Entropy-related chelate effect

It has been pointed out by Adamson (34) and others (35,36) that the entropy-related chelate effect, as manifested in the stability constants, disappears when unit mole fraction replaces unit molality as the standard state of solutes in aqueous systems. On this basis the stability constants assumed for the model compounds in Table II (20) would have to be equivalent in magnitude regardless of the number of chelate rings formed. On the other hand the relative degrees of dissociation of the model compounds in Table II remain an experimental fact, with the larger concentration unit giving smaller numerical concentrations for the solutions illustrated, thus compensating for the disappearance of the chelate effect in the numerical values of the stability constants. 

What could be the cause of such a large difference in thermodynamic stability After all, the number of Ni -N coordinate-covalent bonds is six in both the products of these two reactions, so the enthalpy changes (Ai/) involved when these bonds are formed should be fairly similar. That seems to leave entropy as the major explanation for the effect. Indeed, the rationale for the chelate effect can be understood in two ways, both related to the relative probabilities that the two reactions will occur. First, consider the number of reactants and products in the two cases. As written more explicitly in Equations (6.11) and (6.12), it is apparent that the number of ions and molecules scattered throughout the water structure in the first reaction stays the same (seven in both the reactants and the products). In the second reaction, however, three ethylenediamine molecules replace six water molecules in the coordination sphere, and the number of particles scattered at random throughout the aqueous solution increases from four to seven. The larger number of particles distributed randomly in the solution represents a state of higher probability or higher entropy for the products of the second reaction. Therefore, the second reaction is favored over the first due to this entropy effect. 

Some reactions appear to be driven almost entirely by a positive entropy effect. For the two related reactions (5.15) and (5.16) below, each leading to four Cd—N bonds forming, the A77° values are equivalent (-57kJmol 1 for the monodentate, -56kJmol 1 for the didentate chelate), and only the -7 AS0 terms differ (+20kJmol 1 for the monodentate, -4 kJ mor1 for the didentate chelate), favouring the latter. 

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