Author: Janet Louise Laird
Publisher: National Library of Canada
ISBN: 9780315195219
Category : Chromium ions
Languages : en
Pages : 0

Get Book Here

Book Description
The aquation and chromium(II) reduction reactions of a variety of sulfonamidopentaamminecobalt(III) complexes have been studied in 1 M LiClO4/HClO4. The rate law for the aquation of (NH35CoNHSO2NH22+ has the form -dln[complex]/dt = k2h[H+]/(Ka+[H+]), and the aquation kinetics of (NH3)5CoNHSO2R2+, (R = C6H4CH3-p, C6H4NO2-p), are consistent with the same rate law if Ka ” [H+]. The unusually large rate constants obtained, (e.g., k2h = 1.41 x 10-2 s-1 at 25°C, for (NH3)5CoNHSO2NH22+), are attributed to N-protonation of (NH3)5CoNHSO2R2+ followed by facile release of the neutral ligand. The chromium(II) reduction of (NH3)3CoNH2SO32 produces (H2O)3CrOSO2NH22+. The nitrogen and oxygen bonded linkage isomers are reduced at indistinguishable rates. Reduction of (NH3)5CoNHSO2NH22+ is competitive with aquation, and (NH3)5(2oNH2SO2NH23+ is reduced much more rapidly than the deprotonated species. The chromium(III) product is too unstable to be fully characterized, but a kineticspectrophotometric analysis indicates that ligand transfer does occur. Similar kinetic results were found for (NH3)5CoNHSO2C6H4CH3-p2+, but the chromium(III) product could not be detected. The kinetic trends for the pentaamminecobalt(III) complexes with SO42-, NH2SO3- and SO2(NH2)2 are discussed, and a bridged outer sphere mechanism is suggested to be most consistent with the results. Chromium(II) reduction of (NH3)5CoNHSO2C6H4NO2-p2+ involves reduction of both the Cobalt(III) and the NO2 group. Detection of a p-nitro radical complex has been accomplished by stopped-flow spectrophotometry. A step-wise mechanism consistent with the complex absorbance changes has been proposed and many of the specific rate constants have been measured. Rapid reduction of the nitro group to hydroxylamine occurs concurrently with reduction of the Co(III), the major amount of Co2+ coming from reduction of the p-nitroso radical intermediate. Further reduction of the hydroxylamine derivative is much slower. Similar results were found for the chromium(II) reduction of the free ligand, minus those reactions involving reduction of the metal.