Conformational analysis of coordination compounds. III. Bis- and tris-bidentate complexes containing five-membered diamine chelate rings

Previously published experimental data concerning the energy differences between the various configurations of Co(en) have been reinterpreted, and it has been shown that there is very little energy difference between the D(δδδ) {=L(λλλ)} and D(δδλ) {= L(λλλ)} configurations and that both these structures are considerably preferred over the other configurations. Estimation of these energies based on conformational energies arising from van der Waals, torsional, and angle-bending energy terms has shown that the energy differences are very sensitive to the type of van der Waals equation used. The more established equations for this type of system due to Hill and Mason and Kreevoy were found to be completely inadequate in accounting for the observed energy differences. Although the Bartell nonbonded interaction equation was able to account satisfactorily for the difference in energy between the D(δδδ) and D(λλλ) configurations, it erroneously predicted that the D(δδλ) configuration was significantly more stable than D(δδδ). Empirical equations of the Hill type were developed to account for the energy differences, and these were tested by applying them to the estimation of the energy difference between the axial and equatorial orientations of the methyl group in methylcyclohexane. An energy of the right order was obtained. For cis-bis-ethylenediamine complexes, the a priori calculations showed that there is very little energy difference between the possible configurations, and, for the trans isomer, the chiral and meso configurations were found to have identical energies