Cis-trans isomerization(III) in aquatic solution

Abstract Isomerization of the cationic complex of the type Coen 2 X 3+ 2 (X − = singly charged ligand) in basic solution proceeds through ion-pair formation [1]. The study of isomerization of the present anionic complex was undertaken as with this complex, ion-pair formation is untenable and isomerization may follow a different mechanistic path. The isomeric forms of the complex were prepared according to literature [2]. Isomerization of the cis complex takes place below pH ∼ 6.5. Above pH ∼ 6.5 trans → cis conversion takes place, increasing with pH of the medium. Dependence of isomerization on pH indicates a definite role of the hydroxyl ion, which acts as a catalyst only. In basic medium. ‘amido’ form end, spanning of one of the S 2− 3 (S 2 O 2− 3 may act as a bidentate group also) to adjacent cis position and nucleophilicity and ring closing tendency of the partially released end of ‘en’ gives the required trans → cis conversion. Nearly equal entropies of isomerization of both the forms and equilibrium of 2cis ⇌ trans conversion at pH ∼ 6.5 indicates a common intermediate for the conversion of both the forms.