Copper(II) oxydiacetate hemihydrate: crystal structure and twinning

Abstract

The structure of [Cu{O(CH₂CO₂)₂}],0·5H₂O has been determined by Fourier and least-squares methods from diffractometer data. Crystals, all of which are twinned by reflection across the (101) twin plane or by a 90° rotation about [010], are orthorhombic with a= 9·263(6), b= 14·283(5), c= 9·283(6)Å, Z= 8, space group Pbcn. Volume fractions of the twin components were deduced from the ratios of intensities observed at space-group forbidden positions. The structure of the major component, which represents 88% of the twin volume, was refined to R 0·88 for 1042 reflections.

The Cu atoms are co-ordinated by four carboxy-oxygen atoms, two from the two ends of a chelating oxydiacetate ligand and one from each of two neighbouring Cu{O(CH₂CO₂)} groups, with Cu–O 1·941(9)–1·952(8)Å. Longer contacts to the ether oxygen of the chelating ligand [Cu–O 2·488(8)Å] and to a statistically disordered water molecule [Cu–O 2·73(2)Å] complete a distorted octahedral arrangement. A pseudo-mirror plane passing through Cu, H₂O. and the ether O atom relates the halves of the oxydiacetate group and results in markedly similar packing schemes in the ab and bc unit-cell projections. Stacking faults in the packing along b account for the occurrence of the twinning.