Chemical characterization of the gallium(III) and indium(III) thiocyanate systems in aqueous and dimethylsulfoxide solution

The decomposition of the thiocyanate ion in acidic aqueous solution disturbs the complex formation reactions with gallium(III) and indium(III) to such an extent that any accurate stability constants cannot be obtained. Calorimetric studies have shown that several reactions besides the complex formation take place at the addition of thiocyanate to aqueous solutions of gallium(III) and indium(III). The origin of these reactions has not been possible to establish, but they seem to be metal ion dependent. An EXAFS study on an aqueous gallium(III) thiocyanate solution with excess thiocyanate showed the presence of hydrated gallium(III) ions and a small fraction of gallium(III) hydrolysis products at pH=2. This shows that the complex formation between gallium(III) and thiocyanate is very weak in aqueous solution if it takes place at all. The thiocyanate ion is stable in aprotic solvents as dimethylsulfoxide, and complex formation studies of gallium(III) and indium(III) with thiocyanate can be performed in a normal way. One weak mononuclear complex is formed in the gallium(III)- and indium(III)-thiocyanate systems in dimethylsulfoxide under prevailing conditions with K-1 and Delta H-1, values of 30 +/- 5 dm(3) mol(-1) and 9 +/- 2 kJ mol(-1), and 105 +/- 9 dm(3) mol(-1) and 1.13 +/- 0.05 kJ mol(-1). respectively. An EXAFS study on gallium(III) thiocyanate in dimethylsulfoxide solution showed that the thiocyanate ion is coordinated through the sulfur atom with a Ga-S bond distance of 2.198(6)angstrom, and a change in the coordination number of gallium(III) from six to five, with a mean Ga-O bond distance of 1.902(6) angstrom to the four dimethylsulfoxide molecules. (C) 2008 Elsevier B.V. All rights reserved.