Abstract
Aquation of [Cr(pic)3]0 and [Cr(pic)2(OH)]2 0 in aqueous HClO4 solutions leads to formation of the common product – [Cr(pic)2(H2O)2]+. The first, reversible stage, the ring opening via Cr—N bond breaking in [Cr(pic)3]0 is followed by the second, rate-determining step – one-end bonded pic ligand liberation. In the case of the [Cr(pic)2(OH)]2 0 complex, the first faster stage produces the singly bridged dimer, which undergoes cleavage into the parent monomers in the second, much slower step. The subsequent aquation of [Cr(pic)2(H2O)2]+ is extremely slow and leads to [Cr(pic)(H2O)4]2+ formation, which practically does not undergo further ligand substitution under the conditions applied. Kinetics of the first aquation stage for [Cr(pic)3]0 and of the second step for [Cr(pic)2(OH)]2 0 were studied spectrophotometrically in the 0.1–1.0 M HClO4 range at I = 1.0 M. The observed pseudo-first order rate constant for [Cr(pic)3]0 decreases with [H+] increase according to the rate law: k obs = k 1 + k −1 Q 1/[H+], where k 1 and k −1 are the rate constants of the forward and the reverse processes in the unprotonated substrate and Q 1 is the protonation constant of the pyridine nitrogen atom. In the case of the [Cr(pic)2(OH)]2 0 complex, the rate for the singly bridged dimer cleavage does not depend on [H+]. The activation parameters for the chelate-ring opening in [Cr(pic)3]0 and for the singly bridged dimer cleavage have been determined and discussed. Some kinetic data of the slow, second aquation stage for the [Cr(pic)3]0 complex and of the fast, first aquation stage for the doubly bridged dimer have been studied; for both reactions the rate increases linearly with the increase in [H+].
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Kita, E., Szabłowicz, M. Kinetics and mechanism of the first aquation stage for the [Cr(pic)3]0 and [Cr(pic)2(OH)]2 0 complexes in HClO4 solutions. Transition Metal Chemistry 28, 698–706 (2003). https://doi.org/10.1023/A:1025469431212
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DOI: https://doi.org/10.1023/A:1025469431212