Hydrophobic and charge-dipole interactions in aqueous solutions of highly charged metal chelate cations and nitrobenzene,dinitrobenzenes, and toluene at 25°C

Salting effects of metal chelate electrolytes Fe(phen)₃Br₂, Fe(bpy)₃Br₂, Co(phen)₃Br₂, Co(phen)₃Br₃, Co(en)₃Br₃, and Co(pn)₃Br₃ (where phen=1,10-phenanthroline, bpy=2,2-bipyridyl, en=ethylenediamine, and pn=1,2-propylenediamine) on the solubilities of nitrobenzene,o-,m-,and p-dinitrobenzenes (DNB), and toluene were studied in water at 25°C and compared to the results for sodium bromide and tetrabutylammonium bromide (Bu₄NBr). The Co(phen)₃³⁺, Fe(phen)₃²⁺, and Fe(bpy)₃²⁺ ions showed much stronger salting-in effects than did the Bu₄N⁺ ion, while the effects of the Co(en)₃³⁺ and Co(pn)₃³⁺ ions are comparable with those of Bu₄N⁺. A great dependence of salting-in on the polarity of dinitrobenzene isomers was found for Co(phen)₃³⁺ and Fe(phen)₃²⁺. The results were related to the partial molal volume of the respective cations. The very strong salting-in was considered to be mainly due to hydrophobic hydrations of the metal chelate cations and partly due to van der Waals interactions between the aromatic ligands and the nonelectrolytes. The small salting-in effects by Co(en)₃³⁺ and Co(en)₃³⁺ were interpreted in terms of hydrogen bonding between oxygen atoms of the nitro compounds or the solvent water molecules and hydrogen atoms attached to nitrogens in the complexes.