Epsilon Open Archive

Abstract

Structures of hydrated, solvated and hydrolyzed multicharged metal ions are determined by EXAFS and LAXS in solution, and by X-ray crystallography and EXAFS in the solid state.

During hydrolysis, the nine-coordinate hydrated thorium(IV) ion is first transformed to a dimer, [Th₂(μ²-OH)₂(H₂O)₁₂]⁶⁺, then to a tetramer, [Th₄(μ²-OH)₈(H₂O)₁₆]⁸⁺, and finally to a hexamer, [Th₆(μ³-O₈)(H₂O)n]⁸⁺ with increasing pH and thorium(IV) concentration in an aqueous solution. Coordinated water looses the protons in two steps, first with the formation of the dimer and tetramer, and finally with the formation of the hexamer.

Together thorium(IV) and iron(III) form a stable and highly soluble heteronuclear hydrolysis complex, [Th₂Fe₂(μ²-OH)₈(H₂O)₁₂]⁶⁺, in the pH range 2.9 - 4.8 and metal concentrations 0.02 – 0.4 mol·dm⁻³. In the same solutions at pH < 2.9 two- or six-line ferrihydrite precipitates with time, while thorium(IV) remains in solution.

Palladium(II) and platinum(II) hydrolyze very slowly in acidic aqueous solution and after 25 years of storage tiny oxide-like particles are formed at pH = 0.7. The particle size has been estimated from the number of M···M distances in the formed particles and in the corresponding solid oxides from EXAFS studies. The palladium(II) and platinum(II) oxide particles grow along a or b axies and reach a size of 1 - 1.5 nm³. These results have been supported by SAXS studies.

During hydrolysis, the six-coordinate hydrated chromium(III) ion is first transformed to a dimer, then to a tetramer, [Cr₄(μ²-OH)₂(μ-OH)₄(OH)n(H₂O)₁₂-n](6-n)+ with increasing pH from 0 to 3.7 at chromium concentration of ca. 1 mol·dm⁻³. At pH  15, soluble [Cr(μ²-OH)₂(OH)₂]nn-, n  3, complex is formed, which at pH < 15 slowly precipitates as an amorphous solid with a structure similar to α-CrOOH.

Dimethylsulfoxide (DMSO) solvated thorium(IV) is nine-coordinate both in solution and solid state. N,N’-Dimethylpropyleneurea (DMPU) solvated thorium(IV) is eight-coordinate in solution. The lower coordination number is due to the space-demanding properties of the DMPU molecules upon coordination.