Superimposition of Several Complexation Equilibria

Let s consider the very simple following example. Frequently, the metallic ion M (electrical charge omitted for simplicity) we want to engage into a complex with the ligand L (electrical charge omitted) is initially in the presence of another potential ligand L. For example, L may be in the initial solution in order to maintain the metallic ion in the solution. As another example, the presence of a buffer is often necessary for some complexes to be sufficiently stable. In this case, L may be one of the members of the buffer. In these conditions, the two superimposing equilibria are 

It is evident that the constant K must be inferior to the constant (3 since, by hypothesis, the parasitic complex ML exists that is, P 1. We also notice that, as a limiting case, the constant K may be inferior to the unity (P p, where the initial complex ML is more stable than ML). Then the formation of the complex ML searched for is inhibited. 

Therefore, from a general standpoint, ML is obtained with a weaker yield than would be obtained if there were no interference with another complexation equilibrium. An example, among numerous others, is provided by the complexation ofNi + by the complexone(IV), Y . The expected (and occurring) reaction is 

To obtain satisfactory yields, the reaction must be carried out at pH 9 (see Sect. 25.4). Hence, the ammonia buffer is used. In these conditions and due to the fact that a buffer must be used in a rather important concentration, the following ammine complexes form [Ni(NH3)] +, [Ni(NH3)2] +,. .., [Ni(NH3)6] +, whose overall formation constants are Pj, P2, , Pe- As a result, the formation reactions of the complex of interest must be written as 

We shall see that the yield in the complex [NiY] may be considerably lower than expected after having considered only the following simple reaction  

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