Specific Features of the Crystal Chemistry of Layered Uranyl Compounds with the Ratio UO2: TO4 = 5 : 8 (T = S6+, Cr6+, Se6+, Mo6+)

Complex Uranyl Dichromates Templated by Aza-Crowns

“…The presence of polychromate groups in the structures of three novel reported compounds can be explained, on the one hand, by the relatively high pH and Cr 6+ concentration in the mother liquids [16]. On the other hand, formation of novel moieties is likely to exemplify the particular flexibility and adaptability of the uranyl chromate systems towards the size and shape of the template, as noted in the Introduction.…”

“…It is very likely that the presence of relatively large organic molecules of aza-crowns does not allow uranyl chromate chain complexes to condense into the units of higher dimensionality (layers or frameworks). It may be speculated that the [(UO 2 ) 2 (CrO 4 ) 2 (Cr 2 O 7 ) 2 (H 2 O)] 4− and [(UO 2 ) 3 (CrO 4 ) 4 (Cr 2 O 7 ) 2 ] 6− chains represent the transitional structures between one-and two-dimensional structural topologies very typical for uranyl oxysalts [16]. We also note that 1-3 were formed in the case of relatively nitrogen-rich templates (tetra-aza compounds) where the mutual positioning of the hydrogen donor centers (protonated nitrogen atoms) is more or less fixed.…”

“…Uranyl molybdates tend to form framework structures [5] due to the flexibility and broad variation of U-O-Mo bond angles. Uranyl chromates constitute a particularly versatile class among the compounds considered as they exhibit all the features listed above: both mono- [6][7][8][9][10][11][12][13][14][15][16] and bidentate [17][18][19] linkage modes of CrO 4 tetrahedra to UO 7 uranyl bipyramids, formation of corner-sharing tetrahedral chromate polyanions [19][20][21], flexibility and variation of U-O-Cr angles similar to those in uranyl molybdates and formation of framework structures [22], and variability of coordination environments [23][24][25]. The tendency of the CrO 4 2− to form isopolyanions in acidic media leads to formation of structures involving simultaneously several polychromate moieties such as Cr 2 O 7 2− dimers, Cr 3 O 10 2− trimers and Cr 4 O 13 2− tetramers [26].…”

Organically Templated Layered Uranyl Molybdate [C3H9NH+]4[(UO2)3(MoO4)5] Structurally Based on Mineral-Related Modular Units

“…The layers in the structure of [Co(H 2 O) 6 ] 3 (UO 2 ) 5 (SO 4 ) 8 (H 2 O)•H 2 O can also be viewed as a modular arrangement formed by the assembly of chains [39]. A modular approach was also applied to the description of some uranyl chromates, sulfates and selenates [40,41]. It was shown [41] that layers with the ratio UO 2 :TO 4 = 2:3 can be obtained by the association of two types of fundamental chains.…”

Secondary Role of Aliphatic and Heterocyclic Amines in the Formation of Low-Temperature Amine-Bearing U, Np, and Pu(VI) Chromates

“…In the structure of Li 2 (H 2 O) 5 [(UO 2 )­(CrO 4 ) 2 ], the [Li 2 (H 2 O) 5 ] 2+ unit acts as a guest and is located between the [(UO 2 )­(CrO 4 ) 2 ] 2– layers . The crystal chemistry and topological isomerism of uranyl complexes with XO 4 2– anions are discussed in works refs − .…”