The structure of gallium in strongly alkaline, highly concentrated gallate solutions -: a Raman and 71Ga-NMR spectroscopic study

Highly concentrated alkaline gallate solutions with 0.23 <= [Ga(III)](T) <= 2.32 mol.dm(-3) and 1 <= [NaOH](T) <= 15 mol.dm(-3) have been prepared and investigated by Raman and (71)Ga-NMR spectroscopy. Both the Raman and (71)Ga-NMR spectra are consistent with the presence of only one Ga-bearing species in these solutions, the tetrahedral hydroxocomplex, Ga(OH)(4)(-). Contact ion pairs were found to cause variations in the Raman and 71Ga-NMR parameters that are at the edge of detectability. Other species that have been claimed to exist in the literature, like higher hydroxo complexes (i.e., Ga(OH)(6)(3-)) or the mu-oxo-bridged dimer (i.e., (OH)(3)Ga-O-(OH)(3)(2-)), were not detected by these spectroscopic techniques. If such solution species exist at all, their concentrations are below the detection limit of Raman and 71Ga-NMR spectroscopy. The behavior of gallium appears to be very similar to that of aluminium under identical conditions, except that the dimeric species detected in aluminate solutions is undetectable in analogous gallates.