The chelate effect : is it all entropy?

Chelating ligands and the chelate effect have been known for a long time, and have been covered in a wide selection of texts. This increase in stability of coordination compounds, which results from chelate ring formation, has been attributed to both to an increase in bond strength resulting from formation of the ring, and to a statistical effect associated with chelation. Phrased differently, it has been rationalized by arguing that the chelate effect is caused by an increase in the number of particles that directly affects translational entropy. It has been commonly assumed that the enthalpy term due to the chelate effect is negligible, since it is believed that the monodentate and polydentate ligands are equivalent, such that the energy released upon bond-breaking is balanced out by the energy absorbed during bond formation. The goal of this study is to test this assumption through computational calculations looking at gas-phase reactions. The entropy and enthalpy contributions to a series of chelate reactions involving amine-derived ligands are analyzed and compared. During this analysis, the thermodynamic components of these reactions are broken down into their respective components in an attempt to understand what intrinsic physical factors of a ligand dictates its binding affinity to a metal center. The relative entropy and enthalpy contributions are compared through three models: The first one only looks at enthalpy of reactions, the second one looks at the enthalpy and entropy contributions for singular mode systems, and the third ones considered the entropy and enthalpy contributions for multi-structural systems. Hindered rotations are not corrected for in this v analysis, but previous studies support that contributions from hindered rotations are very small at room temperature and one atmosphere. This analysis of different models demonstrated that the chelate effect is not only due to translational entropy contributions, but also rotation and vibration contributions are at least as important. It has also been found that enthalpy also contributes to the chelate effect. There is no simple explanation of the high stability of chelate complexes; it is a results of both enthalpy and entropy contributions that are highly variable depending on the system under consideration