Entropies of complex formation

In contrast to stability constants, there are very few data for enthalpies and entropies of complex formation for hydroxypyranonate and hydroxypyridinonate complexes. Early studies on zinc-maltolate (190) and first-row transition metal(II) complexes of kojate (191) gave estimates of enthalpies and entropies of formation from temperature variation of stability constants, though as accurate stability constant measurements are only possible over a rather short temperature range the Aff and AS values obtained cannot be of high precision. 

The enthalpy and entropy of complex formation between Zn11 and picolinate and dipicolinate anions in aqueous solution have been determined by calorimetry and from formation constant data. The greater stability of the dipicolinate complex compared to the picolinate complex reflects an entropy effect, and Ais actually less favourable. These anions are well known to have a low basicity to H+ compared to their complexing ability to metals. In the present case, this probably reflects the coplanarity of the carboxylate anions and the pyridine ring, so that the oxygen atoms are in a favourable position to coordinate.800... 

As to Eq. (7), it is to be remembered that AG, in a general case is a function of p. Therefore, the experimental dependencies of p on concentration, chain length of oligomer and temperature may be employed to find thermodynamic parameters only for a fixed value of p, e.g., for p = 0.5 using Eqs. (8 a- b). These equations have been taken by various authors to calculate the enthalpy and entropy of complex formation between simple synthetic oligomers and polymers 28). In a number of cases the correspondence between the values of complex formation enthalpy thus obtained and determined, either by calorimetry or by potentiometric titration 26), has been found satisfactory although it is obvious that in a general case these values do not necessarily coincide. 

For review, see also MacDiarmid (269). S(29Si) and 5(31P), Chemical shift in ppm AH, enthalpy of complex formation inkcal/mol A5, entropy of complex formation in cal/mol degree Py, pyridine NMI, [Pg.283]

The AG term can be determined from knowledge of the enthalpy and entropy of complex formation, AH and AS , using the usual equation ... 

The thermodynamic stability constant k which represents the free energy of complex formation (AF° = — RT In k ) can be subdivided into heat and entropy terms (AF° = AH° — TAS°). The entropy of complex formation has been discussed elsewhere (Cobble, 1953 Schwarzenbach, 1954 Williams, 1954). The factors involved include (1) the size and geometry of metal ions and ligand molecules 2) the change in the number of molecules in the system on complex formation as they affect translational freedom (3) restrictions on the freedom of rings imposed by chelation and other restrictions of internal rotation and (4) the entropy of hydration for the water molecules displaced by ligands. 

Thus, NMR spectroscopy has the potential to provide almost complete information on ligand-CD interactions (stoichiometry, binding constants, free energy, enthalpy and entropy of complex formation, dynamics and structure of the complexes) in a solution. The kinetics of complex formation is usually too fast on the NMR time scale and therefore difficult to follow using this technique. However, an information on the kinetics of complex formation may be also obtained from NMR data. 

The purpose of this paper is to describe the principal factors that influence the stabilities of the complexes formed in solutioa This involves heats and entropies of complex formation and the factors which favor a more negative enthalpy and a more positive entropy. Also, a number of misconceptions that have been adopted in foe recent literature are described and replaced by a more reasonable explanation of foe chelating tendencies involved. 

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