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

Abstract

Two CrIII–picolinato complexes were obtained and characterized in solution. The [Cr(C2O4)(pyac)2]− and [Cr(C2O4)2(pyac)]2− ions (pyac = picolinic acid anion) in acidic solutions undergo a reversible one-end CrIII–picolinato chelate ring opening via CrIII—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+] according to the equation: k obs = a + b[H+] + c/[H+]. A reaction mechanism, which assumes participation of the protonated and unprotonated forms of the reactants, has been proposed. The kinetic parameters a, b, c have been defined as a = k 1, b = k 2 Q 1, c = k −1/Q 2, where k 1, k −1,k 2 are rate constants for the forward and reverse processes and Q 1, Q 2 are the protolytic equilibrium constants in the term of the proposed mechanism. The activation parameters have been determined and discussed.