A thermodynamic study of complex formation in aqueous solution.|nI.|pThe Hg(II)-, Zn(II)-, Cd(II)-, Ag(I)-serine systems.|nII.|pThe Hg(II)-, Zn(II)-, Cd(II)-, In(III)- thioglycollic acid system :|ba study of the effect of d[pi]-d[pi] bonding on these complexes


Thermodynamic equilibrium constants were obtained potentiometrically using a Beckman pH meter for metal-serine and metal-thioglycolloc acid complexes to study the thermodynamic properties of the complexes and the effect of dπ - dπ bonding on the equilibrium constants. The following calculations were made: 1. The stepwise dissociation of protons from serine at 10, 20, 30, and 40°C and the stepwise dissociation of protons from thioglycollic acid at 20, 30, and 40°C. 2. The thermodynamic formation constants for Zn(II), Cd(II), Hg(II), and Ag(I) with serine at 10, 20, 30, and 40°C and at two ionic strengths. 3. The thermodynamic formation constants for Zn(II), Cd(II), Hg(II), and In(III) with thioglycollic acid at 20 and 30°C and at two ionic strengths. 4. The entropy, enthalpy, and free energy change for each of the metal-serine complexes. The order of increaseing stability of log K_1 for the metal-serine systems was as follows: Hg(II) > > Zn(II) > Cd(II) > Ag(I), and for log K_2 the order: Hg(II) > > Zn(II) > Cd(II) ≈ Ag(I). The stability of the Hg(II)-serine was high becasue of the high electronegativity of Hg(II) and an abundance of 4f and 5d electrons that can be polarized by the ligand donor atoms. Values for log K_1 of Zn(II), Cd(II), and Ag(I) with serine decreased in the order of decreasing electronegativity of the metal ions. All log K_2 values were smaller than log K_1 values and followed the same order of stability as log K_1 with the exception of the Ag(I)-serine complex. Linear bonding is more stable for Ag(I) than any other spatial configuration. The second formation constant is this higher than the first because the second ligand completes this stable linear complex. In(III) precipitated as a hydroxide, making it impossible to measure a formation constant for In(II) with serine. The order of increasing stability of log K_1 for the metal-thiogly-colloc acid system was as follows: Hg(II) In(III) Cd(II) Zn(II) and for log K_2 the order: Hg(II) Zn(II) In(III) Cd(II). Log K_1 values for Hg(II) In(III), and Cd(II) were above 10 indicating that these metal ions form an extremely stable bond with sulfur. This enhanced stability is due to the formation of a second bond or a d - d bond between the metal ion and the sulfur atom. The d bonding with Zn(II) is not as great as with the other metal ions. The order is somewhat reversed for log K_2 and follows the order of decreasing electronegativities of the metal ion. This would indicate that the d bonding is not as pronounced in the second formation constant. Hg(II) again has a high log K_2 value because of the electronegativity of Hg(II) and the 4f and 5d polarizable electrons. The Ag(I)-thioglyeollic acid complex could not be calculated because Ag(I) precipitated with thioglycollic acid.



College and Department

Chemistry and Biochemistry



Date Submitted


Document Type





Thermodynamics, Thioglycollic acid, Chemical equilibrium



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