TERNARY COMPLEXES IN SOLUTION .54. STABILITY OF TERNARY METAL-ION COMPLEXES FORMED BY IMIDAZOLE AND THE ANION OF N,N-BIS(2-HYDROXYETHYL)GLYCINE (BICINE) - OBSERVATION OF A RELATIVELY HIGH-STABILITY OF THE ZN(BICINATE)(IMIDAZOLE)+ COMPLEX

The acidity constant of H(imidazole)+ and the stability constants of the binary 1:1 complexes formed between co2+, Ni2+, Cu2+, Zn2+ or Cd2+ (M2+) and imidazole (Im) were determined by potentiometric pH titration in aqueous solution (I=0.1 M, NaNO3; 25-degrees-C). The stabilities of the ternary complexes containing in addition N,N-bis(2-hydroxyethyl)glycinate (Bic-) were also measured. This information allows the position of the following equilibrium to be quantified: M(Bic)+ + M(Im)2+ half arrow right over half arrow left M(Bic)(Im)+ + M2+. The ternary systems with Co2+, Ni2+, Cu2+ and Cd2+ show the properties expected from previous experience. However, the result for the Zn2+ system is most surprising: the position of the mentioned equilibrium is at the right side, i.e. DELTA log K(Zn) = log K(M(Bic)(Im))M(Bic) - log K(M(Im))M = 2.81-2.50 = 0.31 +/- 0.04. This high relative stability of the ternary Zn(Bic)(Im)+ complex is explained by a reduction of the coordination number of Zn2+ from 6 to 4 (or 5) upon coordination of imidazole, which entropically favors the formation of the ternary complex. The equilibrium constants measured earlier for the corresponding systems with nitrilotriacetate (Nta3-) also fit into the indicated picture. The relevance of these observations for biological systems is briefly discussed.