The crystal structures of (ND4)2[Cu(D2O)6](SO4)2 at 295 K, determined by X-ray diffraction at pressures of 1 bar, ∼1.5 kbar and ∼3.0 kbar, are reported. Between 1 bar and 1.5 kbar, the crystal structure changes to one almost identical to that of the corresponding hydrogenous compound at 1 bar. The structural change involves a 90° switch in the direction of the long bonds of the Cu(D2O)6 2+ ion, accompanied by a change in the hydrogen-bonding interactions of the ammonium cations. Comparison of these two structures with those at ∼15 K shows that at room temperature, for both the high (H) and low (L) pressure phases, the Cu complexes are in thermal equilibrium with the other structural isomer. Though one of these is energetically preferred in the H and the other in the L phase, the variation of the powder EPR spectrum with pressure indicates a continuous transition between both within ∼1 kbar and suggests that at ∼0.5 kbar the two orientations of the copper complex are similar in energy. However, the underlying thermally induced equilibrium is little affected by pressure, and this is also the case for the Cu(H2O)6 2+ ion in Cu2+-doped K2[Zn(H2O)6](SO4)2. The crystal structure of K2[Cu(H2O)6](SO4)2 at 15 K and 1.4 kbar, determined by time-of-flight neutron diffraction, is similar to that at 295 K and 1 bar, except for indications that here also a thermal population of the higher energy form in which the directions of the long and intermediate Cu−O bonds interchange occurs at 295 K. The powder EPR spectrum of the potassium salt at 295 K shows little change between 1 bar and 11 kbar.

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