A complex-formation reaction has been studied for the Me2SnCl2 plus pyridine system in dimethyl phthalate (DMP) and dioctyl phthalate (DOP) by means of titration calorimetry. The resulting entropy changes in the reactions of A+B=AB and AB+B=AB2, with A=Me2SnCl2 and B=pyridine, were compared with those calculated for the gas-phase reactions. The values in solution amounted only to 1/3–1/4 of the gas-phase values. Correction for the entropy of solution of each component failed to explain the observed values in the absence of a strong solvation. The solvation model is presented to allow for such a discrepancy, assuming a strong solvation for Me2SnCl2 as well as for the 1:1 pyridine complex by definite numbers of solvents. To test the applicability of the model, calculations were made based on molecular statistical thermodynamics. The results obtained were reasonable, indicating that a free molecule, Me2SnCl2, is solvated by 1.3 molecules of the solvent of DMP, which are released about half by half on the formation of the 1:1 and 1:2 complexes with pyridine.
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