XAFS Analysis of Jahn-Teller Distortions in Tris(1,2-Diaminoethane)Copper(II) Sulfate Cu(en)₃SO₄

Abstract
XAFS analysis Jahn-Teller distortions in tris(1,2-diaminoethane)copper(II) sulfate, Cu(en)₃SO₄ (en = 1,2- diaminoethane) shows that Cu(TI) is elongated octahedral at room and at low temperature: the XANES spectra are similar and the EXAFS signals are fitted with four short and two long Cu-N distances. This result is in apparent contradiction with X-ray diffraction studies: [Cu(en)₃]²⁺ appears regular octahedral at 300 K and compressed at 120 K. The observed EXAFS is clearly incompatible with the EXAFS simulations using FEFF6 code, with the diffraction coordinates. On the contrary, a good agreement is obtained for the isostructural Ni(en)₃SO₄, where Ni is not subject to Jahn-Teller effect. Similar results are obtained with the hexanitrocuprate(II) K₂PbCu(NO₂)₆ and the hexanitronickelate(II) K₂PbNi(NO₂)₆. The results are in agreement with a dynamic Jahn-Teller effect in elongated octahedra [Cu(en)₃]²⁺ and [Cu(NO₂)₆]^4- at high temperature and with a static Jahn-Teller effect combined with an antiferrodistortive order at low temperature.