Determination of contact ion-pair formation in CoCl2 aqueous, methanol, and ethanol dilute solutions by UV-vis and X-ray absorption spectroscopies†

Abstract

A detailed investigation on the coordination environment of the Co²⁺ ion has been carried out with the intent of quantifying the contact ion-pair formation in dilute (0.1 mol L⁻¹) CoCl₂ aqueous, methanol (MeOH), and ethanol (EtOH) solutions. An effective approach has been employed combining UV-vis measurements, X-ray absorption spectroscopy, and density functional theory (DFT). The CoCl₂ metal salt is fully dissociated in aqueous solution with the Co²⁺ cation first hydration shell formed by six water molecules arranged in an octahedral fashion. On the other hand, the chloride anion enters the Co²⁺ coordination sphere giving rise to ionic pairs in MeOH and EtOH solution due to the weaker solvation ability of these solvents. The Co–Cl distances are 2.34(2) and 2.26(3) Å in MeOH and EtOH solutions, respectively, as determined by extended X-ray absorption fine structure data analysis. In MeOH solution the dominant species is the octahedral [CoCl(MeOH)₅]⁺ complex, while for EtOH the spectral evidence can be interpreted with an equilibrium between different four-fold metal-chloro species. Structural distortions in the coordination clusters have been evidenced by the X-ray absorption near-edge structure analysis aided by DFT optimizations and allowed us to rationalize the spectroscopic outcome of the UV-vis measurements. The adopted combined approach provided an all-around structural picture of the coordination complexes formed when the CoCl₂ salt is dissolved in solvents with different coordinating properties.