Abstract

The structures of the hydrated scandium(III) ion and of the hydrated dimeric hydrolysis complex, [Sc-2(mu-OH)(2)](4+), in acidic aqueous solutions have been characterized by X-ray absorption. ne 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) angstrom to six strongly bound prism water molecules, 2.32( 4) angstrom to one capping position, with possibly another capping position at about 2.5 angstrom. The LAXS study supports this structural model and shows furthermore a second hydration sphere with similar to 12 water molecules at a mean Sc center dot center dot center dot O-II distance of 4.27( 3) angstrom. In less acidic concentrated scandium( III) aqueous solutions, the dimeric hydrolysis product, [Sc-2(mu-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 angstrom. 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 (R) over bar with Z = 6 and the unit cell dimensions a = 14.019( 2) and c = 25.3805( 5) angstrom. The Sc - O distances in the two crystallographically unique, but nearly identical, [Sc(H2O)(6)](3+) entities (both with (3) over bar imposed crystallographic symmetry) are 2.085( 6) and 2.086( 5) angstrom, while the mean Sc - O distance in the near octahedral [Sc(OSO2CH3)(6)](3-) entities (with three-fold symmetry) is 2.078 angstrom

Published in

Dalton Transactions
2006, number: 32, pages: 3868-3878
Publisher: ROYAL SOC CHEMISTRY

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