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 PO43- > SeO32-
> (+)-tartrate2- > SO42- > (+)-Sb2(C,4H2O6)22- > Cl- ≈ NO3- > ClO4-. The effects of
association with PO43-, SeO32- and (+)-tartrate2- 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.
