Ternary Complexes in Solution

The stability constants of the ternary Cu 2+ complexes, ethylenediamine‐Cu 2+ ‐pyrocatechol and histamine‐Cu 2+ ‐pyrocatechol dianion, were determined. Both complexes are more stable than one would expect on pure statistical reasons. The imidazole‐containing ternary Cu 2+ complex is, however, especially stable. For the equilibrium, Cu(histamine) 2 + Cu(pyrocatechol) 2 ⇋ 2[(histamine)Cu(pyrocatechnol)], the constant is log X = 4.86 ( I = 0.1; T = 25°), while the statistical value is 0.6. These results were compared with those taken (or calculated) from the literature; it is concluded that the π‐system of the ligands is important for such a great increase in stability. Furthermore, the Cu 2+ ‐histamine 1:1 complex selectively binds to O‐ligands rather than to N‐ligands. Thus, the first coordinated ligand has an influence on the kind of the second to be coordinated, i. e. the Cu 2+ ‐histamine 1:1 complex shows discriminating qualities. The significance of these results is discussed with regard to Cu‐proteins, since it is suggested for several enzymes that the imidazole group of histidine is involved with the binding of metal ions.