Kinetics of anation of hexaaquachromium(III) by thiocyanates - specific salt effects

The kinetics of anation of hexaaquachromium(III) by thiocyanates follows the rate law:) -d[complex]/ dt = k[NCS] (20 - 40 degreesC, [NCS-] = 0.1 - 0.6 M, I = 2.0 M, pH = 1.0). The specific salt effect has been studied for five media: NaCl, NaBr, NaClO4, KCl and CsCl. The series of chloride (Na+, K+ and Cs+) salts show a negligible effect on the anation rate. On the contrary, the series of sodium salts (Cl-, Br- and ClO4-) reveal a marked difference in the reaction rate. The anation rate decreases sharply with the ionic strength increase (I = 0.2 - 2.0 M, NaCl). The results were interpreted within the frame of fast equilibria of ion-pair formation followed by an interchange mechanism step. The difference of reaction rate is a result of competition between anions ( thiocyanates and supporting electrolyte anions) to the complex cation at an ion-pair formation process.