Ternäre Komplexe in Lösung

Abstract

In this paper is discussed the stability of ternary complexes in which one component is a metal ion. The stability constants of the following equilibria are compared:

Me + L ⇌ MeL (1)
MeY+ L ⇌ MeY∙L (2)

This comparison – see the equilibrium scheme (4) – leads to equation (6):

Δlog K = log K^MeY_MeY∙L − log K^Me_MeL = log K^MeY_MeY∙L − log K^Me_MeY (6)

Bhat et al. investigated equilibrium (2) where Y = ethylendiamine-N,N,N',N'-tetraacetic acid and L= ethylendiamine. A comparison of their results with those by Bjerrum et al. for equilibrium (1), shows, as expected, that in this example Δlog K  is strongly negative (table 1).
On the other hand it is surprising that if the binary complex MeY has some “free” (i. e. hydrated) coordination positions, the resulting ternary complex [equilibrium (2)] seems to be more stable than expected (tables 3 and 4).
Of great interest are the observations of DeWitt and Watters, Kida and Pâris et al., that equilibrium (11), from their investigated examples, lie on the right side; i.e. the ternary complexes are more stable, than expected on statistical reasons

MeY₂ + MeL₂ ⇌ 2MeY∙L (11)

For obtaining more examples for comparison, we determined the stability constants [equilibrium (1)] of the Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ complexes of the ligands where L = γ-picoline, methyl-α-picolyl-sulfide and 2-acetyl-3-hydroxythiophene. The stability constants of equilibrium (2) with Y = 2,2'-bipyridine also were determined (tables 6, 7 and 8).
It is of interest, that the stability constants for equilibria (1) and (2) with the Co²⁺, Nⁱ²⁺ and Zn²⁺ complexes are nearly the same for L = 2-acetyl-3-hydroxythiophene and Y = 2,2'-bipyridine (i.e. Δlog K = 0; table 8); the results with L = γ-picoline and methyl-α-picolyl-sulfide are similar, i. e. Δlog K = 0 to −0.3 (table 6 and 7). In these cases, therefore evidence is given that equilibrium (11) lies on the right side.
An important result from these investigations is the observation that the Cu²⁺-complex, with L = 2-acetyl-3-hydroxythiophene and Y = 2,2'-bipyridine of equilibrium (2) is more stable than that of (1) (Δlog K = +0.5). This results has been confirmed by L’Heureux and Martell.
The results obtained with Cu²⁺-complexes by these authorsand others, as with our own (table 9) were discussed and compared. One of the intersting points may be that two NH₃ molecules are not able to replace 2,2'-bipyridine without a great decrease in the complex stability, i.e. the Δlog K value.
Attemps were made to explain this observation and also the finding that the mixed complexes of 2,2'-bipyridine and O- ligands with Cu²⁺ are more stable than those with Co²⁺, Nⁱ²⁺ and Zn²⁺.
Since mixed complexes are very important in biochemistry some biological aspects of these results are discussed. Especially it is pointed out that a metal ion coordinated to a first ligand may cause discrimination for a second ligand.