AbstractThe formation constants (logK MAMAB) of ternary mixed ligand complex formed from 1,10‐phenanthroline (A, phen) andN‐(p‐substituted phenyl) glycines,RNHCH2COOH (B,pRPhG, CH3, H, CL), with copper (II), nickel (II), cobalt (II) and zinc (II) were determined by pH method at 25°C in 30% (V/V) ethanol solution in presence of 0.1MNaClO4. It was found that linear free energy relationships exist between the stability of M(II)‐phen‐p‐RPhG ternary complex and the base strengths of the ligandsp‐RPhG, conforming to the equation logK MAMA= αpKB+C. The factors influencing the stability of the ternary complex have been discussed. The enhancement of stability of ternary complex was explained in terms of formation of π back bonding between metal ion and the ligand phen. In contrast to the binary system, the ternary ni (II) and zn (II) complexes showed higher stability which could be accounted for by the formation of π back bonding and by intramolecular ligand‐ligand interaction, poss