Figure 2.

Model structures of the aqueous species (a) UO₂(CO₃)₃^4-•28(H₂O), (b) UO₂(CO₃)₃^5-•28(H₂O) and (c) Ca₂UO₂(CO₃)₃•28(H₂O) in-plane configuration. (a) Strong H-bonding to the carbonate groups weakens the U-carbonate bonding by approximately 0.1 Å to bring the calculated value in better agreement with observation (Table 1; [42]). H-bonds to the O atoms of the UO₂²⁺ group are relatively weak. Reduction of the U atom to U(V) causes a slight twisting of the carbonate ligands as proposed by Docrat et al. [43]. H-bonding to the uranyl O atoms becomes relatively stronger compared to the analogous U(VI) complex. (c) Addition of Ca²⁺ ions to charge-balance this model aqueous species results in a configuration close to the observed crystal structure of the mineral liebigite [45].