Spectroscopic studies of gallium complexes in solution

Abstract

High-field 71Ga NMR spectra at 122.03 MHz of aqueous solutions of Ga 3+ in the presence of oxyanions reveal sulphate, phosphate or citrate complexes. In halogen acid solutions the [Ga(H 2O) 6] 3+ signal is accompanied by that of a mixture of [GaX n (H 2O) 4- n ] 3- n (X = Cl or Br; n = 1–4) from which a single NMR peak is observed. Aqueous solutions of [GaCl 4] − and [GaBr 4] − in combination yield one peak due to mixed halides undergoing ligand exchange. The diethyl ether extract of this solution displays the five signals of the complex acids H +(aq)[GaCl n Br 4− n ] − ( n = 0–4). The Raman spectrum of this system provides a set of v(GaCl) sym and v(GaBr) sym frequencies. The 17O chemical shift of the [H 9O 4] + ion occurs at 15 ppm. Attempts to investigate the halogen exchange process of gallium mixed-halide species by a two-dimensional 71Ga NMR technique are described. 71Ga NMR spectra of mixed-ligand halide/thiocyanate complexes are attributable to species [GaX n (NCS) 4- n ] − (X = Cl or Br; n = 0–4). Chemical shift trends show the gallium(III) shielding order, Cl < NCS < Br < I.