Interactions Of Chromium(Iii) Complexes With Hydrous Delta-Al2o3 - Rearrangements In The Coordination Sphere Studied By Electron-Spin-Resonance And Electron Spin-Echo Spectroscopies

Chromium(III) complex binding mechanisms to hydrous delta-Al2O3 are investigated by applying ESR and ESEEM spectroscopies. Adsorption isotherms of the Cr(H2O)6(3+) and the Cr(oxalate)3(3+) complexes can be monitored adequately either by conventional analysis (AAS) or by measuring the spin concentration of the adsorbed species. Because of the inertness of chromium(III) species, time-resolved changes of ligand exchange reactions can be identified. Optical reflectance and ESR spectra indicate the gradual transition from outer-sphere to inner-sphere binding mechanisms. In the case of the Cr(H2O)6(3+) ion, hydrolysis and/or polynuclear complex formation are potential competitive reactions, whereas for the precursor complex Cr(oxalate)3(3-) no hydrolyzable functional groups are present. For the latter example, an initially charge-reversed (anionic) type of isotherm gradually interconverts into a cationic type yet is different from that of the aquo complex.