Abstract: The stabilities of binary complexes of type MB, MB₂ and the mixed ligand complexes of type MABhave been studied by potentiometric pHtitration technique at 37℃ and at ionic strength I=0.02(NaCI), where M=Cd(Ⅱ)orCo(Ⅱ); A=phenanthroline; B=glycine, alanine, valine, leucine, phenylalanine and tryptophan.The logarithm of stability constants IgK_MAB^M, the logarithm of equilibrium constants △lgKof reaction MA+MB=MAB+Mand the logarithm of equilibrium constants IgXof reaction M A₂ + MB₂=2MABhave been calculated.The results show that the six mixed ligand complexes containing leucine, phenylalanine or tryptophan as one of the ligands have an enhanced stability, because their △lgKand IgXare distinctly higher than the corresponding statistically expected values, even becoming positive values.The possible reasons that lead to these special results have been discussed from the viewpoints of π electronic back donation from the metal ions to the aromatic amine molecule and of the intramolecular ligand-ligand hydrophobic interactions presented in the mixed ligand complexes.After analysing the experimental data, we come to the conclusion that the major reason for the enhanced stability of above mentioned six mixed ligand complexes is the intramolecular hydrophobic interaction between the aromatical ring of phenanthroline and hydrophobic moieties of these amino-acids.The variation order of △lgKor IgXis Try>Phe>Leu, which is consistent with the size of hydrophobic moieties of amino-acids.The equilibrium constants, K₁ of transfer between the opened species (absence of ligand-ligand interaction) and the closed species (hydrophobic interaction having occured) and the percentage of the.closed species have been calculated for each of the six mixed ligand complexes (Table 3).The results show that the closed species of the mixed ligand complexes containing amino-acid with aromatical ring moiety as a ligand have become the major components in solution.

Key words: Coordination compounds, Cadmium(Ⅱ), Cobalt(Ⅱ), Amino-acid, Hydrophobic interaction