Variation of the An-O bond lengths in NaAnO2(OOCCH3)3 and (NH4)4AnO2(CO3)3, An = U(VI), Np(VI), and Pu(VI)

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For studying actinide contraction in triacetate and tricarbonate complexes of hexavalent actinides in the series U-Np-Pu, single crystals were prepared and crystal structures of NaPuO2(OOCCH3)3 and (NH4)4. AnO2(CO3)3 (An = Np, Pu) were determined. These compounds are isostructural with NaAnO2(OOCCH3)3 (An = U, Np) and (NH4)4UO2(CO3)3 studied previously. The coordination surrounding of An(VI) is a distorted hexagonal bipyramid. The actinide contraction in An(VI) triacetate and tricarbonate complexes is manifested in shortening of the bond lengths in the AnO 2 2+ cation in the series U-Np-Pu. In triacetate complexes NaAnO2(OOCCH3)3, the average bond lengths in AnO 2 2+ decrease by approximately 2.3% in passing from Np(VI) to Pu(VI). In tricarbonate complexes (NH4)4AnO2(CO3)3, the bond lengths in AnO 2 2+ decrease by approximately 0.7% in passing from U to Np and by approximately 0.4% in passing from Np to Pu. The actinide contraction is less noticeable in the equatorial planes of the bipyramids. The lengths of the An-O bonds with oxygen atoms located around actinyl groups depend on the ligand nature and also on participation of oxygen atoms in the coordination with Na+ cations in the triacetate complexes or in hydrogen bonds with NH 4 + ions in the tricarbonate complexes.