Charge density in (NH4)2Cr(SO4)2.6H2O at 84 K: a Jahn–Teller distorted complex

Abstract

Diammonium hexaaquachromium(II) disulfate, [NH412[Cr(H20)6](SO4)2, Mr = 38829. At 84(2)K, the structure is monoclinic, P2~/a, a = 9.490 (2), b 12.816 (3), c = 6.104 (1) A, /3 = 107.09 (2) °, V = 710 (1) A 3, Z = 2, Dx = 182 Mg m -3, a(Mo Ka) = 071069 ]1, /x = 1196 mm -~, F(000) = 405.2, R(/) = 0-022, R(F)= 0-030 for 7490 reflections. The CrO6 octahedron has a large Jahn-Teller distortion with a tetragonal elongation of Cr--O(8) 2.389(1)]t; Cr--O(7) 2.079 (1), Cr--O(9) 2.053 (1) ]1. At 295 (2)K, the structure is monoclinic, P2/a, a = 9.424 (2), b = 12.699 (3), c = 6.204 (1) ]1, /3 = 106-60 (1) °, V= 712 (1) ]13, Z= 2, Dm = 1.80 (1), Dx = 1.81 Mg m -3, A(Mo K~r) = 0.71069 ]1, /1, = l'199mm -I, F(000)=405.2, R(/) = 0.020, R(F)= 0.024 for 4339 reflections. The frO6 octahedron has a slightly smaller Jahn-Teller distortion with tetragonal elongation Cr---O(8) 2.323(2)]1; Cr---O(7) 2.125 (2), Cr--O(9) 2-054 (1) ]1. The CrO6 geometry has an unusual temperature dependence. The valence-orbital populations from the refinement give, in the Cr(OH2)6 unit, Cr-atom populations of 3dl.26 (3) qd 2.04 (4) 2,,/0.88 (4)2,40-28 (3)A,~- 0.32 (16)4pO-39 (15) xy ~ XZ, yZ Jlzt 22 3t~ x2--y2 l'~-I x, y which show the substantial differences expected on the grounds of the large Jahn-Teller effect present. The two short Cr--O bonds show much greater covalent charge transfer from the O lone pairs to the Cr atom than does the longer bond. For the shorter bonds distinct accummulation of charge in the midbond region is observed. The populations within the ammonium and sulfate ions present the expected features and preserve ideal tetrahedral symmetry accurately, and the charge distributions within them are unremarkable. The ionic charges depart from the ideal values, apparently owing to charge transfers associated with the strong hydrogen bonding in the crystal. They are, approximately, [Cr(OH2)6] z2+, [SO4] ~5- and [NH4] °'9+, with some variation according to the method of analysis, as expected.