Interaction between alkaline earth cations and oxo ligands: a DFT study of the affinity of Mg2+ for carbonyl ligands

The affinities of Mg2+ for various substituted carbonyl ligands were determined at the DFT (B3LYP/6-31+G(d)) and semi-empirical (PM6) levels of theory. Two sets of carbonyl ligands were studied: monosubstituted [aldehydes R-CHO and RPh-CHO] and homodisubstituted [ketones R2C=O and (RPh)(2)C=O], where R = NH2, OCH3, OH, CH3, H, F, Cl, Br, CN, or NO2). In the (RPh)(2)CO case, the R group was bonded to the para position of a phenyl ring. The enthalpies of interaction between the ligands and a pentaaquomagnesium(II) complex were calculated to determine the affinity of each ligand for the Mg2+ cation and to correlate with geometrical and electronic parameters. These parameters exhibited the same trends for all of the ligands studied, showing that the affinity of Mg2+ for electron-donating ligands is higher than its affinity for electron-withdrawing ligands. In the complexes, electron-donating groups increase both the electrostatic and the covalent components of the Mg-ligand interaction. This behavior correlates with the Mg-O(carbonyl) distance and the ligand electron-donor strength.