Abstract: Thermodynamic properties of the complexation of 18-crown-6 with KCI, KBr, KI, KClO_4, KNO_3, KBrO_3,K_2CO_3 and K_2S_2O_8 in water solution were studied at 25 ℃ by calorimetric titration. The stability constants of coordination compounds and enthalpies of coordination were calculated directly.It was shown that among the calculated volues the stability constants of coordination compounds formed by 18-crown-6 with KNO_3 and K_2S_2O_8 were the highest.
The influence of anions on the stability of coordination compounds may be that:(1) The concomitant anions have effection the ligand conformation, which will cause considerable changes in the entropy of complexation and the enthalpy of complexation; (2) The strength of bonding force of anion-K~+ effects the strength of ion-dipole bond of K~+-ligand; (3) The hydration of anion itself also effects the complexation of K~+ with crown ether.
The sequence of the stability constants of coordination compounds of 18-crown-6 with the 8 potassium salts is S_2O_8~(2-)>NO_3~->CO_3~(2-)>Br~->Cl~->I~->ClO_4~->BrO_3~-. The order of salts in the complexation enthalpy sequence is, however, different.