XAFS ANALYSIS OF JAHN-TELLER DISTORTIONS IN TRIS(1,2-DIAMINOETHANE)COPPER(II) SULFATE CU(EN)(3)SO4

XAFS analysis Jahn-Teller distortions in tris(1,2-diaminoethane)copper (II) sulfate, Cu(en)(3)SO4 (en = 1,2-diaminoethane) shows that Cu(II) is elongated octahedral at loom and at low temperature: the XANES spec tra are similar and the EXAFS signals are fitted with four short and t wo 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 incomp atible with the EXAFS simulations using FEFF6 code, with the diffracti on coordinates. On the contrary, a good agreement is obtained for the isostructural Ni(en)(3)SO4, where Ni is not subject to Jahn-Teller eff ect. Similar results are obtained with the hexanitrocuprate(II) K2PbCu (NO2)(6) and the hexanitronickelate(II) K2PbNi(NO2)(6). The results ar e in agreement with a dynamic Jahn-Teller effect in elongated octahedr a [Cu(en)(3)](2+) and [CU(NO2)(6)](4-) at high temperature and with a static Jahn-Teller effect combined with an antiferrodistortive order a t low temperatur.