Equilibrium and enthalpy measurements on the copper(I) and silver(I) chloride, bromide, iodide and thiocyanate systems in tetrahydrothiophene

Abstract

Potentiometric and calorimetric measurements have been carried out for the determination of the stability constants and the enthalpy changes of the formation of copper(I) and silver(I) halide and thiocyanate complexes in tetrahydrothiophene (THT). THT is a sulfur donor solvent, which solvates the soft acceptors copper(I) and silver(I) well. THT has a low dielectric constant ϵ≈8, which affects the stability constants and the complex distribution, since the solvent has limited ability to neutralize ionic charges. Two mononuclear complexes are formed in all systems. The stabilities of the stepwise silver(I) halide complexes increase in the order Cl −<Br −<I −. The stabilities of the first copper(I) halide complexes are almost identical, while the stabilities of the second copper(I) halide complexes follow the sequence Cl −>Br −<I −. The neutral complexes of these systems are considerably more stable in THT than in any other previously studied solvent, in spite of the strong solvating properties of THT. This is because the electrostatic forces become predominant in a solvent with a low dielectric constant where charges are neutralized through ion pair formation. The enthalpy changes of the formation of the first complexes are small and negative, except for copper- (I) chloride and iodide where small positive values are found. For all systems, the second complexes are formed in strongly endothermic reactions.