Sigma donor and pi acceptor characteristics of certain NN-bidentate ligands: a DFT Study

Abstract

Metal ion binding affinity of three NN bidentate ligands in terms of simple parameters of the conceptual density functional theory is reported. Role of ligand framework for chelate stabilization for ethylenediamine (en) bipyridyl (bpy), and 1,10 phenanthroline (phen) is quantified on the basis of NCCN dihedral angle () and N–N spatial distance. We find that the sigma (σ) donor character of three NN-bidentate ligands follow the order phen < bpy < en, which is quit opposite to their experimentally observed stability constant data. However, the overall binding affinity order is correlative with the chelate stabilization and pi back acceptance characteristics; thereby explaining the experimental stability order. We also studied the effect of some electron donating and electron withdrawing substituents on the sigma bonding affinity and pi (π) bonding behavior of the various substituted phen derivatives. The overall binding affinity of these derivatives was correlated on the basis of electrophilicity index (ω). σ donor character for the ligands was observed to vary in the order NH2 > Ph > OH > CH3 > H > Br > Cl > COOH > NO2 for the substitution at 2 and 9 positions. The trend was found in synergism with their Ortho, Para directing efficiencies.