Abstract
The acid-catalyzed aquation of [Cr(pic)(H2O)4]2 2+ and [Cr(dpic)(H2O)3]+(pic = picolinic acid anion, dpic = dipicolinic acid dianion) in nitrate(V) media was studied. The reaction is reversible in the case of the pic-complex and practically irreversible in the case of the dpic-complex. It is assumed that the reactive form of the substrate undergoes fast chelate ring-opening followed by protolytic equilibria, followed by the rate of the Cr—O bond breaking of the monodentate bonded ligand which is the rate-determining step. The kinetics of pic/dpic ligand liberation were followed spectrophotometrically in the 0.4–2.0 M HNO3 range at I= 2.0 M. The following dependences of the pseudo-first order rate constants on [H+] have been established:k obs=a+b[H+](where b and a are apparent rate constants for the forward and the reverse reaction of the pic-complex) and k obs=b[H+]+c[H+]2(where b and c are apparent rate constants for the dpic liberation). Fast protolytic pre-equilibria, leading to protonation of the carboxylic oxygen atom on the monodentate bonded ligand, preceeds ligand liberation.
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Szabłowicz, M., Kita, E. Kinetics and Mechanism of Chromium(III)-Picolinato and Chromium(III)-Dipicolinato Complexes Aquation in HNO3 Solutions. Transition Metal Chemistry 29, 345–351 (2004). https://doi.org/10.1023/B:TMCH.0000027441.48508.90
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DOI: https://doi.org/10.1023/B:TMCH.0000027441.48508.90