EXAFS DETERMINATION OF THE COPPER OXALATE STRUCTURE - RELATION BETWEEN STRUCTURE AND MAGNETIC-PROPERTIES

The structure of copper oxalate(CuIIC2O4-1/3H2O) is studied by extended X-ray absorption fine structure spectroscopy. Two structures were previously proposed: one similar to the iron oxalate chain (FeC2O4-2H2O) or one close to the copper acetate coordination site. The main difference between these two models is the value of the first metal-metal distance: greater than 5 Å in the first case and less than 3 Å in the second case. The comparative study of X-ray absorption spectra of CUC2O41/3H2O and of compounds of known structure, either with a long or with a short first metal-metal distance, indicates clearly that the second shell surrounding a Cu(II) ion is exclusively composed of light atoms. For the first shell, the four Cu-O distances are within 1.98 ± 0.02 A. These results are consistent with the first model and do not agree with the second one. The knowledge of this structure is essential to include CuIIC2O4-1/3H2O in the discussion of the magnetic exchange in Cu(II) binuclear and chain compounds with oxalate ions as bridging ligands. The published values of J for such compounds indicate that (i) an important antiferromagnetic coupling does not involve necessarily a short metal-metal distance (for instance, J = -272 cm-1 for CuC2O4-1/3H2O) and (ii) the intensity of this exchange is very sensitive to the symmetry of the Cu(II) site (J = 0 for Cu2C2O4(dien)2(C104)2). These facts are explained by the preponderance of the σ-type dx2-y2 pathway for the magnetic exchange. © 1979, American Chemical Society. All rights reserved.