Kinetics and mechanism of dissociation of metal chelates. V. Dissociation of tris‐acetylacetonato‐chromium(III). With 4 figures

Abstract

From spectral observations it has been shown that in acid (HClO4) media tris‐acetyl‐acetonato‐chromium(III), Cr(acac)3, appears to undergo dissociation in stages forming [Cr(acac)2(OH2)2]+, [Cr(acac)(OH2)4]2+ and [Cr(OH2)6]3+. The rate of dissociation of the tris‐ to the bis‐ complex has been investigated. The dissociation occurs by two concurrent paths in accordance with the following rate law: Rate = k′[Complex] + k″ H +[Complex][H+]. It has been suggested that the H+ independent path involves a slow one‐ended solvent assisted dissociation of a Cr(III)–Ligand chelate ring followed by a very much more rapid dissociation of the other Cr(III)–Ligand bond leading to the loss of the ligand. The H+ dependent path involves a similar set of changes of the protonated complex (the conjugated acid) formed in a rapid pre‐equilibrium step. Values of the activation parameters ΔH≠ and ΔS≠ corresponding to the rate constants k′ and k″ H + are in agreement with the assigned mechanisms. The highly negative ΔS≠ values further suggest that, in addition to solvent participation of the type already mentioned, considerable solvation of the ligand being released must also be important. Increase in ionic strength of the medium under otherwise identical conditions increases k″ H + without affecting k′ as expected.