The role of ion-association in the chromatographic separation of isomeric cationic cobalt(III)-amine complexes on cation-exchange resins, particularly SP-Sephadex

Abstract

Chromatographic separations of a set of closely similar or isomeric cobalt(III) complexes, of the kinds [Co(dien)₂]³⁺, [Co(dien)(medien)]³⁺ and [Co(medien)₂]³⁺ [dien = diethylenetriamine; medien = 4-methyldiethylenetriamine or 2,2'-methyliminodi(ethylamine)] on SP-Sephadex C-25 and Dowex 50W-X2 cation-exchange resins have been examined with different eluents as sodium salts. The separations obtained appear to be due primarily to differences in the association constants for ion-pairs formed betwen the complex cations and the eluent anions, rather than due to differences in affinities of the free cations for the resins. The strengths of the associations depend on: (1) the availability on the complex cations of appropriately disposed N-H bonds for hydrogen-bonding to oxyanions, and probably on the acidities of these hydrogens (acidity sec-NH > NH₂), rather than on polarity of charge distribution, and (2) the basicities of anions, in the order PO₄^3- > SeO₃^2- > (+)-tartrate^2- > SO₄^2- > (+)-Sb₂(C,₄H₂O₆)₂^2- > Cl^- ≈ NO₃^- > ClO₄^-. The effects of association with PO₄^3-, SeO₃^2- and (+)-tartrate^2- may be so great that complex cations of different charges are not necessarily eluted in the order of their increasing charges. Determinations of charges and polarities (cis/trans) of cationic complexes by chromatographic procedures must therefore be carried out under conditions where association is minimized, as with sodium perchlorate as eluent. Under these conditions, separations are determined largely by cation-resin interactions which are less specific and less discriminating than cation-anion interactions. SP-Sephadex is a particularly sensitive medium for separating different cationic complexes, and tri- sodium orthophosphate and sodium (+)-tartrate are the most effective and generally useful eluents.