Structural Dynamics in (ND₄)₂[Cu(D₂O)₆](SO₄)₂

The extended X-ray absorption fine structure spectroscopy (EXAFS) of (ND₄)₂[Cu(D₂O)₆](SO₄)₂ at 5, 14, 100, 200, and 298 K is reported. This indicates that the Cu−O bond lengths of the Cu(D₂O)₆²⁺ ion do not change significantly within this temperature range, which contrasts with EPR results and X-ray and neutron diffraction experiments, which imply that two of the Cu−(D₂O) bonds converge in length as the temperature is raised. The EXAFS measurements thus confirm that the bond distances yielded by the diffraction experiments refer to the average positions of ligands involved in a dynamic equilibrium in which the directions of the long and intermediate bonds of the Jahn−Teller distorted Cu(D₂O)₆²⁺ ion are interchanged in the crystal lattice. Analysis of the displacement parameters is consistent with this interpretation, as are the wave functions calculated using a model involving Jahn−Teller vibronic coupling and the influence of lattice strain interactions.