Kinetics and mechanism of the chromium(III)-picolinato chelate ring opening in some chromium(III)-oxalato-picolinato complexes in acidic aqueous solutions

Two Cr-III-picolinato complexes were obtained and characterized in solution . The [Cr(C2O4)(pyac)(2)](-) and [Cr(C2O4)(2)(pyac)](2-) ions (pyac = picol inic acid anion) in acidic solutions undergo a reversible one-end Cr-III-pi colinato chelate ring opening via Cr-III-N bond breaking. The reaction rate was determined spectrophotometrically in the 0.1-1.0 M HClO4 range at I = 1.0 M. The observed pseudo-first order rate constant depends on [H+] accord ing to the equation: k(obs) = a + b[H+] + c/[H+]. A reaction mechanism, whi ch assumes participation of the protonated and unprotonated forms of the re actants, has been proposed. The kinetic parameters a, b, c have been define d as a = k(1), b = k(2)Q(1), c = k(-1)/Q(2), where k(1), k(-1), k(2) are ra te constants for the forward and reverse processes and Q(1), Q(2) are the p rotolytic equilibrium constants in the term of the proposed mechanism. The activation parameters have been determined and discussed.