Pair distribution function and (71)Ga NMR study of aqueous Ga(3+) complexes

The atomic structures, and thereby the coordination chemistry, of metal ions in aqueous solution represent a cornerstone of chemistry, since they provide first steps in rationalizing generally observed chemical information. However, accurate structural information about metal ion solution species is often surprisingly scarce. Here, the atomic structures of Ga(3+) ion complexes were determined directly in aqueous solutions across a wide range of pH, counter anions and concentrations by X-ray pair distribution function analysis and (71)Ga NMR. At low pH (<2) octahedrally coordinated gallium dominates as either monomers with a high degree of solvent ordering or as Ga-dimers. At slightly higher pH (pH ≈ 2–3) a polyoxogallate structure is identified as either Ga(30) or Ga(32) in contradiction with the previously proposed Ga(13) Keggin structures. At neutral and slightly higher pH nanosized GaOOH particles form, whereas for pH > 12 tetrahedrally coordinated gallium ions surrounded by ordered solvent are observed. The effects of varying either the concentration or counter anion were minimal. The present study provides the first comprehensive structural exploration of the aqueous chemistry of Ga(3+) ions with atomic resolution, which is relevant for both semiconductor fabrication and medical applications.