Isomerization of the cationic complex of the type Coen?Xi (X-= singly charged anionic ligand) in basic solution proceeds through ion-pair formation [l], 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 fruns + 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, 'en' transforms to 'amid0 form at one end. The combined effect of trans labilization of this 'amid0 end, spanning of one of the SzO:-(S,O:-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 cis + trans conversion at pH -6.5 indicates a common intermediate for the conversion of both the forms.