Crystal- and Solution-Structure Characteristics of Ethylenediaminetetraacetatoaluminate(III) and Gallate(III)

The structures of complex anions, [Al(edta)]- and [Ga(edta)]- (H4edta = ethylenediaminetetraacetic acid), were investigated both in the solid state (X-ray crystallography) and in solution (1H and 13C NMR). Two complexes, (NH4)[Al(edta)]·2H2O (1) and (NH4)[Ga(edta)]·2H2O (2), are isostructural in the solid state. In both metal complexes, the central metal atoms are coordinated by six donor atoms (N2O4) in distorted octahedral coordination geometries in which two nitrogen atoms and two oxygen atoms of two acetate groups occupy the equatorial positions, and the axial positions are occupied by two oxygen atoms of two acetate groups. The 1H and 13C NMR spectra of 1 and 2 showed that acetate groups undergo rapid interchanges between the axial and equatorial positions in solution, its rate in D2O being much faster than that in dimethyl sulfoxide(dmso)-d6 for each complex, and its rate for 1 being slower than that for 2. Based on the NMR spectra, the structures of complex anions [Al(edta)]- and [Ga(edta)]- in solution are concluded to be the same as those in the solid state.