Polydentate Complexing Agents

It is also observed that chelating agents (polydentate ligands) form more stable complexes than a comparable number of donor atoms in monodentate ligands. Consider the following example of Ni+ complexes with four ammonia molecules (each monodentate) and two ethylenediamine... 

Such exchange reactions have been used to prepare scores of TcNP+ complexes with complex, usually polydentate ligands for trial as possible "mTc imaging agents. 

We have seen how the pH and the presence of several complexing agents can be taken into account in equilibrium calculations. If, as happens often, we can identify one reaction in the array of reactions as the principal reaction, then all the others can be properly be called side reactions, and treated in a convenient manner. For example, in complexometric titrations of metal ions with EDTA or some other polydentate chelating titrant, the presence of auxiliary ligands like NHj, citrate anion, etc., can best be accounted for by the use of the conditional constant, first introduced by Schwarzenbach and widely applied by Ringbom. 

Another polydentate complexing agent that forms complexes with many metal ions is EDTA (ethylenediaminetetra-acetic acid), with formula... 

The kinetic stability of the lanthanide complexes wifli polydentate aminocarboxylate ligands has been extensively studied mainly for gadoUunium complexes given their use as MRI contrast agents. In this case the literature clearly shows a relationship between the rigidity of the ligand and the kinetics of formation and dissociation of the metal complexes. Due to their flexible stmcture, the complexation of lanthanide ions by EDTA and DTPA is a fast process. In the case of DTPA and its derivatives, the kinetics of complexation is... 

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