Seven uranyl(VI) complexes, [UO2(acac)2(L)] [L = 4-methylpyridine (1), 4-ethylpyridine (2), 2,4-dimethylpyridine (3), (−)-nicotine (4), and imidazole (5)], [{UO2(acac)2}2(4,4′-bipyridine)] (6), and [(2,2′-bipyridine)2H][UO2(acac)(NO3)2] (7) have been synthesized and characterized crystallographically. The coordination geometry of U has a UNO6 pentagonal-bipyramidal coordination in 1–6, and a UO8 hexagonal-bipyramidal coordination in 7.
We now report the 237 Np Mö ssbauer spectra for [NpO 2 (acac) 2 py] I. The neptunium(VI) ion in I is coordinated by six oxygen atoms and the one nitrogen atom of pyridine. The isomer shift value obtained for I is j40 mm/s, which falls in the range for the Np(VI) oxidation state. The relationship between the isomer shift and Np-O bond distance of the neptunyl group for oxygen coordination compounds is in good agreement with the reported structure of I. It is well known that the mean Np-O bond distance of the neptunium compounds is shorter by 0.01 Å than that of analogous uranium compounds. The good agreement in the 237 Np-Mö ssbauer parameters and the Np-O distance has indicated that the reported analogous [UO 2 (acac) 2 py] structure may have some inaccuracies. Therefore, we have redetermined the crystal structure of [UO 2 (acac) 2 py]. The U-O bond distance we have obtained is reasonable.
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