Brendebach B, Altmaier M, Rothe J, Neck V, Denecke M A
Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung, Karlsruhe, Germany.
Inorg Chem. 2007 Aug 6;46(16):6804-10. doi: 10.1021/ic070318t. Epub 2007 Jul 17.
A hitherto unknown type of aqueous complex, ternary Ca-MIV-OH complexes (M = Zr and Th), causes unexpectedly high solubilities of zirconium(IV) and thorium(IV) hydrous oxides in alkaline CaCl2 solutions (pHc = 10-12, [CaCl2] > 0.05 mol.L(-1), and pHc = 11-12, [CaCl2] > 0.5 mol.L(-1), respectively). The dominant aqueous species are identified as Ca3[Zr(OH)6]4+ and Ca4[Th(OH)8]4+ and characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. The number of OH- ligands in the first coordination sphere detected by EXAFS, NO = 6 (6.6 +/- 1.2) for Zr and NO = 8 (8.6 +/- 1.2) for Th, are consistent with the observed slopes of 2 and 4 in the solubility curves log [M]tot vs pHc. The presence of polynuclear hydrolysis species and the formation of chloride complexes can be excluded. EXAFS spectra clearly show a second coordination shell of calcium ions. The [Zr(OH)6]2- and [Th(OH)8]4- complexes with an unusually large number of OH- ligands are stabilized by the formation of associates or ion pairs with Ca2+ ions. The number of neighboring Ca2+ ions around the [Zr(OH)6]2- and [Th(OH)8]4- units is determined to be NCa = 3 (2.7 +/- 0.6) at a distance of RZr-Ca = 3.38 +/- 0.02 A and NCa = 4 (3.8 +/- 0.5) at a distance of RTh-Ca = 3.98 +/- 0.02 A. The Ca3[Zr(OH)6]4+ and Ca4[Th(OH)8]4+ complexes have first (M-O) and second (M-Ca) coordination spheres with the Ca2+ ions bound to coordination polyhedra edges.
一种迄今未知的水相络合物，即三元Ca-MIV-OH络合物（M = Zr和Th），导致锆(IV)和钍(IV)水合氧化物在碱性CaCl2溶液（分别为pHc = 10 - 12，[CaCl2] > 0.05 mol·L(-1)，以及pHc = 11 - 12，[CaCl2] > 0.5 mol·L(-1)）中具有出乎意料的高溶解度。主要的水相物种被鉴定为Ca3[Zr(OH)6]4+和Ca4[Th(OH)8]4+，并通过扩展X射线吸收精细结构（EXAFS）光谱进行了表征。通过EXAFS检测到的第一配位层中OH-配体的数量，Zr为NO = 6 (6.6 ± 1.2)，Th为NO = 8 (8.6 ± 1.2)，这与溶解度曲线log [M]tot对pHc中观察到的斜率2和4一致。可以排除多核水解物种的存在以及氯络合物的形成。EXAFS光谱清楚地显示了钙离子的第二配位层。具有异常大量OH-配体的[Zr(OH)6]2-和[Th(OH)8]4-络合物通过与Ca2+离子形成缔合物或离子对而得以稳定。[Zr(OH)6]2-和[Th(OH)8]4-单元周围相邻Ca2+离子的数量在距离RZr-Ca = 3.38 ± 0.02 Å时确定为NCa = 3 (2.7 ± 0.6)，在距离RTh-Ca = 3.98 ± 0.02 Å时为NCa = 4 (3.8 ± 0.5)。Ca3[Zr(OH)6]4+和Ca4[Th(OH)8]4+络合物具有第一（M-O）和第二（M-Ca）配位层，其中Ca2+离子与配位多面体的边缘相连。
