Metal complexes, entropy effect

Electrostatically-controlled pre-association interactions have an important effect on rates for [Pd(dien)Cl]+ reacting with thione-containing nucleosides, nucleotides and oligonucleotides, as is often the case for reactions between metal complexes and this type of biological ligand. Interaction between the charged complex and the polyanionic oligonucleotide surface leads to an increase in both enthalpy and entropy of activation in the DNA or model environment (252). 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

Kx for copper (II)-diamine complex is 10.36 and 9.45 for 1,2-ethanediamine and 1,3-propanediamine, respectively (-2)]. The large difference in the stabilities of the two copper (II)-diamine complexes is attributed to an unfavorable entropy effect associated with an increase in the size of the metal-chelate ring (2). Extrapolating to the / -ketoimine derivatives, it seems reasonable to expect that the stability of bisacetylacetonetrimethylenediiminocopper(II) would be less than that of the ethylenediamine analog and to suspect that the former compound is less stable than bis-(4-iminopentane-2-ono) copper (II). That this is reasonable is... 

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... 

Thus, it is seen that the effect described by Schwarzenbach has precise thermodynamic meaning—the change in the entropy of translation that accompanies metal chelate ring formation. The entropy effects estimated by Schwarzenbach, up to 2.0 log K units, agree quite well with the value obtained with the thermodynamic approximation. Experimentally, one would expect wide deviations from this value (7.9 entropy units per chelate ring) because of the variations in solvation and internal entropies of complexes and ligands that occur in the displacement reaction. 

The entropies of formation of the 1 1 bivalent metal complexes of the structurally related ligands, Formulas IX-XIII, reported by Martell (10) show a linear correlation with the square of the reciprocal of the effective ionic radius of the metal ions, r , in Equation 8, and with the number of negative carboxylate groups of the ligand that combine with the metal ion. 

It is well known that alkynes and diolefins are more strongly chemisorbed during their hydrogenation on metal surfaces than are olefins, although the eflFect can arise from quite small differences in heats of chemisorption (5). The analogy with organometallic complexes is quite close, but the stronger coordination of diolefins compared with monoolefins is almost entirely an entropy effect. 

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