Solution X-ray Absorption Fine Structure Study of the Eu²⁺ and Sr²⁺ Ions:  Unexpected Solvent and Metal Ion Dependencies of the Solvation Numbers

Abstract

Structural parameters of the Sr2+ and, for the first time, of the Eu2+ ions in nonaqueous solutions were determined by the X-ray absorption fine structure (XAFS) method and compared with those of the aqua ions. For both Eu2+ and Sr2+ ions, a decrease in coordination number (N) and metal-to-solvent distances was found along the increasingly solvating MeCN 2O ≪ DMF N = 8 with [Sr(MeCN)8]2+ (2.665(6) Å) and [Sr(H2O)8]2+ (2.600(3) Å) and N = 7 with [Sr(DMF)7]2+ (2.555(5) Å) and [Sr(DMSO)7]2+ (2.540(7) Å); for europium(II), N = 8 with [Eu(MeCN)8]2+ (2.640(4) Å), N = 7 with [Eu(H2O)7]2+ (2.584(5) Å), and N = 6 with [Eu(DMF)6]2+ (2.541(3) Å) and by extrapolation [Eu(DMSO)6]2+ (2.525 Å). Smaller coordination numbers are observed for the Eu2+ ion in O-coordinating solvents. The ionic radii of both Sr2+ and Eu2+ ions are very similar, but the slightly softer character of the Eu2+ ion leads to shorter M−N and longer M−O bonds