CHARGE-DENSITY AROUND A JAHN-TELLER-DISTORTED SITE - (ND4)2CU(SO4)2.6D2O AT 85-K

Diammonium hexaaquacopper(II) disulfate-d20, [ND4]2[Cu(D2O)6](SO4)2, M(r) = 419.7, monoclinic, P2(1)/a, a = 9.399 (2), b = 12.673 (2), c = 6.071 (1) angstrom, beta = 107.13 (1)-degrees, V = 691.1 (4) angstrom 3, Z = 2, D(x) = 2.02 Mg m-3, Mo K-alpha radiation, lambda = 0.71069 angstrom, mu = 2.008 mm-1, F(000) = 415.3, T = 85 (2) K, R(I) = 0.020, R(F) = 0.014 for 7287 reflections. The CuO6 octahedron has a large Jahn-Teller distortion; Cu-O(8) 2.301 (1), Cu-O(7) 2.010 (1), Cu-O(9) 1.960 (1) angstrom. It was necessary to use quartic anharmonic thermal parameters at the Cu(II) site. These modelled potential softening associated with the Jahn-Teller distortions, important even at this temperature. At the ammonium, the sulfate and the hexaaqua-ion sites the charge densities, at this experimental accuracy, can be described by simple valence functions which reflect only local symmetry. Large lower-symmetry densities are not observed; any small such effects are comparable with uncertainties in the treatment of the thermal motion. The valence refinement gave Cu 3d populations of 3d(xy)1.58 (6)3d(xz,yz)2.12 (3)3d(z2)2.18 (6)3d(x2-y2)0.82 (7). This shows the hole expected in 3d(x2-y2), modified, taking overlap into account, by a covalent sigma-acceptance of 0.52 (1) and pi back donation of 0.18 (10) e. The rhombic distortion causes 3d(z2)/3d(x2-y2) mixing corresponding to a 3d(z2) coefficient of -0.21 (7) in a spin-hole wavefunction. Ionic charges from the model are Cu(OD2)6(1.7+), ND4(0.6+) and SO4(1.5-). These are more ionic than for the isomorphous Cr(II) salt, where sigma and pi charge flows are both to the metal, not opposed as here. Qualitatively these results agree with the conclusions from ESR, NMR and optical spectra.