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 indic ates that the Cu-O bond lengths of the Cu(D2O)(6)(2+) ion do not change sig nificantly 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 m easurements thus confirm that the bond distances yielded by the diffraction experiments refer to the average positions of ligands involved in a dynami c equilibrium in which the directions of the long and intermediate bonds of the Jahn-Teller distorted Cu(D2O)(6)(2+) ion are interchanged in the cryst al lattice. Analysis of the displacement parameters is consistent with this interpretation, as are the wave functions calculated using a model involvi ng Jahn-Teller vibronic coupling and the influence of lattice strain intera ctions.