Liquid adsorption chromatography of chelates : The effect of structure on the thin-layer chromatographic behaviour of chelates

The thin-layer chromatographic behaviour of chelates belonging to different classes is discussed. The major adsorption interactions of chelates are hydrogen bonding between the ligand donor atoms and the surface hydroxyl groups and reactions between the metal atom and the electron-donor active centres of the sorbent. Predominance of either of these general mechanisms depends on the chelate structure and particularly on the coordination saturation of the chelate. Coordination-saturated chelates are retained because of hydrogen bonding, while the metal atom does not participate directly but can influence sorption by affecting the electron density distribution in the chelating ring. Atomic electronegativity is used as a measure of the electron-acceptor ability of the metal. Electronegative atoms located outside the functional group of the chelate can participate in the adsorption either directly or by affecting the proton-acceptor ability of the donor atoms as a result of induction and steric effects. The relationship between chelate retention factors and the parameters characterizing the electron and spatial structure of ligands can be described quantitatively by an equation of the type log [(/R_f)?1] = A + Σσ. In the case of coordination-unsaturated chelates, adsorption interactions with participation of the metal atom predominate, either by ion exchange (with ligand replacement) or by a donor-acceptor mechanism (with introduction of the adsorption centre into the coordination sphere without decomposition). In general, the adsorbability of chelates is directly related to the proton-acceptor ability of donor atoms and the acceptor ability of the metal atom. Classification of chelates by their adsorption interactions is proposed. Recommendations are given for selecting the optimal chelating reagent for the separation of metals by liquid-adsorption chromatography.