The protonation constants of 3, 3, 9, 9-tetramethyl-4, 8- diazaundecane-2, 10-dioxime (PnAO), the stability constants of Zn(Ⅱ), Cd(Ⅱ), Hg(Ⅱ)-PnAO, Zn(Ⅱ), Cd(Ⅱ), Hg(Ⅱ)-5-substituted o- phenanthroline binary complexes and Zn(Ⅱ), Cd(Ⅱ), Hg(Ⅱ)-PnAO- 5- substituted o-phenanthroline ternary complexes have been determined by pH titration method at (35±0.1)℃, I=0.1mol·dm^-^3 KNO~3. The results show that here exist Linear Free Energy Relationships (LFER) between the protonation constants of 5-substituted o-phenanthroline and the corresponding stability constants of Zn(Ⅱ), Cd(Ⅱ), Hg(Ⅱ) binary complexes as well as the relevant constants for the Zn(Ⅱ), Cd (Ⅱ), Hg(Ⅱ)-PnAO-5-substituted o-phenanthroline ternary system. The stability order for the title systems is generally shown by the following: Zn(Ⅱ)＞Cd(Ⅱ)＜ Hg(Ⅱ). The internal cause has been explored in terms of the charge/radius effect of the metal ions.

Key words: GROUP IIB ELEMENT, METAL ION, PHENANTHROLINE P, DIKETONE, OXIME, TERNARY COMPLEX, STABILITY, STABILITY CONSTANT