CRYSTAL CHEMISTRY OF URANYL MOLYBDATES. III. NEW STRUCTURAL THEMES IN Na6[(UO2)2O(MoO4)4], Na6[(UO2)(MoO4)4] AND K6[(UO2)2O(MoO4)4]

Two Na uranyl molybdates, Na 6 [(UO 2 ) 2 O(MoO 4 ) 4 ] and Na 6 [(UO 2 )(MoO 4 ) 4 ], and one K uranyl molybdate, K 6 [(UO 2 ) 2 O(MoO 4 ) 4 ], have been synthesized by high-temperature solid-state reactions of mixtures of Na 2 CO 3 , K 2 CO 3 , UO 3 and MoO 3 . All three structures were solved by direct methods and refined on the basis of F 2 for all unique data collected with monochromatic Mo K α X-radiation and a CCD (charge-coupled device) detector. The structure of Na 6 [(UO 2 ) 2 O(MoO 4 ) 4 ] was efined to an agreement factor ( R 1) of 4.1%, calculated using the 1969 unique observed reflections ( F o ≥ 4σ F ), and a goodness-of-fit ( S ) of 1.06. It is triclinic, space group P 1, a 7.637(2), b 8.164(2), c 8.746(2) A, α 72.329(5), β 79.364(5), γ 65.795(4)°, V 472.7(2) A 3 , Z = 1. The structure of Na 6 [(UO 2 )(MoO 4 ) 4 ] was refined to an R 1 of 3.3%, calculated using the 3268 unique observed reflections ( F o ≥ 4σ F ), and a S of 0.97. It is triclinic, space group P 1, a 7.0958(8), b 9.566(1), c 13.415(2) A, α 73.692(2), β 86.621(2), γ 82.940(2)°, V 867.0(2) A 3 , Z = 2. The structure of K 6 [(UO 2 ) 2 O(MoO 4 ) 4 ] was refined to an R 1 index of 4.9%, calculated using the 1833 unique observed reflections ( F o ≥ 4σ F ), and a S of 0.90. It is triclinic, space group P 1, a 7.8282(8), b 7.8298(8), c 10.302(1) A, α 83.893(2), β 73.131(2), γ 80.338(2)°, V 594.6(1) A 3 , Z = 1. The structural connectivity of these three compounds is notable in that each exhibits themes not previously observed in uranyl compounds. The structures of both Na 6 [(UO 2 ) 2 O(MoO 4 ) 4 ] and K 6 [(UO 2 ) 2 O(MoO 4 ) 4 ] contain chains of polyhedra of composition [(UO 2 ) 2 O(MoO 4 ) 4 ] 6− . These chains have not been previously observed and are composed of [ Ur O 4 O Ur O 4 ] dimers consisting of two Ur O 5 ( Ur : uranyl ion) pentagonal bipyramids sharing a common O atom. The dimers are connected into a chain by sharing vertices with MoO 4 tetrahedra, with the chains linked by Na + or K + cations. The structure of Na 6 [(UO 2 )(MoO 4 ) 4 ] consists of finite clusters of composition [(UO 2 ) 2 (MoO 4 ) 8 ] 12− composed of two Ur O 5 pentagonal bipyramids connected by sharing vertices with MoO 4 tetrahedra. The clusters in Na 6 [(UO 2 )(MoO 4 ) 4 ] are oriented parallel to the (001) plane and are linked through Na + cations.