Thermodynamic relationships in complex formation. IX. ΔH – ΔS and free energy relationships in mixed ligand complex formation reactions of Cd(II) in aqueous solution

Abstract

Standard thermodynamic parameters ΔG, ΔH and ΔS for several series of coordination reactions between mixed ligand complexes of Cd(II) and single entering groups in aqueous solution at T= 298.15 K and constant ionic strength μ= 1 for KNO3 were inspected for the occurrence of enthalpy–entropy and free energy relationships. Both the series of reactions between constant substratum and variable ligands ( Sub + L(i)= Sub– L(i);i= variable) and the series of reactions between variable Cd(II) complex substrata and constant entering ligand (Sub(i)+ L = Sub(i)– L; i= variable) disclose linear compensativeΔH – ΔS interplay (ΔH=ΔHres+τΔS). The parameters (ΔHres and τ) of the linear relationships do not discriminate within the coordination levels of the reacting substrata. The series Sub(i)+ S2O32−= Sub(i)− S2O32− is distinguished from the other series for more a favourable ΔHres.The series of reactionsSub(i)+ L = Sub(i)– L ( i= variable) shows a non-linear dependence of the substratum-to-ligand affinities on the overall thermodynamic stability of the reacting substrata. The exponential relationship Y=A+Bexp (CX) is proposed for describing the dependencies. The general interpretation of the parameters A, B and C are given.