Chemical Speciation of Ternary Complexes of Co (II), Ni (II) and Cu (II) with L-Histidine and L-Glutamic Acid in Low Dielectric Media

Chemical speciation of ternary complexes of Co(II), Ni(II) and Cu(II) with L-histidine and L-glutamic acid was studied in various concentrations (0-60% v/v) of DMSO-water mixtures maintaining an ionic strength of 0.16 mol L^-1 (NaCl) at 303.0 K. Alkalimetric titrations were carried out in different relative concentrations (M: L: X = 1:2.5:2.5, 1:2.5:5.0, 1:5.0:2.5) of metal (M) to histidine (L) to glutamic acid (X) with sodium hydroxide as titrant. Stability constants of ternary complexes were calculated and various models were refined with MINIQUAD75. The trend of the variation in the stability constants with changing dielectric constant of the medium was explained based on the electrostatic interactions of the side chains of the ligands, charge neutralization, chelate effect, stacking interactions and hydrogen bonding. The species detected are MLXH, MLX₂H and MLX₂ for Ni(II) and Cu(II) and MLX₂H and MLX₂ for Co(II). Distribution diagrams with pH at different compositions of DMSO and structures of plausible ternary complexes were also presented.