Thermodynamics of ion solvation in mixed aqueous solvents. Part 2.—Effect of steric hindrance on free energies of transfer of cations; correlations with structural determinations

Abstract

Precise values of the standard free energies of transfer, ΔG°_t, from water to mixtures of water with methanol, t-butyl alcohol (TBA), acetone and dioxane are available for the alkali-metal and silver chlorides and for hydrochloric acid, together with a few values for chlorides of bivalent metals in the two alcohol systems. These values are discussed from the standpoint of steric hindrance to solvation. When a more basic ligand replaces water in a cation–solvent complex, the lowering of free energy arising from the stronger acid–base interaction is offset by the effects of any resultant steric hindrance to solvation. The Feakins–Watson ‘acid–base’ theory suggests a simple electrostatic model, which is applied quantitatively to Li⁺, Na⁺, H⁺ and Ag⁺. The behaviour of Ag⁺ and H⁺ is quantitatively reproducible from system to system and is consistent with no steric hindrance of either ion up to ca. 60%(w/w) organic solvent. Li⁺ and Na⁺ are unhindered in methanol–water mixtures; steric hindrance of Li⁺ in other mixtures increases in the order dioxane < acetone < TBA; Na⁺ is hindered only in TBA–water. Geometries recently found for the aqueous complexes from neutron diffraction experiments and MD calculations are reviewed. The coordination numbers (n): Li⁺(6), Na⁺(6) and Ag⁺(4) yield O—O distances for adjacent ligands which are completely consistent with the above observations. Since the ion–ligand interaction energy is proportional to n, Ca²⁺(10) should have a lower ΔG°_t than an alkali-metal ion of the same size. The alkaline-earth-metal ions should also be vulnerable to steric hindrance by TBA if Na⁺ is. Both conclusions are verified. The values for Cs⁺ also fit the electrostatic model fairly well, although they would also be consistent with an additional non-electrostatic interaction in this case. The O—O distance in the Cs⁺ complex (n= 8) is about the same as in the Ag⁺ complex, but the lower O—M—O will make Cs⁺ more vulnerable to steric hindrance. Cs⁺ is unhindered in aqueous methanol, dioxane and acetone, but hindered in TBA–water, although less than Li⁺ and Na⁺. For Ag⁺ the structural and thermodynamic findings, of uncrowded stereochemistry and strong metal–oxygen bonds relative to those of the alkali metals, suggest weak and stereochemically selective shielding of the nucleus by the d electrons. The limited thermodynamic evidence suggests that this may also be true for Zn²⁺ and Cd²⁺, which also have d¹⁰ configurations, and would also support a relatively low n (ca. 7) for Cd²⁺.