Spectrophotometric study of thiocyanato complexation of cobalt(II) and nickel(II) ions in micellar solutions of a nonionic surfactant Triton X-100

Complexation of cobalt(II) and nickel(II) with thiocyanate ions has been studied by precise spectrophotometry in aqueous and micellar solutions of a nonionic surfactant Triton X-100 of varying concentrations (20-100 mmol-dm(-3)). With regard to cobalt(II), the formation of [Co(NCS)](+), [Co(NCS)(2)], and [Co(NCS)(4)](2-) was established. The formation constant of [Co(NCS)(4)](2-) is increased with increasing concentration of the surfactant, suggesting that the [Co(NCS)(4)](2-) complex is formed in micelles. In contrast, the formation constants of [Co(NCS)](+) and [Co(NCS)(2)] are remained practically unchanged. On the other hand, with nickel(II), the formation of sole [Ni(NCS)](+) and [Ni(NCS)(2)] was established in both aqueous and micellar solutions examined, their formation constants being also remained unchanged. interestingly no higher complex was confirmed in the nickel(ll) system, unlike cobalt(II). The unusual affinity of the [Co(NCS)(4)](2-) complex with micelles will be discussed from thermodynamic and structural points of view.