Kinetics and mechanism of the reaction between chromium(III) and picolinic acid in weak acidic aqueous solution

The interaction between chromium(III) and picolinic acid in weak acid aqueous solution was studied, resulting in the formation of a complex upon substitution of water molecules in the chromium(III) coordination sphere. Experimental results show that the reaction takes place in multiple steps. The first step is the formation of an ion pair, the second step (two consecutive steps) is the slow one corresponding to substitution of the first water molecule from the chromium aqueous complex coordination sphere by a picolinic acid molecule via oxygen atom of the carboxylic acid group and substitution of the second water molecule via nitrogen of the pyridine ring forming an 1:1 complex. Both consecutive steps were independent of chromium concentration. The rate constants of the 1st and 2nd consecutive steps were increased by increasing picolinic acid concentration. The corresponding activation parameters are a dagger H (1obs) (*) = 28.4 +/- A 4 kJ mol(-1), a dagger S (1obs) (*) = -202 +/- A 26 J K-1 mol(-1), a dagger H (2obs) (*) = 39.6 +/- A 5 kJ mol(-1), and a dagger S (2obs) (*) = -175 +/- A 19 J K-1 mol(-1). The third step is fast, corresponding to formation of the final complex [Cr(pic)(3)]. The logarithms of the formation constants of 1:1 and 1:3 complexes were found to be 1.724 and 4.274, respectively.