A Model for Trivalent Actinides in Media Containing High Carbonate Concentrations − Structural Characterization of the Lanthanide Tetracarbonate [Co(NH3)6][Na(μ-H2O)(H2O)4]2[Ho(CO3)4]·4H2O

Abstract

The identity of the limiting HoIII species in aqueous solutions with high carbonate concentrations has been determined to be Ho(CO3)45−. Single crystals of [Co(NH3)6][Na(μ−H2O)(H2O)4]2[Ho(CO3)4]·4H2O were obtained by the addition of [Co(NH3)6]3+ to an aqueous 0.04 M solution of HoIII in 2.1 M Na2CO3. The asymmetric unit contains the anion, [Ho(CO3)4]5−, a [Co(NH3)6]3+ cation and two Na+ cations, which are bound to H2O molecules in an edge‐sharing bioctahedral geometry. The [Ho(CO3)4]5− anion is eight coordinate with four bidentate carbonate ligands bound to the Ho atom. The molecule has essentially C2v symmetry with two coplanar carbonates making a vane, which is perpendicular to a similar vane produced by the other two carbonate ligands. An alternative way to this view molecule is through the geometry of the C atoms, which are found in a distorted tetrahedron. The average Ho−O distance was determined to be 2.361(5) Å, while the average Ho−C distance was 2.784(6) Å. The IR and Raman spectra were determined in both the solid state and solution in order to confirm the solution speciation. The Raman data show a single CO32− stretch for the solid at 1062 cm−1. The solution data show multiple peaks with the most prominent being at 1048 cm−1, which is consistent with the literature reports of an equilibrium mixture. The IR data for the solids confirm the X‐ray results showing bidentate carbonate ligands by the splitting of the ν3 band of the CO32−. The crystal data for [Co(NH3)6][Na(μ‐H2O)(H2O)4]2[Ho(CO3)4]·4H2O are as follows: monoclinic space group P2/n, a = 8.7091(5) Å, b = 10.8744(6) Å, c = 15.7971(9) Å, β = 93.117(1)°, Z = 2, R1 = 0.0307, wR2 = 0.0756.