Geometrical isomers of bis(benzimidazol-2-ylethyl)sulfide)cobalt(II) diperchlorates: synthesis, structure, spectra and redox behavior of pink-[Co(bbes)2](ClO4)2 and blue-[Co(bbes)2](ClO4)2

1:1 and 1:2 cobalt complexes of bis(benzimidazol-2-ylmethyl)amine (bbma) bis(benzimidazol-2-ylmethyl)sulfide (bbms), bis(benzimidazol-2-ylethyl)sulfide (bbes) and diethylenetriamine (dien) were prepared and their spectral and redox behavior studied. Two geometrical isomers pink-[Co(bbes)2]2+ and blue-[Co(bbes)2]2+ were obtained when the complexes were prepared by using with bbes and they were separated manually and recrystallized. The octahedral structure of pink-[Co(bbes)2]2+ was resolved by X-ray analysis. The electronic spectra show the presence of two geometrical isomers for Co(bbes)22+ in the solid state; for example, the spectral bands of pink-[Co(bbes)2]2+ differs markedly with those of blue-[Co(bbes)2]2+. This is consistent with the results obtained from magnetic measurements (5.10 BM for pink-Co(bbes)22+ and 4.72 BM for blue-[Co(bbes)2]2+). Further, the behavior of the ligands (bbma, bbms, bbes) at different pH conditions was determined on the basis of 13C NMR studies. The redox potentials [Co(II)/Co(I)] of the complexes follow the trend [Co(bbma)2]2+ < [Co(bbms)2]2+ ≈ [Co(bbes)2]2+ which demonstrates the stabilization of the Co(II) ion is more by both weak σ-donor and weak π-acceptor ligands rather than by σ-donor ligand.