Density Functional Theory studies on interactions of phosphoryl ligands with a pentaaqua Ca2+ complex: Bond interaction analysis

Quantum chemical calculations using the B3LYP/6-31 + G(d) method were carried out for the pentaaqua Ca2+-phosphoryl complexes [Ca(H2O)5L]2+. Two sets of phosphoryl ligands were studied: with trivalent (OPR and OPPhR) and with pentavalent (OP(R)3 and OP(PhR)3) phosphorous atoms, where RNH2, OCH3, OH, CH3, H, F, Cl, Br, CN and NO2. In the OPPhR and OP(PhR)3 cases the R group is bonded to the para position of a phenyl ring. The nature of the metal–ligand bond was investigated with an energy decomposition analysis (EDA). The decomposition of the bond energy into its five components (electrostatic, exchange, polarization, dispersion and repulsion) shows that the electronic nature of the substituent is strongly correlated with the total interaction energy. Complexes with electron donating substituents (RNH2, OCH3, OH, CH3) have the strongest metal–ligand interaction energies. The electrostatic, polarization and repulsion energies are the components of the interaction most strongly affected by inductive and resonance effects of the R groups, whereas the exchange and dispersion components are almost constants along each set of complexes. Both electrostatic and the covalent terms are important components to dictate the magnitude of the interaction. NBO analysis shows that the nature of the substituent is also correlated with the s/p character of the orbital in the ligand that is responsible for the interaction.