Electrostatic and Hydrophobic Interactions in the Ion Associations between Tripositive Complex Ions and Sulfonate Anions

Abstract

Abstract The ion associations of [Co(NH3)6]3+, [Cr(en)3]3+(en=ethylenediamine), lel3-[Cr(R-chxn)3]3+ lel3=lel,lel,lel-, and chxn=trans-1,2-cyclohexanediamine), [Co(bpy)3]3+ (bpy=2,2′-bipyridine), [Co(phen)3]3+(phen=1,10-phenanthroline), and [Cr(phen)3]3+ with Cl−, SO42−, CH3SO3−, C6H5SO3−, (CH2SO3)22−, and m-C6H4(SO3)22− were investigated at 25°C by means of conductivity measurements of aqueous mixed electrolyte solutions with a stoichiometrically constant ionic strength of 0.01 mol dm−3. The ion-association constants thus obtained were compared with each other and also with those estimated by the use of electrostatic theories of ion association. Some specific interactions besides such usual electrostatic ones were found to play important roles in the ion associations of SO42− with [Co(NH3)6]3+ and [Cr(en)3]3+, in those of C6H5SO3− and m-C6H4(SO3)22− with [Co(phen)3]3+, [Cr(phen)3]3+, and [Co(bpy)3]3+, and in those of SO42−, C6H5SO3−, and m-C6H4(SO3)22− with lel3-[Cr(R-chxn)3]3+. The specific interactions existing in the first group were presumed to be short-range electrostatic ones, such as hydrogen-bonding, and in the second group, the so-called hydrophobic ones. The complex ion, lel3-[Cr(R-chxn)3]3+, seemed to have both interactions, depending on the counter anions.