Incremental Binding Free Energies in Mg²⁺ Complexes:  A DFT Study

Density functional theory has been employed to evaluate the incremental binding energies, enthalpies, entropies, and free energies for the reactions of Mg 2+ with water, methanol, formamide, and formate. The B3LYP/6-31+G* calculations on the Mg 2+ complexes show that the metal ion can accommodate no more than three negatively charged formates. For the neutral-ligand complexes, magnesium prefers to bind to methanol and formamide rather than water when the number of ligands is less than four, but it prefers water to methanol and formamide for complexes with five or six ligands. These results have been rationalized in terms of steric crowding of the ligands around the metal ion and charge transfer from the ligand(s) to Mg 2+ . These two factors result in attenuation of the Mg−O bond distance and, hence, reduction in the electrostatic and polarization energies, which dominate the incremental binding energy. An empirical scheme, employing the incremental binding energies for Mg 2+ −single-type-ligand complexes, has been developed to accurately predict the total binding energy of Mg 2+ −mixed-ligand clusters.