Formation of Metal Complexes with Malate Anions: Quantum-Chemical Modeling

Abstract

Quantum-chemical modeling of anionic forms of malic acid H₂mal and the model complex [V^V(O)₂(mal)]^– in which the mal^2– ligand is coordinated to the metal ion through its carboxyl and hydroxyl oxygen atoms has been performed by the DFT M06/631G(d,p) method. It has been demonstrated that the participation of the hydroxyl group, which loses its proton only in an alkaline medium, in the formation of malate complexes in an acidic medium is not due to its acid dissociation, but to the process of competitive replacement by a more electropositive metal ion. Calculations show that the simultaneous absence of the protons at the carboxyl and hydroxyl groups leads to electron excess in the region of the –CH₂–CHO– bonding. The formation of bonds with electropositive metal ions decreases the electron density in this region, which results in the stabilization of the doubly charged anion.