Chromatographic separations of fac- and mer-tris(β-diketonato)chromium(III) complexes

In order to find the factors that affect the separation of metal chelates, the elution and distribution behaviour of chromium(III) complexes coordinated with unsymmetrical β-diketones were studied in various solvents using silica gel as an adsorbent. The differences in the specific retention volumes or adsorption distribution coefficients of metal chelates are ascribed to the differences in the total energies of interaction between the chelate molecule and the adsorbent surface. The total energy S⁰ is expressed by three independent terms as follows: S⁰ aG_i + bG_j + c where G_i and G_j are parameters representing the interaction energies of the functional groups containing the substituents in a β-diketone. Using the relative values of G_i and G_j for the functional groups, the ratio of the distribution coefficients of Cr(acac)₃ to a tris(β-diketonato)chromium(III) was evaluated. On the basis of a linear relationship between the experimental and calculated distribution coefficients, the quantitative separation of metal chelates in the silica gel adsorption chromatography is discussed.