Comparison of Cadmium II Complexes with Other Transition Elements

Comparison of Cadmium(II) Complexes with Other Transition Elements 

The results reported in the literature and summarized in the previous sections reveal that cadmium(II) can form complexes with all natural amino acids and peptides. The thermodynamic stabilities of the cadmium(II) complexes are, however, relatively low as compared to those of the corresponding copper(II) and nickel(II) complexes. On the other hand, these data are comparable to those of the essential zinc(II) and the small differences are dependent on the effects of various side chain residues. These effects together with the structural characteristics of the cadmium(II) complexes can be evaluated separately for amino acids and peptides. 

The other general characteristic feature of cadmium(ll) complexes with thiol derivatives is the formation of di- or trinuclear species, in which thiol sulfur atoms behave as bridging ligands. The existence of polynuclear species was observed in nickel(ll)- and zinc(ll)-cysteine systems with similar stoichiometry, but cadmium(ll) complexes have usually an octahedral geometry, while in the nickel(ll) complexes this coordination mode results in four-coordinated square planar complexes. 

Comparison of the stability of cadmium(ll) complexes of cysteine and related ligands with those of the corresponding nickel(ll) and zinc(ll) clearly shows that the stability order follows just opposite trends with respect to that of other amino acids log jS (Cd(ll)-complexes) log jS (Ni(ll)-complexes) log jS (Zn(ll)-complexes). 

In the case of the common peptides the comparison of the stability constants of zinc(II) and cadmium(II) complexes reveals the same trends as reported for the amino acids Favored complexation with zinc(II) in almost all cases, except for the thiolate ligands. The binding of several side chain residues may enhance the stability of cadmium(II) complexes but the extent of this stabihzation is relatively low. This is true even for the aspartyl or histidyl residues which are generally considered as the most strongly coordinating side chains in the corresponding 

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