Spectroscopic studies of aqueous gallium(III) and aluminum(III) citrate complexes

Aqueous gallium(III) citrate complexes have been studied in the 10−2 M concentration range with extended X-ray absorption fine structure (EXAFS) and FTIR techniques. From EXAFS data, one mononuclear and one oligomeric species were identified at different Ga(III) to citrate ratios. The first shell of the mononuclear complex was found to be distorted, with average Ga-O bond lengths of 1.95 and 2.06 Å, in agreement with the solid-state structure of Ga(Cit)23- (Cit = citrate). Also the oligomeric species was found to have a distorted first shell, with average Ga-O bond lengths of 1.95 and 2.04 Å. This complex was found to contain two Ga-Ga distances at 3.03 and 3.56 Å, typical for edge and corner sharing GaO6 octahedra, respectively. The gallium(III) and aluminum(III) citrate systems were compared by means of FTIR, and were found to be analogous. The IR results suggest that the bond lengths derived from EXAFS for the 1:2 gallium(III) citrate complex also provide a good estimate of the corresponding distances in the mononuclear 1:1 complex. Direct coordination of citrate to the metal ions in the oligomeric gallium(III) citrate complex was indicated from both EXAFS and IR results, and this complex is stoichiometrically analogous to the Al3(H−1Cit)3(OH)(H2O)4− complex, which has been structurally determined. However, while the formation of the aluminum trimer has been shown to be slow, the gallium trimer was significantly more labile with a rate of formation indicated to be in the order of seconds or faster.