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, P2₁/a, a = 9.399 (2), b = 12.673 (2), c = 6.071 (1) Å, [beta]= 107.13 (1)°, V= 691.1 (4) Å³, Z = 2, D_x= 2.02 Mg m^-3, Mo K[alpha] radiation, [lambda] = 0.71069 Å, [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) Å. 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^{1.58 (6)}_xy3d^{2.12 (3)}_{xz, yz}3d^2.18(6)_z²3d^{0.82 (7)}_{x² - y²}. This shows the hole expected in 3d_x²y², modified, taking overlap into account, by a covalent or 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)¹⁷⁺₆, ND⁶⁺_° and SO^1.5-₄. 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.

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