Solution x-ray absorption fine structure study of the Eu²⁺ and Sr²⁺ ions: Unexpected solvent and metal ion dependencies of the solvation numbers

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 < H₂O ≪ DMF < DMSO solvent series: for strontium, 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.