Crystal, molecular, and electronic structure of catena-µ-oxalato-ammine-copper(II)

Abstract

Crystals of formula Cu(NH₃)(C₂O₄), obtained from very dilute ammonia solutions of copper(II) oxalate, are orthorhombic, space-group Pbca, with a= 11·19(1), b= 9·43(1), c= 8·38(1)Å, Z= 8. The structure was refined by least-squares methods using 587 independent reflections, collected on a diffractometer, to a conventional R of 0·050. The structure is polymeric and co-ordination around copper is distorted square bipyramidal, with the ammonia molecule bonding in the more strict co-ordination plane. One oxygen of an oxalate group functions as a bridge between two atoms of copper, and this gives rise to strong antiferromagnetism, the magnetic moment at 302 K being 1·46 and at 94·5 K 0·53 BM. A superexchange mechanism operating through these oxygen atoms is invoked to explain this and a value for J of ca.–265 cm^–1 is obtained. E.s.r. spectra of powders confirm this behaviour, and do not show the resonance at 3·1 cm characteristic of binuclear species such as copper(II) acetate. No bands other than those expected for the copper(II) chromophore itself could be found in the reflectance and single-crystal optical spectra. These latter could be interpreted in terms of ca. D_4h symmetry, and it is found that the ²A_1gâ†�²B_1g transition lies at rather low energy, as expected from the relatively short axial copper(II)–oxygen bond lengths. Solution optical spectra indicate the possible existence of binuclear species.