Electronic spectroscopy of some transition metal ion compounds in a new ionic liquid

The behavior of some transition metal cations [Cr (III), Fe (III), Fe (II), Co (II), Ni (II) and Cu (II)] were investigated in ammonium alum-urea ionic liquid. The study was followed by UV-Visible electronic spectroscopy to establish their coordination geometry in the liquid and estimate the ionic species attached to the transition metal cations. The reactivity of the metal cations were found to vary from one to another. Chromium (III) cation was found, (as expected) to be in octahedral geometry with ionic liquid species and was inert toward the addition of ligands. Similarly iron (III) did not show a tendency to react with added ligands, yet charge transfer was prevailed. However, interesting iron (II) showed high spin octahedral coordination with thiocyanate ion but a low spin octahedral complex with nitrite ion. Cobalt (II) showed an interesting behavior in alum-urea ionic liquid, as it coordinated in an octahedral geometry with its ionic species. A gradual geometrical changes were appeared as a major change in their spectra most probably forming a mixture of two species tetrahedral with some original octahedral complex when its solution was studied with increasing thiocyanate ion concentration up to 35 thiocyanate:1 cobalt mole ratio. The solubility limit of nitrite in ionic liquid ceased the concentration ratio of nitrite : cobalt to be 5:1. Nickel (II) also showed an octahedral geometry behavior in ionic liquid species. It reacts with added ligands particularly when the
concentration of thiocyanate ion : Ni (II) increased up to 30 to 1 mole ratio, the color changed from pale green to olive-green solution. Cupper (II) cation showed a clear blue solution and its spectra are assigned to be octahedral geometry, while the added nitrite did not show color change but less absorbance was recorded. However, thiocyanate ion seems to react with cupper (II) ionic liquid solution producing suspended green solution with much less absorbance than with ionic liquid. Interestingly, the water molecules present in the original ammonium alum was not found to coordinate with metal cations as their spectra was found to differ when compared with those obtained in aqueous solution.