Cooperative interactions between ions and water in the chloride ion and water coordination to a square-planar nickel complex ion in concentrated aqueous solutions

In concentrated aqueous solutions (1–6 mol dm–3) of various kinds of salts such as sodium chloride, calcium chloride, tetramethylammonium chloride and sodium perchlorate, salt effects on coordination of the chloride ion and water to [Ni(tmc)]2+(tmc: 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) have been investigated spectrophotometrically. In water at 25 °C 63% of the nickel complex is coordinated by water as a five-coordinate species. Addition of perchlorates to the complex solution decreases the fraction of the five-coordinate complex without shifting the absorption peaks and 6.5 mol dm–3 perchlorate gives 100% of the four-coordinate species. With sodium chloride, the fraction of the four-coordinate species is increased with concomitant red shifts in wavelength and decreases in absorbance at the absorption peaks for the five-coordinate species. On the other hand, tetramethylammonium chloride decreases the fraction of the four-coordinate species with similar red shifts of the five-coordinate peaks. The red shift is attributed to coordination of the chloride ion. The different effect caused by sodium chloride compared with tetramethylammonium chloride is discussed in terms of the difference in cooperative interactions between the chloride ion and the counter-cations through hydrated water molecules.