The hydration of the scandium(iii) ion in aqueous solution and crystalline hydrates studied by XAFSspectroscopy, large-angle X-ray scattering and crystallography

Abstract

The structures of the hydrated scandium(III) ion and of the hydrated dimeric hydrolysis complex, [Sc₂(µ-OH)₂]⁴⁺, in acidic aqueous solutions have been characterized by X-ray absorption fine structure (XAFS) and large-angle X-ray scattering (LAXS) methods. Comparisons with crystalline reference compounds containing hydrated scandium(III) ions in well characterized six-, seven- and eight-coordinated polyhedra have been used to evaluate the coordination numbers and configurations in aqueous solution. In strongly acidic aqueous solution the structure of the hydrated scandium(III) ion is found to be similar to that of the eight-coordinated scandium(III) ion with distorted bicapped trigonal prismatic coordinating geometry in the crystalline [Sc(H₂O)_8.0](CF₃SO₃)₃ compound. The EXAFS data reveal for the solution, as for the solid, a mean Sc–O bond distance of 2.17(1) Å to six strongly bound prism water molecules, 2.32(4) Å to one capping position, with possibly another capping position at about 2.5 Å. The LAXS study supports this structural model and shows furthermore a second hydration sphere with ∼12 water molecules at a mean Sc⋯O_II distance of 4.27(3) Å. In less acidic concentrated scandium(III) aqueous solutions, the dimeric hydrolysis product, [Sc₂(µ-OH)₂(H₂O)₁₀]⁴⁺, is the predominating species with seven-coordinated scandium(III) ions in a double hydroxo bridge and five terminal water molecules at a mean Sc–O bond distance of 2.145 Å. Hexahydrated scandium(III) ions are found in the crystal structure of the double salt [Sc(H₂O)₆][Sc(CH₃SO₃)₆], which crystallizes in the trigonal space group R with Z = 6 and the unit cell dimensions a = 14.019(2) and c = 25.3805(5) Å. The Sc–O distances in the two crystallographically unique, but nearly identical, [Sc(H₂O)₆]³⁺ entities (both with imposed crystallographic symmetry) are 2.085(6) and 2.086(5) Å, while the mean Sc–O distance in the near octahedral [Sc(OSO₂CH₃)₆]³⁻ entities (with three-fold symmetry) is 2.078 Å.