Charge-density study of deuterated ammonium ferrous Tutton salt at 85 K and comparison with CrII and CuII salts

Diammonium hexaaquairon(II) disulfate (ND4)2- [Fe(D2O)6](SO4)2, Mr = 412.16. At 85(2) K, the structure is monoclinic. The valence-orbital population analysis gives, for Fe, the near crystal-field 3d configuration dxz1.30(2) dyz1.43(2) dxy1.40(2) dx2-y21.22(2) dz20.98(2). The near equality of the t2g and eg populations respectively contrasts, as expected, with the results from the Jahn-Teller-distorted CrII and CuII cases. The M 3d populations follow the expectations of crystal-field and/or local-density functional calculations moderately well. When the results for all three metals are compared the charges deduced for the M, D2O, S, So4 and ND4 units are found to be reduced by about 35% from the formal values. The charges and populations in sulfate and hexaaquametal(II) ions differ in the CuII case from those of FeII and CrII. The differences indicate greater charge transfer from sulfate to hexaaquametal(II) ion via hydrogen bonding for CrII and FeII. This transfer is driven by the need to partly fill empty metal 3d t2g orbitals in CrII and FeII, orbitals which are full in CuII. This comparison strengthens belief in the reliability of valence features in transition-metal complexes deduced from X-ray data. (orig./GSCH)