The linear free-energy relation in the thermodynamics of complex formation. Part I. Correlations involving enthalpy changes on complex formation of silver(I) and mercury(II) with ‘soft’ donor ligands

Abstract

The free energies of complex formation of various metal ions with ligands having ‘Soft’ donor atoms P, As, S, Se, and I^–, Br^–, Cl^–, and CN^– form linear free-energy relations (l.f.e.r.s) involving all these ligands. A potentiometric and calorimetric study has been made in which one l.f.e.r., for the pair of cations Ag⁺ and Hg²⁺, has been analyzed in terms of the separate enthalpy and entropy contributions. A linear enthalpy relation (l.e.r.) has been found which lies parallel to the l.f.e.r., but, unlike the l.f.e.r., does not pass through the origin. The intercept of the l.e.r. is rationalized in terms of the Powell, Latimer, and Cobble model for the partial molar entropies of complexes in aqueous solution.