The stability constants of the mixed ligand ternary complexes of nickel(II) of the type MAB, MA 2 B and MAB 2 (where A = histidine, B = diols, viz. ethane diol, propane-1,2-diol, butan-1,3-diol, 2-butene-1,4-diol, pentane-2,4-diol and hexane-1,6-diol) were determined by potentiometric method. The difference in the stability constants of binary and ternary complexes (ΔlogK) and total enhancement of stability due to mixed ligand complex formation (log X) were of greater magnitude in MA 2 B systems than those in MAB 2 systems. This appeared to be due to the greater number of nitrogen atoms coordinating to metal ion in the MA 2 B species. Some changes in the stability constants have been found to be due to the variation in the size of the ring formed by diols. The stability of 2-Butene-1,4-diol containing species exhibited the highest stability in this series. It was concluded that the π-system of the ligands is important for such a great increase in stability.

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