Tricarbonate complex of hexavalent Am with guanidinium: synthesis and structural characterization of [C(NH2)3]4[AmO2(CO3)3]•2H2O, comparison with [C(NH2)3]4[AnO2(CO3)3] (An = U, Np, Pu)

A formation of Am(VI) carbonate complexes has been observed under ozonation of Am(III) in guanidinium carbonate solutions. New [C(NH2)(3)](4)[AmO2(CO3)(3)]center dot 2H(2)O tricarbonate complex has been isolated and studied by the X-ray analysis. The Am atom has oxygen environment as distorted hexagonal bipyramid with three CO32- ions in equatorial plane and oxygen atoms of the AmO2 group in the apical positions. Averaged Am-O-CO3 bond length is 2.431(13) angstrom, the length of Am=O bond is 1.750(12) angstrom. In crystal hydrogen bonds between [C(NH2)(3)](+) cations, water molecules and [AmO2(CO3)(3)](4-) complex anions link the fragments of structure into the three-dimensional network. To compare the peculiarities of the structure of tricarbonate complexes it was prepared [C(NH2)(3)](4)[AnO(2)(CO3)(3)], where An= U(VI), Np(VI) and Pu(VI). In structure of [C(NH2)(3)](4)[AnO(2)(CO3)(3)] an actinide contraction appears as a slight shortening of the bond lengths in the actinyl groups AnO(2): 1.80 angstrom for U, 1.78 angstrom for Np 1.76 angstrom Pu. The An-OCO3 bond length in the equatorial plane of hexagonal bipyramids remains almost unchanged in the series from U to Am. An analysis of the structures showed that hydrogen bonding played an important role in crystal formation.