The Thermodynamics of Complex Formation

Hydration enthalpy Stability or formation constant Overall and stepwise stability constants Chelate effect Macrocyclic effect Preorganization Equilibrium template effect Kinetic template effect Self-assembly 

The chapter will assume a knowledge of basic thermodynamics, including the meaning of the terms equilibrium constant, enthalpy, entropy, free energy and electrode potential, and the relationships between them. 

One of the first questions one might ask about forming a metal complex is how strong is the metal ion to ligand binding In other words, what is the equilibrium constant for complex formation A consideration of thermodynamics allows us to quantify this aspect of complex formation and relate it to the electrode potential at which the complex reduces or oxidizes. This will not be the same as the electrode potential of the simple solvated metal ion and will depend on the relative values of the equilibrium constants for forming the oxidized and reduced forms of the complex. The basic thermodynamic equations which are needed here show the relationships between the standard free energy (AG ) of the reaction and the equilibrium constant (K), the heat of reaction, or standard enthalpy (A// ), the standard entropy (AS ) and the standard electrode potential (E for standard reduction of the complex (equations 5.1-5.3). 

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The thermodynamics of complex formation between the chelates ML2 (M = Zn or Hg HL = RC(SH)=CHCOCF3, R = Me or Ph) and py and bipy have been investigated.759 Zinc forms 1 1 complexes with both ligands, the bipy being bidentate. Mercury gives a 1 1 complex with py, but only an extremely unstable adduct is formed with bipy. 

Especially for the phosphine, the thermodynamics of complex formation follows very much the same pattern as has been found before for the likewise very soft cyanide ligand. Evidently the bonds formed by these ligands must be of a rather similar nature, all markedly covalent according to the arguments advanced above. 

Marsicano, F., and Hancock, R. D. The linear free energy relation in the thermodynamics of complex formation. Part 2,... 

The roles of enthalpy and entropy in the thermodynamics of complex formation... 

Marsicano, E, and R. Hancoc k. 1978. The linear free energy relation in the thermodynamics of complex formation, 2. Analysi.s of the formation constants of complexes of the large metal ions Ag Hg- and Cd with ligands having soft" and nitrogen-donor atoms. J. Chem. Soc. Dalton 1978 228-34. 

A detailed discussion of the thermodynamics of complex formation in aqueous solution lies beyond the scope of this book, but we discuss briefly entropy changes that accompany the formation of coordination compounds in solution, and the so-called chelate ejfect. In Chapter 20, we look further at the thermodynamics of complex formation. 

A detailed discussion of the thermodynamics of complex formation in aqueous solution lies beyond the scope of... 

A fundamental concept in metal complexation is that unfavorable steric interactions lead to weaker complexes. Steric strain that is associated with complex formation can be measured by the difference in MM steric energy between the products and reactants, Af/. Such Af/ values should provide a way to predict the influence of steric effects on the thermodynamics of complex formation. Specific examples of structure-reactivity relationships obtained with this approach are discussed below. 

The thermodynamics of complex formation of PMAA and PAA with PEG were studied by Papisov et al. [34] using calorimetry, potentiometry and viscometry. Complexation enthalpy and entropy values were obtained from titration and calorimetric measurements. The enthalpy of complexation of PMAA and PEG was 0.33 kcal/mole and the entropy of complexation 1.32 cal/mole°K. Both values are higher than for complexes involving PAA, indicating a stronger hydrophobic association in the PMAA complex than in PAA. The equimolar complex of PMAA and PEG as measured by viscometry started to dissociate between pH=5.1 and pH=5.4, with no complex detected at pH=5.9. Here, PMAA concentrations of O.lg/dl were used. 

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