Thermodynamic stability of metal-aminoacid complexes in water is discussed in terms of the Gibbs free energy of water-ligand exchange processes, and the electronic stabilizing factors thoroughly investigated by means of 1-electron and 2-electron density properties. Hexacoordinated complexes formed between iron cations and glycine molecules acting as monodentate or bidentate ligands have been chosen as targets for the current study. Results agree with experimental findings, and complexes formed with bidentate ligands are found to be more stable than those formed with monodentate ones. The larger the number of the coordinated glycine molecules the more stable is the complex. Fe(III) complexes are more stable than Fe(II) ones, but differences are small and the Fe(3+)/Fe(2+) exchange process appears to be energetically feasible for these complexes. Formation of the second glycine-iron interaction involving the amino nitrogen in the bidentate ligands is enthalpycally unfavorable but takes place due to the large entropy rise of the process. The larger stability of Fe(III) complexes is due however to the balance between energetic and solvation terms, which is favorable to these complexes. Electron density properties account satisfactorily for the electronic energy changes along the complex formation in terms of ligand-metal electron transfer and covalent bond orders.
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