2]2involving corner-sharing UrO5 pentagonal bipyramids and MoO4 tetrahedra. The structure of Rb2[(UO2)2(MoO4)3] (orthorhombic, Pna21, Z=4, a=20.214(1) ?, b=8.3744(4) ?, c= 9.7464(5) ?, V=1649.9(1) ?3, R1=0.027) consists of a framework with composition [(UO2) 2(MoO4)3]2- that involves corner-sharing UrO5 pentagonal bipyramids and MoO4 tetrahedra. The structure of Rb2[(UO2)6 (MoO4)7 (H2O)2] (orthorhombic, Pbcm, Z=4, a=13.961(2) ?, b=10.7515(16) ?, c=25.579(4) ?, V=3839(1) ?3, R1=0.037) consists of a framework of corner-sharing UrO5 pentagonal bipyramids, UrO4(H2O) pentagonal bipyramids and MoO4 tetrahedra. These structures are compared to those of other uranyl molybdates. The structural diversity and variability of uranyl molybdates is related to the high degree of flexibility of the U-O-Mo linkages.